Snap for 7263304 from 16514b8f2782005df002eb2e0e0cb0e957b6e694 to sc-d1-releaseandroid-12.0.0_r7android-12.0.0_r6android-12.0.0_r5android-12.0.0_r4android-12.0.0_r15android-12.0.0_r14android-12.0.0_r13android-12.0.0_r12android12-d1-s6-releaseandroid12-d1-s5-releaseandroid12-d1-s4-releaseandroid12-d1-s3-releaseandroid12-d1-s2-releaseandroid12-d1-s1-releaseandroid12-d1-release

Change-Id: I82bd4311011acad38c7e04f56e68d893868d1e19

52 files changed, 3263 insertions, 730 deletions

diff --git a/.cargo_vcs_info.json b/.cargo_vcs_info.json
index ca20d51..e778cc0 100644
--- a/.cargo_vcs_info.json
+++ b/.cargo_vcs_info.json
@@ -1,5 +1,5 @@
 {
   "git": {
-    "sha1": "b085c523681146151866ead66ae8a4b8967fd005"
+    "sha1": "4dac4c718895d8c0347d90be70f478673dd28193"
   }
 }
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..efc779f
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,51 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - staging
+      - trying
+
+jobs:
+  msrv:
+    name: Rust MSRV
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@1.36.0
+      - run: cargo check --no-default-features
+      - run: cargo check --no-default-features --features "use_alloc"
+      - run: cargo check
+
+  stable:
+    name: Rust Stable
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo check --no-default-features
+      - run: cargo check --no-default-features --features "use_alloc"
+      - run: cargo test
+
+  # https://github.com/rust-lang/crater/blob/9ab6f9697c901c4a44025cf0a39b73ad5b37d198/.github/workflows/bors.yml#L125-L149
+  end-success:
+    name: bors build finished
+    if: success()
+    runs-on: ubuntu-latest
+    needs: [msrv,stable]
+
+    steps:
+      - name: Mark the job as successful
+        run: exit 0
+
+  end-failure:
+    name: bors build finished
+    if: "!success()"
+    runs-on: ubuntu-latest
+    needs: [msrv,stable]
+
+    steps:
+      - name: Mark the job as a failure
+        run: exit 1
+
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index 8c4a598..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,20 +0,0 @@
-language: rust
-sudo: false
-matrix:
-  include:
-    - rust: 1.32.0
-    - rust: stable
-    - rust: beta
-    - rust: nightly
-branches:
-  only:
-    - master
-    # bors branches
-    - staging
-    - trying
-script:
-  - |
-      cargo build --verbose --no-default-features &&
-      cargo build --verbose --features "$FEATURES" &&
-      cargo test --verbose --features "$FEATURES" &&
-      cargo bench --verbose --features "$FEATURES"
diff --git a/Android.bp b/Android.bp
index cc98d26..963b929 100644
--- a/Android.bp
+++ b/Android.bp
@@ -45,6 +45,7 @@ rust_library {
     edition: "2018",
     features: [
         "default",
+        "use_alloc",
         "use_std",
     ],
     rustlibs: [
diff --git a/Cargo.toml b/Cargo.toml
index d8a9b8c..ffe2f37 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,7 +13,7 @@
 [package]
 edition = "2018"
 name = "itertools"
-version = "0.9.0"
+version = "0.10.0"
 authors = ["bluss"]
 exclude = ["/bors.toml"]
 description = "Extra iterator adaptors, iterator methods, free functions, and macros."
@@ -54,11 +54,22 @@ harness = false
 [[bench]]
 name = "bench1"
 harness = false
+
+[[bench]]
+name = "combinations"
+harness = false
+
+[[bench]]
+name = "powerset"
+harness = false
 [dependencies.either]
 version = "1.0"
 default-features = false
 [dev-dependencies.criterion]
-version = "=0.3.0"
+version = "=0"
+
+[dev-dependencies.paste]
+version = "1.0.0"
 
 [dev-dependencies.permutohedron]
 version = "0.2"
@@ -72,4 +83,5 @@ version = "0.7"
 
 [features]
 default = ["use_std"]
-use_std = []
+use_alloc = []
+use_std = ["use_alloc"]
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
index 472a27a..46b2094 100644
--- a/Cargo.toml.orig
+++ b/Cargo.toml.orig
@@ -1,6 +1,6 @@
 [package]
 name = "itertools"
-version = "0.9.0"
+version = "0.10.0"
 
 license = "MIT/Apache-2.0"
 repository = "https://github.com/bluss/rust-itertools"
@@ -27,7 +27,8 @@ either = { version = "1.0", default-features = false }
 
 [dev-dependencies]
 rand = "0.7"
-criterion = "=0.3.0" # TODO update criterion version once it becomes compatible with our minimum required Rust verision or bump minimum required rust version
+criterion = "=0" # TODO how could this work with our minimum supported rust version?
+paste = "1.0.0" # Used in test_std to instanciate generic tests
 
 [dev-dependencies.quickcheck]
 version = "0.9"
@@ -38,7 +39,8 @@ version = "0.2"
 
 [features]
 default = ["use_std"]
-use_std = []
+use_std = ["use_alloc"]
+use_alloc = []
 
 [profile]
 bench = { debug = true }
@@ -66,3 +68,11 @@ harness = false
 [[bench]]
 name = "bench1"
 harness = false
+
+[[bench]]
+name = "combinations"
+harness = false
+
+[[bench]]
+name = "powerset"
+harness = false
diff --git a/METADATA b/METADATA
index a2c5dd7..b617903 100644
--- a/METADATA
+++ b/METADATA
@@ -7,13 +7,13 @@ third_party {
   }
   url {
     type: ARCHIVE
-    value: "https://static.crates.io/crates/itertools/itertools-0.9.0.crate"
+    value: "https://static.crates.io/crates/itertools/itertools-0.10.0.crate"
   }
-  version: "0.9.0"
+  version: "0.10.0"
   license_type: NOTICE
   last_upgrade_date {
-    year: 2020
-    month: 12
-    day: 21
+    year: 2021
+    month: 4
+    day: 1
   }
 }
diff --git a/README.rst b/README.rst
index 24c99d3..4e37007 100644
--- a/README.rst
+++ b/README.rst
@@ -21,7 +21,7 @@ How to use with cargo:
 .. code:: toml
 
     [dependencies]
-    itertools = "0.8"
+    itertools = "0.9"
 
 How to use in your crate:
 
diff --git a/benches/combinations.rs b/benches/combinations.rs
new file mode 100644
index 0000000..e7433a4
--- /dev/null
+++ b/benches/combinations.rs
@@ -0,0 +1,125 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use itertools::Itertools;
+
+// approximate 100_000 iterations for each combination
+const N1: usize = 100_000;
+const N2: usize = 448;
+const N3: usize = 86;
+const N4: usize = 41;
+const N14: usize = 21;
+
+fn comb_for1(c: &mut Criterion) {
+    c.bench_function("comb for1", move |b| {
+        b.iter(|| {
+            for i in 0..N1 {
+                black_box(vec![i]);
+            }
+        })
+    });
+}
+
+fn comb_for2(c: &mut Criterion) {
+    c.bench_function("comb for2", move |b| {
+        b.iter(|| {
+            for i in 0..N2 {
+                for j in (i + 1)..N2 {
+                    black_box(vec![i, j]);
+                }
+            }
+        })
+    });
+}
+
+fn comb_for3(c: &mut Criterion) {
+    c.bench_function("comb for3", move |b| {
+        b.iter(|| {
+            for i in 0..N3 {
+                for j in (i + 1)..N3 {
+                    for k in (j + 1)..N3 {
+                        black_box(vec![i, j, k]);
+                    }
+                }
+            }
+        })
+    });
+}
+
+fn comb_for4(c: &mut Criterion) {
+    c.bench_function("comb for4", move |b| {
+        b.iter(|| {
+            for i in 0..N4 {
+                for j in (i + 1)..N4 {
+                    for k in (j + 1)..N4 {
+                        for l in (k + 1)..N4 {
+                            black_box(vec![i, j, k, l]);
+                        }
+                    }
+                }
+            }
+        })
+    });
+}
+
+fn comb_c1(c: &mut Criterion) {
+    c.bench_function("comb c1", move |b| {
+        b.iter(|| {
+            for combo in (0..N1).combinations(1) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c2(c: &mut Criterion) {
+    c.bench_function("comb c2", move |b| {
+        b.iter(|| {
+            for combo in (0..N2).combinations(2) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c3(c: &mut Criterion) {
+    c.bench_function("comb c3", move |b| {
+        b.iter(|| {
+            for combo in (0..N3).combinations(3) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c4(c: &mut Criterion) {
+    c.bench_function("comb c4", move |b| {
+        b.iter(|| {
+            for combo in (0..N4).combinations(4) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c14(c: &mut Criterion) {
+    c.bench_function("comb c14", move |b| {
+        b.iter(|| {
+            for combo in (0..N14).combinations(14) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+criterion_group!(
+    benches,
+    comb_for1,
+    comb_for2,
+    comb_for3,
+    comb_for4,
+    comb_c1,
+    comb_c2,
+    comb_c3,
+    comb_c4,
+    comb_c14,
+);
+criterion_main!(benches);
diff --git a/benches/fold_specialization.rs b/benches/fold_specialization.rs
index 53319a5..5de4671 100644
--- a/benches/fold_specialization.rs
+++ b/benches/fold_specialization.rs
@@ -9,7 +9,7 @@ where I: Iterator
     type Item = I::Item;
 
     #[inline(always)]
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
 
diff --git a/benches/powerset.rs b/benches/powerset.rs
new file mode 100644
index 0000000..074550b
--- /dev/null
+++ b/benches/powerset.rs
@@ -0,0 +1,44 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use itertools::Itertools;
+
+// Keep aggregate generated elements the same, regardless of powerset length.
+const TOTAL_ELEMENTS: usize = 1 << 12;
+const fn calc_iters(n: usize) -> usize {
+    TOTAL_ELEMENTS / (1 << n)
+}
+
+fn powerset_n(c: &mut Criterion, n: usize) {
+    let id = format!("powerset {}", n);
+    c.bench_function(id.as_str(), move |b| {
+        b.iter(|| {
+            for _ in 0..calc_iters(n) {
+                for elt in (0..n).powerset() {
+                    black_box(elt);
+                }
+            }
+        })
+    });
+}
+
+fn powerset_0(c: &mut Criterion) { powerset_n(c, 0); }
+
+fn powerset_1(c: &mut Criterion) { powerset_n(c, 1); }
+
+fn powerset_2(c: &mut Criterion) { powerset_n(c, 2); }
+
+fn powerset_4(c: &mut Criterion) { powerset_n(c, 4); }
+
+fn powerset_8(c: &mut Criterion) { powerset_n(c, 8); }
+
+fn powerset_12(c: &mut Criterion) { powerset_n(c, 12); }
+
+criterion_group!(
+    benches,
+    powerset_0,
+    powerset_1,
+    powerset_2,
+    powerset_4,
+    powerset_8,
+    powerset_12,
+);
+criterion_main!(benches);
\ No newline at end of file
diff --git a/benches/tuple_combinations.rs b/benches/tuple_combinations.rs
index 84411ef..4e26b28 100644
--- a/benches/tuple_combinations.rs
+++ b/benches/tuple_combinations.rs
@@ -7,8 +7,8 @@ const N2: usize = 448;
 const N3: usize = 86;
 const N4: usize = 41;
 
-fn comb_for1(c: &mut Criterion) {
-    c.bench_function("comb for1", move |b| {
+fn tuple_comb_for1(c: &mut Criterion) {
+    c.bench_function("tuple comb for1", move |b| {
         b.iter(|| {
             for i in 0..N1 {
                 black_box(i);
@@ -17,8 +17,8 @@ fn comb_for1(c: &mut Criterion) {
     });
 }
 
-fn comb_for2(c: &mut Criterion) {
-    c.bench_function("comb for2", move |b| {
+fn tuple_comb_for2(c: &mut Criterion) {
+    c.bench_function("tuple comb for2", move |b| {
         b.iter(|| {
             for i in 0..N2 {
                 for j in (i + 1)..N2 {
@@ -29,8 +29,8 @@ fn comb_for2(c: &mut Criterion) {
     });
 }
 
-fn comb_for3(c: &mut Criterion) {
-    c.bench_function("comb for3", move |b| {
+fn tuple_comb_for3(c: &mut Criterion) {
+    c.bench_function("tuple comb for3", move |b| {
         b.iter(|| {
             for i in 0..N3 {
                 for j in (i + 1)..N3 {
@@ -43,8 +43,8 @@ fn comb_for3(c: &mut Criterion) {
     });
 }
 
-fn comb_for4(c: &mut Criterion) {
-    c.bench_function("comb for4", move |b| {
+fn tuple_comb_for4(c: &mut Criterion) {
+    c.bench_function("tuple comb for4", move |b| {
         b.iter(|| {
             for i in 0..N4 {
                 for j in (i + 1)..N4 {
@@ -59,8 +59,8 @@ fn comb_for4(c: &mut Criterion) {
     });
 }
 
-fn comb_c1(c: &mut Criterion) {
-    c.bench_function("comb c1", move |b| {
+fn tuple_comb_c1(c: &mut Criterion) {
+    c.bench_function("tuple comb c1", move |b| {
         b.iter(|| {
             for (i,) in (0..N1).tuple_combinations() {
                 black_box(i);
@@ -69,8 +69,8 @@ fn comb_c1(c: &mut Criterion) {
     });
 }
 
-fn comb_c2(c: &mut Criterion) {
-    c.bench_function("comb c2", move |b| {
+fn tuple_comb_c2(c: &mut Criterion) {
+    c.bench_function("tuple comb c2", move |b| {
         b.iter(|| {
             for (i, j) in (0..N2).tuple_combinations() {
                 black_box(i + j);
@@ -79,8 +79,8 @@ fn comb_c2(c: &mut Criterion) {
     });
 }
 
-fn comb_c3(c: &mut Criterion) {
-    c.bench_function("comb c3", move |b| {
+fn tuple_comb_c3(c: &mut Criterion) {
+    c.bench_function("tuple comb c3", move |b| {
         b.iter(|| {
             for (i, j, k) in (0..N3).tuple_combinations() {
                 black_box(i + j + k);
@@ -89,8 +89,8 @@ fn comb_c3(c: &mut Criterion) {
     });
 }
 
-fn comb_c4(c: &mut Criterion) {
-    c.bench_function("comb c4", move |b| {
+fn tuple_comb_c4(c: &mut Criterion) {
+    c.bench_function("tuple comb c4", move |b| {
         b.iter(|| {
             for (i, j, k, l) in (0..N4).tuple_combinations() {
                 black_box(i + j + k + l);
@@ -101,13 +101,13 @@ fn comb_c4(c: &mut Criterion) {
 
 criterion_group!(
     benches,
-    comb_for1,
-    comb_for2,
-    comb_for3,
-    comb_for4,
-    comb_c1,
-    comb_c2,
-    comb_c3,
-    comb_c4,
+    tuple_comb_for1,
+    tuple_comb_for2,
+    tuple_comb_for3,
+    tuple_comb_for4,
+    tuple_comb_c1,
+    tuple_comb_c2,
+    tuple_comb_c3,
+    tuple_comb_c4,
 );
 criterion_main!(benches);
diff --git a/examples/iris.rs b/examples/iris.rs
index 25ab373..987d9e9 100644
--- a/examples/iris.rs
+++ b/examples/iris.rs
@@ -55,7 +55,7 @@ fn main() {
     // using Itertools::fold_results to create the result of parsing
     let irises = DATA.lines()
                      .map(str::parse)
-                     .fold_results(Vec::new(), |mut v, iris: Iris| {
+                     .fold_ok(Vec::new(), |mut v, iris: Iris| {
                          v.push(iris);
                          v
                      });
diff --git a/src/adaptors/coalesce.rs b/src/adaptors/coalesce.rs
new file mode 100644
index 0000000..116f688
--- /dev/null
+++ b/src/adaptors/coalesce.rs
@@ -0,0 +1,232 @@
+use std::fmt;
+use std::iter::FusedIterator;
+
+use crate::size_hint;
+
+pub struct CoalesceBy<I, F, T>
+where
+    I: Iterator,
+{
+    iter: I,
+    last: Option<T>,
+    f: F,
+}
+
+impl<I: Clone, F: Clone, T: Clone> Clone for CoalesceBy<I, F, T>
+where
+    I: Iterator,
+{
+    clone_fields!(last, iter, f);
+}
+
+impl<I, F, T> fmt::Debug for CoalesceBy<I, F, T>
+where
+    I: Iterator + fmt::Debug,
+    T: fmt::Debug,
+{
+    debug_fmt_fields!(CoalesceBy, iter);
+}
+
+pub trait CoalescePredicate<Item, T> {
+    fn coalesce_pair(&mut self, t: T, item: Item) -> Result<T, (T, T)>;
+}
+
+impl<I, F, T> Iterator for CoalesceBy<I, F, T>
+where
+    I: Iterator,
+    F: CoalescePredicate<I::Item, T>,
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        // this fuses the iterator
+        let mut last = match self.last.take() {
+            None => return None,
+            Some(x) => x,
+        };
+        for next in &mut self.iter {
+            match self.f.coalesce_pair(last, next) {
+                Ok(joined) => last = joined,
+                Err((last_, next_)) => {
+                    self.last = Some(next_);
+                    return Some(last_);
+                }
+            }
+        }
+        Some(last)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (low, hi) = size_hint::add_scalar(self.iter.size_hint(), self.last.is_some() as usize);
+        ((low > 0) as usize, hi)
+    }
+
+    fn fold<Acc, FnAcc>(self, acc: Acc, mut fn_acc: FnAcc) -> Acc
+    where
+        FnAcc: FnMut(Acc, Self::Item) -> Acc,
+    {
+        if let Some(last) = self.last {
+            let mut f = self.f;
+            let (last, acc) = self.iter.fold((last, acc), |(last, acc), elt| {
+                match f.coalesce_pair(last, elt) {
+                    Ok(joined) => (joined, acc),
+                    Err((last_, next_)) => (next_, fn_acc(acc, last_)),
+                }
+            });
+            fn_acc(acc, last)
+        } else {
+            acc
+        }
+    }
+}
+
+impl<I: Iterator, F: CoalescePredicate<I::Item, T>, T> FusedIterator for CoalesceBy<I, F, T> {}
+
+/// An iterator adaptor that may join together adjacent elements.
+///
+/// See [`.coalesce()`](../trait.Itertools.html#method.coalesce) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub type Coalesce<I, F> = CoalesceBy<I, F, <I as Iterator>::Item>;
+
+impl<F, Item, T> CoalescePredicate<Item, T> for F
+where
+    F: FnMut(T, Item) -> Result<T, (T, T)>,
+{
+    fn coalesce_pair(&mut self, t: T, item: Item) -> Result<T, (T, T)> {
+        self(t, item)
+    }
+}
+
+/// Create a new `Coalesce`.
+pub fn coalesce<I, F>(mut iter: I, f: F) -> Coalesce<I, F>
+where
+    I: Iterator,
+{
+    Coalesce {
+        last: iter.next(),
+        iter,
+        f,
+    }
+}
+
+/// An iterator adaptor that removes repeated duplicates, determining equality using a comparison function.
+///
+/// See [`.dedup_by()`](../trait.Itertools.html#method.dedup_by) or [`.dedup()`](../trait.Itertools.html#method.dedup) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub type DedupBy<I, Pred> = CoalesceBy<I, DedupPred2CoalescePred<Pred>, <I as Iterator>::Item>;
+
+#[derive(Clone)]
+pub struct DedupPred2CoalescePred<DP>(DP);
+
+pub trait DedupPredicate<T> {
+    // TODO replace by Fn(&T, &T)->bool once Rust supports it
+    fn dedup_pair(&mut self, a: &T, b: &T) -> bool;
+}
+
+impl<DP, T> CoalescePredicate<T, T> for DedupPred2CoalescePred<DP>
+where
+    DP: DedupPredicate<T>,
+{
+    fn coalesce_pair(&mut self, t: T, item: T) -> Result<T, (T, T)> {
+        if self.0.dedup_pair(&t, &item) {
+            Ok(t)
+        } else {
+            Err((t, item))
+        }
+    }
+}
+
+#[derive(Clone)]
+pub struct DedupEq;
+
+impl<T: PartialEq> DedupPredicate<T> for DedupEq {
+    fn dedup_pair(&mut self, a: &T, b: &T) -> bool {
+        a == b
+    }
+}
+
+impl<T, F: FnMut(&T, &T) -> bool> DedupPredicate<T> for F {
+    fn dedup_pair(&mut self, a: &T, b: &T) -> bool {
+        self(a, b)
+    }
+}
+
+/// Create a new `DedupBy`.
+pub fn dedup_by<I, Pred>(mut iter: I, dedup_pred: Pred) -> DedupBy<I, Pred>
+where
+    I: Iterator,
+{
+    DedupBy {
+        last: iter.next(),
+        iter,
+        f: DedupPred2CoalescePred(dedup_pred),
+    }
+}
+
+/// An iterator adaptor that removes repeated duplicates.
+///
+/// See [`.dedup()`](../trait.Itertools.html#method.dedup) for more information.
+pub type Dedup<I> = DedupBy<I, DedupEq>;
+
+/// Create a new `Dedup`.
+pub fn dedup<I>(iter: I) -> Dedup<I>
+where
+    I: Iterator,
+{
+    dedup_by(iter, DedupEq)
+}
+
+/// An iterator adaptor that removes repeated duplicates, while keeping a count of how many
+/// repeated elements were present. This will determine equality using a comparison function.
+///
+/// See [`.dedup_by_with_count()`](../trait.Itertools.html#method.dedup_by_with_count) or
+/// [`.dedup_with_count()`](../trait.Itertools.html#method.dedup_with_count) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub type DedupByWithCount<I, Pred> =
+    CoalesceBy<I, DedupPredWithCount2CoalescePred<Pred>, (usize, <I as Iterator>::Item)>;
+
+#[derive(Clone)]
+pub struct DedupPredWithCount2CoalescePred<DP>(DP);
+
+impl<DP, T> CoalescePredicate<T, (usize, T)> for DedupPredWithCount2CoalescePred<DP>
+where
+    DP: DedupPredicate<T>,
+{
+    fn coalesce_pair(
+        &mut self,
+        (c, t): (usize, T),
+        item: T,
+    ) -> Result<(usize, T), ((usize, T), (usize, T))> {
+        if self.0.dedup_pair(&t, &item) {
+            Ok((c + 1, t))
+        } else {
+            Err(((c, t), (1, item)))
+        }
+    }
+}
+
+/// An iterator adaptor that removes repeated duplicates, while keeping a count of how many
+/// repeated elements were present.
+///
+/// See [`.dedup_with_count()`](../trait.Itertools.html#method.dedup_with_count) for more information.
+pub type DedupWithCount<I> = DedupByWithCount<I, DedupEq>;
+
+/// Create a new `DedupByWithCount`.
+pub fn dedup_by_with_count<I, Pred>(mut iter: I, dedup_pred: Pred) -> DedupByWithCount<I, Pred>
+where
+    I: Iterator,
+{
+    DedupByWithCount {
+        last: iter.next().map(|v| (1, v)),
+        iter,
+        f: DedupPredWithCount2CoalescePred(dedup_pred),
+    }
+}
+
+/// Create a new `DedupWithCount`.
+pub fn dedup_with_count<I>(iter: I) -> DedupWithCount<I>
+where
+    I: Iterator,
+{
+    dedup_by_with_count(iter, DedupEq)
+}
diff --git a/src/adaptors/map.rs b/src/adaptors/map.rs
new file mode 100644
index 0000000..ff377f7
--- /dev/null
+++ b/src/adaptors/map.rs
@@ -0,0 +1,120 @@
+use std::iter::FromIterator;
+use std::marker::PhantomData;
+
+#[derive(Clone)]
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub struct MapSpecialCase<I, F> {
+    iter: I,
+    f: F,
+}
+
+pub trait MapSpecialCaseFn<T> {
+    type Out;
+    fn call(&mut self, t: T) -> Self::Out;
+}
+
+impl<I, R> Iterator for MapSpecialCase<I, R>
+where
+    I: Iterator,
+    R: MapSpecialCaseFn<I::Item>,
+{
+    type Item = R::Out;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iter.next().map(|i| self.f.call(i))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+
+    fn fold<Acc, Fold>(self, init: Acc, mut fold_f: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut f = self.f;
+        self.iter.fold(init, move |acc, v| fold_f(acc, f.call(v)))
+    }
+
+    fn collect<C>(self) -> C
+    where
+        C: FromIterator<Self::Item>,
+    {
+        let mut f = self.f;
+        self.iter.map(move |v| f.call(v)).collect()
+    }
+}
+
+impl<I, R> DoubleEndedIterator for MapSpecialCase<I, R>
+where
+    I: DoubleEndedIterator,
+    R: MapSpecialCaseFn<I::Item>,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter.next_back().map(|i| self.f.call(i))
+    }
+}
+
+impl<I, R> ExactSizeIterator for MapSpecialCase<I, R>
+where
+    I: ExactSizeIterator,
+    R: MapSpecialCaseFn<I::Item>,
+{
+}
+
+/// An iterator adapter to apply a transformation within a nested `Result::Ok`.
+///
+/// See [`.map_ok()`](../trait.Itertools.html#method.map_ok) for more information.
+pub type MapOk<I, F> = MapSpecialCase<I, MapSpecialCaseFnOk<F>>;
+
+/// See [`MapOk`](struct.MapOk.html).
+#[deprecated(note = "Use MapOk instead", since = "0.10.0")]
+pub type MapResults<I, F> = MapOk<I, F>;
+
+impl<F, T, U, E> MapSpecialCaseFn<Result<T, E>> for MapSpecialCaseFnOk<F>
+where
+    F: FnMut(T) -> U,
+{
+    type Out = Result<U, E>;
+    fn call(&mut self, t: Result<T, E>) -> Self::Out {
+        t.map(|v| self.0(v))
+    }
+}
+
+#[derive(Clone)]
+pub struct MapSpecialCaseFnOk<F>(F);
+
+/// Create a new `MapOk` iterator.
+pub fn map_ok<I, F, T, U, E>(iter: I, f: F) -> MapOk<I, F>
+where
+    I: Iterator<Item = Result<T, E>>,
+    F: FnMut(T) -> U,
+{
+    MapSpecialCase {
+        iter,
+        f: MapSpecialCaseFnOk(f),
+    }
+}
+
+/// An iterator adapter to apply `Into` conversion to each element.
+///
+/// See [`.map_into()`](../trait.Itertools.html#method.map_into) for more information.
+pub type MapInto<I, R> = MapSpecialCase<I, MapSpecialCaseFnInto<R>>;
+
+impl<T: Into<U>, U> MapSpecialCaseFn<T> for MapSpecialCaseFnInto<U> {
+    type Out = U;
+    fn call(&mut self, t: T) -> Self::Out {
+        t.into()
+    }
+}
+
+#[derive(Clone)]
+pub struct MapSpecialCaseFnInto<U>(PhantomData<U>);
+
+/// Create a new [`MapInto`](struct.MapInto.html) iterator.
+pub fn map_into<I, R>(iter: I) -> MapInto<I, R> {
+    MapSpecialCase {
+        iter,
+        f: MapSpecialCaseFnInto(PhantomData),
+    }
+}
diff --git a/src/adaptors/mod.rs b/src/adaptors/mod.rs
index 7d61f11..8a8697b 100644
--- a/src/adaptors/mod.rs
+++ b/src/adaptors/mod.rs
@@ -4,12 +4,17 @@
 //! option. This file may not be copied, modified, or distributed
 //! except according to those terms.
 
+mod coalesce;
+mod map;
 mod multi_product;
-#[cfg(feature = "use_std")]
+pub use self::coalesce::*;
+pub use self::map::{map_into, map_ok, MapInto, MapOk};
+#[allow(deprecated)]
+pub use self::map::MapResults;
+#[cfg(feature = "use_alloc")]
 pub use self::multi_product::*;
 
 use std::fmt;
-use std::mem::replace;
 use std::iter::{Fuse, Peekable, FromIterator};
 use std::marker::PhantomData;
 use crate::size_hint;
@@ -32,13 +37,7 @@ pub struct Interleave<I, J> {
 ///
 /// `IntoIterator` enabled version of `i.interleave(j)`.
 ///
-/// ```
-/// use itertools::interleave;
-///
-/// for elt in interleave(&[1, 2, 3], &[2, 3, 4]) {
-///     /* loop body */
-/// }
-/// ```
+/// See [`.interleave()`](trait.Itertools.html#method.interleave) for more information.
 pub fn interleave<I, J>(i: I, j: J) -> Interleave<<I as IntoIterator>::IntoIter, <J as IntoIterator>::IntoIter>
     where I: IntoIterator,
           J: IntoIterator<Item = I::Item>
@@ -56,7 +55,7 @@ impl<I, J> Iterator for Interleave<I, J>
 {
     type Item = I::Item;
     #[inline]
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         self.flag = !self.flag;
         if self.flag {
             match self.a.next() {
@@ -113,23 +112,12 @@ impl<I, J> Iterator for InterleaveShortest<I, J>
     type Item = I::Item;
 
     #[inline]
-    fn next(&mut self) -> Option<I::Item> {
-        match self.phase {
-            false => match self.it0.next() {
-                None => None,
-                e => {
-                    self.phase = true;
-                    e
-                }
-            },
-            true => match self.it1.next() {
-                None => None,
-                e => {
-                    self.phase = false;
-                    e
-                }
-            },
+    fn next(&mut self) -> Option<Self::Item> {
+        let e = if self.phase { self.it1.next() } else { self.it0.next() };
+        if e.is_some() {
+            self.phase = !self.phase;
         }
+        e
     }
 
     #[inline]
@@ -221,7 +209,7 @@ impl<I> Iterator for PutBack<I>
 {
     type Item = I::Item;
     #[inline]
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         match self.top {
             None => self.iter.next(),
             ref mut some => some.take(),
@@ -310,14 +298,14 @@ pub fn cartesian_product<I, J>(mut i: I, j: J) -> Product<I, J>
     }
 }
 
-
 impl<I, J> Iterator for Product<I, J>
     where I: Iterator,
           J: Clone + Iterator,
           I::Item: Clone
 {
     type Item = (I::Item, J::Item);
-    fn next(&mut self) -> Option<(I::Item, J::Item)> {
+
+    fn next(&mut self) -> Option<Self::Item> {
         let elt_b = match self.b.next() {
             None => {
                 self.b = self.b_orig.clone();
@@ -401,15 +389,9 @@ impl<B, F, I> Iterator for Batching<I, F>
 {
     type Item = B;
     #[inline]
-    fn next(&mut self) -> Option<B> {
+    fn next(&mut self) -> Option<Self::Item> {
         (self.f)(&mut self.iter)
     }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        // No information about closue behavior
-        (0, None)
-    }
 }
 
 /// An iterator adaptor that steps a number elements in the base iterator
@@ -419,7 +401,7 @@ impl<B, F, I> Iterator for Batching<I, F>
 /// then skipping forward *n-1* elements.
 ///
 /// See [`.step()`](../trait.Itertools.html#method.step) for more information.
-#[deprecated(note="Use std .step_by() instead", since="0.8")]
+#[deprecated(note="Use std .step_by() instead", since="0.8.0")]
 #[allow(deprecated)]
 #[derive(Clone, Debug)]
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
@@ -448,7 +430,7 @@ impl<I> Iterator for Step<I>
 {
     type Item = I::Item;
     #[inline]
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         let elt = self.iter.next();
         if self.skip > 0 {
             self.iter.nth(self.skip - 1);
@@ -577,7 +559,7 @@ impl<I, J, F> Iterator for MergeBy<I, J, F>
 {
     type Item = I::Item;
 
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         let less_than = match self.fused {
             Some(lt) => lt,
             None => match (self.a.peek(), self.b.peek()) {
@@ -606,203 +588,6 @@ impl<I, J, F> Iterator for MergeBy<I, J, F>
     }
 }
 
-#[derive(Clone, Debug)]
-pub struct CoalesceCore<I>
-    where I: Iterator
-{
-    iter: I,
-    last: Option<I::Item>,
-}
-
-impl<I> CoalesceCore<I>
-    where I: Iterator
-{
-    fn next_with<F>(&mut self, mut f: F) -> Option<I::Item>
-        where F: FnMut(I::Item, I::Item) -> Result<I::Item, (I::Item, I::Item)>
-    {
-        // this fuses the iterator
-        let mut last = match self.last.take() {
-            None => return None,
-            Some(x) => x,
-        };
-        for next in &mut self.iter {
-            match f(last, next) {
-                Ok(joined) => last = joined,
-                Err((last_, next_)) => {
-                    self.last = Some(next_);
-                    return Some(last_);
-                }
-            }
-        }
-
-        Some(last)
-    }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let (low, hi) = size_hint::add_scalar(self.iter.size_hint(),
-                                              self.last.is_some() as usize);
-        ((low > 0) as usize, hi)
-    }
-}
-
-/// An iterator adaptor that may join together adjacent elements.
-///
-/// See [`.coalesce()`](../trait.Itertools.html#method.coalesce) for more information.
-#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
-pub struct Coalesce<I, F>
-    where I: Iterator
-{
-    iter: CoalesceCore<I>,
-    f: F,
-}
-
-impl<I: Clone, F: Clone> Clone for Coalesce<I, F>
-    where I: Iterator,
-          I::Item: Clone
-{
-    clone_fields!(iter, f);
-}
-
-impl<I, F> fmt::Debug for Coalesce<I, F>
-    where I: Iterator + fmt::Debug,
-          I::Item: fmt::Debug,
-{
-    debug_fmt_fields!(Coalesce, iter);
-}
-
-/// Create a new `Coalesce`.
-pub fn coalesce<I, F>(mut iter: I, f: F) -> Coalesce<I, F>
-    where I: Iterator
-{
-    Coalesce {
-        iter: CoalesceCore {
-            last: iter.next(),
-            iter,
-        },
-        f,
-    }
-}
-
-impl<I, F> Iterator for Coalesce<I, F>
-    where I: Iterator,
-          F: FnMut(I::Item, I::Item) -> Result<I::Item, (I::Item, I::Item)>
-{
-    type Item = I::Item;
-
-    fn next(&mut self) -> Option<I::Item> {
-        self.iter.next_with(&mut self.f)
-    }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
-    }
-}
-
-/// An iterator adaptor that removes repeated duplicates, determining equality using a comparison function.
-///
-/// See [`.dedup_by()`](../trait.Itertools.html#method.dedup_by) or [`.dedup()`](../trait.Itertools.html#method.dedup) for more information.
-#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
-pub struct DedupBy<I, Pred>
-    where I: Iterator
-{
-    iter: CoalesceCore<I>,
-    dedup_pred: Pred,
-}
-
-pub trait DedupPredicate<T> { // TODO replace by Fn(&T, &T)->bool once Rust supports it
-    fn dedup_pair(&mut self, a: &T, b: &T) -> bool;
-}
-
-#[derive(Clone)]
-pub struct DedupEq;
-
-impl<T: PartialEq> DedupPredicate<T> for DedupEq {
-    fn dedup_pair(&mut self, a: &T, b: &T) -> bool {
-        a==b
-    }
-}
-
-impl<T, F: FnMut(&T, &T)->bool> DedupPredicate<T> for F {
-    fn dedup_pair(&mut self, a: &T, b: &T) -> bool {
-        self(a, b)
-    }
-}
-
-/// An iterator adaptor that removes repeated duplicates.
-///
-/// See [`.dedup()`](../trait.Itertools.html#method.dedup) for more information.
-pub type Dedup<I>=DedupBy<I, DedupEq>;
-
-impl<I: Clone, Pred: Clone> Clone for DedupBy<I, Pred>
-    where I: Iterator,
-          I::Item: Clone,
-{
-    clone_fields!(iter, dedup_pred);
-}
-
-/// Create a new `DedupBy`.
-pub fn dedup_by<I, Pred>(mut iter: I, dedup_pred: Pred) -> DedupBy<I, Pred>
-    where I: Iterator,
-{
-    DedupBy {
-        iter: CoalesceCore {
-            last: iter.next(),
-            iter,
-        },
-        dedup_pred,
-    }
-}
-
-/// Create a new `Dedup`.
-pub fn dedup<I>(iter: I) -> Dedup<I>
-    where I: Iterator
-{
-    dedup_by(iter, DedupEq)
-}
-
-impl<I, Pred> fmt::Debug for DedupBy<I, Pred>
-    where I: Iterator + fmt::Debug,
-          I::Item: fmt::Debug,
-{
-    debug_fmt_fields!(Dedup, iter);
-}
-
-impl<I, Pred> Iterator for DedupBy<I, Pred>
-    where I: Iterator,
-          Pred: DedupPredicate<I::Item>,
-{
-    type Item = I::Item;
-
-    fn next(&mut self) -> Option<I::Item> {
-        let ref mut dedup_pred = self.dedup_pred;
-        self.iter.next_with(|x, y| {
-            if dedup_pred.dedup_pair(&x, &y) { Ok(x) } else { Err((x, y)) }
-        })
-    }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
-    }
-
-    fn fold<Acc, G>(self, mut accum: Acc, mut f: G) -> Acc
-        where G: FnMut(Acc, Self::Item) -> Acc,
-    {
-        if let Some(mut last) = self.iter.last {
-            let mut dedup_pred = self.dedup_pred;
-            accum = self.iter.iter.fold(accum, |acc, elt| {
-                if dedup_pred.dedup_pair(&elt, &last) {
-                    acc
-                } else {
-                    f(acc, replace(&mut last, elt))
-                }
-            });
-            f(accum, last)
-        } else {
-            accum
-        }
-    }
-}
-
 /// An iterator adaptor that borrows from a `Clone`-able iterator
 /// to only pick off elements while the predicate returns `true`.
 ///
@@ -832,7 +617,7 @@ impl<'a, I, F> Iterator for TakeWhileRef<'a, I, F>
 {
     type Item = I::Item;
 
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         let old = self.iter.clone();
         match self.iter.next() {
             None => None,
@@ -848,8 +633,7 @@ impl<'a, I, F> Iterator for TakeWhileRef<'a, I, F>
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let (_, hi) = self.iter.size_hint();
-        (0, hi)
+        (0, self.iter.size_hint().1)
     }
 }
 
@@ -873,7 +657,7 @@ impl<I, A> Iterator for WhileSome<I>
 {
     type Item = A;
 
-    fn next(&mut self) -> Option<A> {
+    fn next(&mut self) -> Option<Self::Item> {
         match self.iter.next() {
             None | Some(None) => None,
             Some(elt) => elt,
@@ -881,8 +665,7 @@ impl<I, A> Iterator for WhileSome<I>
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let sh = self.iter.size_hint();
-        (0, sh.1)
+        (0, self.iter.size_hint().1)
     }
 }
 
@@ -1012,115 +795,163 @@ macro_rules! impl_tuple_combination {
     )
 }
 
-impl_tuple_combination!(Tuple2Combination Tuple1Combination ; A, A, A ; a);
-impl_tuple_combination!(Tuple3Combination Tuple2Combination ; A, A, A, A ; a b);
-impl_tuple_combination!(Tuple4Combination Tuple3Combination ; A, A, A, A, A; a b c);
-
-/// An iterator adapter to apply `Into` conversion to each element.
+// This snippet generates the twelve `impl_tuple_combination!` invocations:
+//    use core::iter;
+//    use itertools::Itertools;
+//
+//    for i in 2..=12 {
+//        println!("impl_tuple_combination!(Tuple{arity}Combination Tuple{prev}Combination; {tys}; {idents});",
+//            arity = i,
+//            prev = i - 1,
+//            tys = iter::repeat("A").take(i + 1).join(", "),
+//            idents = ('a'..'z').take(i - 1).join(" "),
+//        );
+//    }
+// It could probably be replaced by a bit more macro cleverness.
+impl_tuple_combination!(Tuple2Combination Tuple1Combination; A, A, A; a);
+impl_tuple_combination!(Tuple3Combination Tuple2Combination; A, A, A, A; a b);
+impl_tuple_combination!(Tuple4Combination Tuple3Combination; A, A, A, A, A; a b c);
+impl_tuple_combination!(Tuple5Combination Tuple4Combination; A, A, A, A, A, A; a b c d);
+impl_tuple_combination!(Tuple6Combination Tuple5Combination; A, A, A, A, A, A, A; a b c d e);
+impl_tuple_combination!(Tuple7Combination Tuple6Combination; A, A, A, A, A, A, A, A; a b c d e f);
+impl_tuple_combination!(Tuple8Combination Tuple7Combination; A, A, A, A, A, A, A, A, A; a b c d e f g);
+impl_tuple_combination!(Tuple9Combination Tuple8Combination; A, A, A, A, A, A, A, A, A, A; a b c d e f g h);
+impl_tuple_combination!(Tuple10Combination Tuple9Combination; A, A, A, A, A, A, A, A, A, A, A; a b c d e f g h i);
+impl_tuple_combination!(Tuple11Combination Tuple10Combination; A, A, A, A, A, A, A, A, A, A, A, A; a b c d e f g h i j);
+impl_tuple_combination!(Tuple12Combination Tuple11Combination; A, A, A, A, A, A, A, A, A, A, A, A, A; a b c d e f g h i j k);
+
+/// An iterator adapter to filter values within a nested `Result::Ok`.
 ///
-/// See [`.map_into()`](../trait.Itertools.html#method.map_into) for more information.
+/// See [`.filter_ok()`](../trait.Itertools.html#method.filter_ok) for more information.
 #[derive(Clone)]
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
-pub struct MapInto<I, R> {
+pub struct FilterOk<I, F> {
     iter: I,
-    _res: PhantomData<fn() -> R>,
+    f: F
 }
 
-/// Create a new [`MapInto`](struct.MapInto.html) iterator.
-pub fn map_into<I, R>(iter: I) -> MapInto<I, R> {
-    MapInto {
+/// Create a new `FilterOk` iterator.
+pub fn filter_ok<I, F, T, E>(iter: I, f: F) -> FilterOk<I, F>
+    where I: Iterator<Item = Result<T, E>>,
+          F: FnMut(&T) -> bool,
+{
+    FilterOk {
         iter,
-        _res: PhantomData,
+        f,
     }
 }
 
-impl<I, R> Iterator for MapInto<I, R>
-    where I: Iterator,
-          I::Item: Into<R>,
+impl<I, F, T, E> Iterator for FilterOk<I, F>
+    where I: Iterator<Item = Result<T, E>>,
+          F: FnMut(&T) -> bool,
 {
-    type Item = R;
+    type Item = Result<T, E>;
 
-    fn next(&mut self) -> Option<R> {
-        self.iter
-            .next()
-            .map(|i| i.into())
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            match self.iter.next() {
+                Some(Ok(v)) => {
+                    if (self.f)(&v) {
+                        return Some(Ok(v));
+                    }
+                },
+                Some(Err(e)) => return Some(Err(e)),
+                None => return None,
+            }
+        }
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
+        (0, self.iter.size_hint().1)
     }
 
-    fn fold<Acc, Fold>(self, init: Acc, mut fold_f: Fold) -> Acc
+    fn fold<Acc, Fold>(self, init: Acc, fold_f: Fold) -> Acc
         where Fold: FnMut(Acc, Self::Item) -> Acc,
     {
-        self.iter.fold(init, move |acc, v| fold_f(acc, v.into()))
+        let mut f = self.f;
+        self.iter.filter(|v| {
+            v.as_ref().map(&mut f).unwrap_or(true)
+        }).fold(init, fold_f)
     }
-}
 
-impl<I, R> DoubleEndedIterator for MapInto<I, R>
-    where I: DoubleEndedIterator,
-          I::Item: Into<R>,
-{
-    fn next_back(&mut self) -> Option<Self::Item> {
-        self.iter
-            .next_back()
-            .map(|i| i.into())
+    fn collect<C>(self) -> C
+        where C: FromIterator<Self::Item>
+    {
+        let mut f = self.f;
+        self.iter.filter(|v| {
+            v.as_ref().map(&mut f).unwrap_or(true)
+        }).collect()
     }
 }
 
-impl<I, R> ExactSizeIterator for MapInto<I, R>
-where
-    I: ExactSizeIterator,
-    I::Item: Into<R>,
-{}
-
-/// An iterator adapter to apply a transformation within a nested `Result`.
+/// An iterator adapter to filter and apply a transformation on values within a nested `Result::Ok`.
 ///
-/// See [`.map_results()`](../trait.Itertools.html#method.map_results) for more information.
-#[derive(Clone)]
+/// See [`.filter_map_ok()`](../trait.Itertools.html#method.filter_map_ok) for more information.
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
-pub struct MapResults<I, F> {
+pub struct FilterMapOk<I, F> {
     iter: I,
     f: F
 }
 
-/// Create a new `MapResults` iterator.
-pub fn map_results<I, F, T, U, E>(iter: I, f: F) -> MapResults<I, F>
+fn transpose_result<T, E>(result: Result<Option<T>, E>) -> Option<Result<T, E>> {
+    match result {
+        Ok(Some(v)) => Some(Ok(v)),
+        Ok(None) => None,
+        Err(e) => Some(Err(e)),
+    }
+}
+
+/// Create a new `FilterOk` iterator.
+pub fn filter_map_ok<I, F, T, U, E>(iter: I, f: F) -> FilterMapOk<I, F>
     where I: Iterator<Item = Result<T, E>>,
-          F: FnMut(T) -> U,
+          F: FnMut(T) -> Option<U>,
 {
-    MapResults {
+    FilterMapOk {
         iter,
         f,
     }
 }
 
-impl<I, F, T, U, E> Iterator for MapResults<I, F>
+impl<I, F, T, U, E> Iterator for FilterMapOk<I, F>
     where I: Iterator<Item = Result<T, E>>,
-          F: FnMut(T) -> U,
+          F: FnMut(T) -> Option<U>,
 {
     type Item = Result<U, E>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        self.iter.next().map(|v| v.map(&mut self.f))
+        loop {
+            match self.iter.next() {
+                Some(Ok(v)) => {
+                    if let Some(v) = (self.f)(v) {
+                        return Some(Ok(v));
+                    }
+                },
+                Some(Err(e)) => return Some(Err(e)),
+                None => return None,
+            }
+        }
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
+        (0, self.iter.size_hint().1)
     }
 
-    fn fold<Acc, Fold>(self, init: Acc, mut fold_f: Fold) -> Acc
+    fn fold<Acc, Fold>(self, init: Acc, fold_f: Fold) -> Acc
         where Fold: FnMut(Acc, Self::Item) -> Acc,
     {
         let mut f = self.f;
-        self.iter.fold(init, move |acc, v| fold_f(acc, v.map(&mut f)))
+        self.iter.filter_map(|v| {
+            transpose_result(v.map(&mut f))
+        }).fold(init, fold_f)
     }
 
     fn collect<C>(self) -> C
         where C: FromIterator<Self::Item>
     {
         let mut f = self.f;
-        self.iter.map(move |v| v.map(&mut f)).collect()
+        self.iter.filter_map(|v| {
+            transpose_result(v.map(&mut f))
+        }).collect()
     }
 }
 
diff --git a/src/adaptors/multi_product.rs b/src/adaptors/multi_product.rs
index 4a31713..6938014 100644
--- a/src/adaptors/multi_product.rs
+++ b/src/adaptors/multi_product.rs
@@ -1,8 +1,10 @@
-#![cfg(feature = "use_std")]
+#![cfg(feature = "use_alloc")]
 
 use crate::size_hint;
 use crate::Itertools;
 
+use alloc::vec::Vec;
+
 #[derive(Clone)]
 /// An iterator adaptor that iterates over the cartesian product of
 /// multiple iterators of type `I`.
@@ -161,7 +163,7 @@ impl<I> Iterator for MultiProduct<I>
     }
 
     fn count(self) -> usize {
-        if self.0.len() == 0 {
+        if self.0.is_empty() {
             return 0;
         }
 
@@ -183,7 +185,7 @@ impl<I> Iterator for MultiProduct<I>
 
     fn size_hint(&self) -> (usize, Option<usize>) {
         // Not ExactSizeIterator because size may be larger than usize
-        if self.0.len() == 0 {
+        if self.0.is_empty() {
             return (0, Some(0));
         }
 
diff --git a/src/combinations.rs b/src/combinations.rs
index 8759518..e6ba4ac 100644
--- a/src/combinations.rs
+++ b/src/combinations.rs
@@ -1,6 +1,7 @@
 use std::fmt;
 
 use super::lazy_buffer::LazyBuffer;
+use alloc::vec::Vec;
 
 /// An iterator to iterate through all the `k`-length combinations in an iterator.
 ///
@@ -30,13 +31,8 @@ impl<I> fmt::Debug for Combinations<I>
 pub fn combinations<I>(iter: I, k: usize) -> Combinations<I>
     where I: Iterator
 {
-    let mut pool: LazyBuffer<I> = LazyBuffer::new(iter);
-
-    for _ in 0..k {
-        if !pool.get_next() {
-            break;
-        }
-    }
+    let mut pool = LazyBuffer::new(iter);
+    pool.prefill(k);
 
     Combinations {
         indices: (0..k).collect(),
@@ -45,6 +41,45 @@ pub fn combinations<I>(iter: I, k: usize) -> Combinations<I>
     }
 }
 
+impl<I: Iterator> Combinations<I> {
+    /// Returns the length of a combination produced by this iterator.
+    #[inline]
+    pub fn k(&self) -> usize { self.indices.len() }
+
+    /// Returns the (current) length of the pool from which combination elements are
+    /// selected. This value can change between invocations of [`next`].
+    ///
+    /// [`next`]: #method.next
+    #[inline]
+    pub fn n(&self) -> usize { self.pool.len() }
+
+    /// Returns a reference to the source iterator.
+    #[inline]
+    pub(crate) fn src(&self) -> &I { &self.pool.it }
+
+    /// Resets this `Combinations` back to an initial state for combinations of length
+    /// `k` over the same pool data source. If `k` is larger than the current length
+    /// of the data pool an attempt is made to prefill the pool so that it holds `k`
+    /// elements.
+    pub(crate) fn reset(&mut self, k: usize) {
+        self.first = true;
+
+        if k < self.indices.len() {
+            self.indices.truncate(k);
+            for i in 0..k {
+                self.indices[i] = i;
+            }
+
+        } else {
+            for i in 0..self.indices.len() {
+                self.indices[i] = i;
+            }
+            self.indices.extend(self.indices.len()..k);
+            self.pool.prefill(k);
+        }
+    }
+}
+
 impl<I> Iterator for Combinations<I>
     where I: Iterator,
           I::Item: Clone
@@ -52,11 +87,11 @@ impl<I> Iterator for Combinations<I>
     type Item = Vec<I::Item>;
     fn next(&mut self) -> Option<Self::Item> {
         if self.first {
-            if self.pool.is_done() {
+            if self.k() > self.n() {
                 return None;
             }
             self.first = false;
-        } else if self.indices.len() == 0 {
+        } else if self.indices.is_empty() {
             return None;
         } else {
             // Scan from the end, looking for an index to increment
diff --git a/src/combinations_with_replacement.rs b/src/combinations_with_replacement.rs
index d511545..7e7a802 100644
--- a/src/combinations_with_replacement.rs
+++ b/src/combinations_with_replacement.rs
@@ -1,3 +1,4 @@
+use alloc::vec::Vec;
 use std::fmt;
 
 use super::lazy_buffer::LazyBuffer;
@@ -11,10 +12,7 @@ where
     I: Iterator,
     I::Item: Clone,
 {
-    k: usize,
     indices: Vec<usize>,
-    // The current known max index value. This increases as pool grows.
-    max_index: usize,
     pool: LazyBuffer<I>,
     first: bool,
 }
@@ -24,7 +22,7 @@ where
     I: Iterator + fmt::Debug,
     I::Item: fmt::Debug + Clone,
 {
-    debug_fmt_fields!(Combinations, k, indices, max_index, pool, first);
+    debug_fmt_fields!(Combinations, indices, pool, first);
 }
 
 impl<I> CombinationsWithReplacement<I>
@@ -44,13 +42,11 @@ where
     I: Iterator,
     I::Item: Clone,
 {
-    let indices: Vec<usize> = vec![0; k];
+    let indices: Vec<usize> = alloc::vec![0; k];
     let pool: LazyBuffer<I> = LazyBuffer::new(iter);
 
     CombinationsWithReplacement {
-        k,
         indices,
-        max_index: 0,
         pool,
         first: true,
     }
@@ -66,7 +62,7 @@ where
         // If this is the first iteration, return early
         if self.first {
             // In empty edge cases, stop iterating immediately
-            return if self.k != 0 && !self.pool.get_next() {
+            return if self.indices.len() != 0 && !self.pool.get_next() {
                 None
             // Otherwise, yield the initial state
             } else {
@@ -77,14 +73,12 @@ where
 
         // Check if we need to consume more from the iterator
         // This will run while we increment our first index digit
-        if self.pool.get_next() {
-            self.max_index = self.pool.len() - 1;
-        }
+        self.pool.get_next();
 
         // Work out where we need to update our indices
         let mut increment: Option<(usize, usize)> = None;
         for (i, indices_int) in self.indices.iter().enumerate().rev() {
-            if indices_int < &self.max_index {
+            if *indices_int < self.pool.len()-1 {
                 increment = Some((i, indices_int + 1));
                 break;
             }
diff --git a/src/concat_impl.rs b/src/concat_impl.rs
index 6048d18..0233d01 100644
--- a/src/concat_impl.rs
+++ b/src/concat_impl.rs
@@ -18,5 +18,5 @@ pub fn concat<I>(iterable: I) -> I::Item
     where I: IntoIterator,
           I::Item: Extend<<<I as IntoIterator>::Item as IntoIterator>::Item> + IntoIterator + Default
 {
-    iterable.into_iter().fold1(|mut a, b| { a.extend(b); a }).unwrap_or_else(|| <_>::default())
+    iterable.into_iter().fold1(|mut a, b| { a.extend(b); a }).unwrap_or_else(<_>::default)
 }
diff --git a/src/cons_tuples_impl.rs b/src/cons_tuples_impl.rs
index 3cdfe0d..ae0f48f 100644
--- a/src/cons_tuples_impl.rs
+++ b/src/cons_tuples_impl.rs
@@ -35,7 +35,7 @@ macro_rules! impl_cons_iter(
     );
 );
 
-impl_cons_iter!(A, B, C, D, E, F, G, H,);
+impl_cons_iter!(A, B, C, D, E, F, G, H, I, J, K, L,);
 
 /// An iterator that maps an iterator of tuples like
 /// `((A, B), C)` to an iterator of `(A, B, C)`.
@@ -57,8 +57,8 @@ impl<I, J> Clone for ConsTuples<I, J>
 
 /// Create an iterator that maps for example iterators of
 /// `((A, B), C)` to `(A, B, C)`.
-pub fn cons_tuples<I, J>(iterable: I) -> ConsTuples<I, J>
-    where I: Iterator<Item=J>
+pub fn cons_tuples<I, J>(iterable: I) -> ConsTuples<I::IntoIter, J>
+    where I: IntoIterator<Item=J>
 {
     ConsTuples { iter: iterable.into_iter() }
 }
diff --git a/src/exactly_one_err.rs b/src/exactly_one_err.rs
index e4925ff..63485c9 100644
--- a/src/exactly_one_err.rs
+++ b/src/exactly_one_err.rs
@@ -1,5 +1,11 @@
+#[cfg(feature = "use_std")]
+use std::error::Error;
+use std::fmt::{Debug, Display, Formatter, Result as FmtResult};
+
 use std::iter::ExactSizeIterator;
 
+use either::Either;
+
 use crate::size_hint;
 
 /// Iterator returned for the error case of `IterTools::exactly_one()`
@@ -10,12 +16,12 @@ use crate::size_hint;
 ///
 /// This is very similar to PutBackN except this iterator only supports 0-2 elements and does not
 /// use a `Vec`.
-#[derive(Debug, Clone)]
+#[derive(Clone)]
 pub struct ExactlyOneError<I>
 where
     I: Iterator,
 {
-    first_two: (Option<I::Item>, Option<I::Item>),
+    first_two: Option<Either<[I::Item; 2], I::Item>>,
     inner: I,
 }
 
@@ -24,9 +30,17 @@ where
     I: Iterator,
 {
     /// Creates a new `ExactlyOneErr` iterator.
-    pub(crate) fn new(first_two: (Option<I::Item>, Option<I::Item>), inner: I) -> Self {
+    pub(crate) fn new(first_two: Option<Either<[I::Item; 2], I::Item>>, inner: I) -> Self {
         Self { first_two, inner }
     }
+
+    fn additional_len(&self) -> usize {
+        match self.first_two {
+            Some(Either::Left(_)) => 2,
+            Some(Either::Right(_)) => 1,
+            None => 0,
+        }
+    }
 }
 
 impl<I> Iterator for ExactlyOneError<I>
@@ -36,23 +50,61 @@ where
     type Item = I::Item;
 
     fn next(&mut self) -> Option<Self::Item> {
-        self.first_two
-            .0
-            .take()
-            .or_else(|| self.first_two.1.take())
-            .or_else(|| self.inner.next())
+        match self.first_two.take() {
+            Some(Either::Left([first, second])) => {
+                self.first_two = Some(Either::Right(second));
+                Some(first)
+            },
+            Some(Either::Right(second)) => {
+                Some(second)
+            }
+            None => {
+                self.inner.next()
+            }
+        }
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let mut additional_len = 0;
-        if self.first_two.0.is_some() {
-            additional_len += 1;
+        size_hint::add_scalar(self.inner.size_hint(), self.additional_len())
+    }
+}
+
+
+impl<I> ExactSizeIterator for ExactlyOneError<I> where I: ExactSizeIterator {}
+
+impl<I> Display for ExactlyOneError<I> 
+    where I: Iterator,
+{
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        let additional = self.additional_len();
+        if additional > 0 {
+            write!(f, "got at least 2 elements when exactly one was expected")
+        } else {
+            write!(f, "got zero elements when exactly one was expected")
         }
-        if self.first_two.1.is_some() {
-            additional_len += 1;
+    }
+}
+
+impl<I> Debug for ExactlyOneError<I> 
+    where I: Iterator + Debug,
+          I::Item: Debug,
+{
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        match &self.first_two {
+            Some(Either::Left([first, second])) => {
+                write!(f, "ExactlyOneError[First: {:?}, Second: {:?}, RemainingIter: {:?}]", first, second, self.inner)
+            },
+            Some(Either::Right(second)) => {
+                write!(f, "ExactlyOneError[Second: {:?}, RemainingIter: {:?}]", second, self.inner)
+            }
+            None => {
+                write!(f, "ExactlyOneError[RemainingIter: {:?}]", self.inner)
+            }
         }
-        size_hint::add_scalar(self.inner.size_hint(), additional_len)
     }
 }
 
-impl<I> ExactSizeIterator for ExactlyOneError<I> where I: ExactSizeIterator {}
+#[cfg(feature = "use_std")]
+impl<I> Error for ExactlyOneError<I>  where I: Iterator + Debug, I::Item: Debug, {}
+
+
diff --git a/src/format.rs b/src/format.rs
index f72ed39..ec2dc7a 100644
--- a/src/format.rs
+++ b/src/format.rs
@@ -28,7 +28,7 @@ pub struct Format<'a, I> {
     inner: RefCell<Option<I>>,
 }
 
-pub fn new_format<'a, I, F>(iter: I, separator: &'a str, f: F) -> FormatWith<'a, I, F>
+pub fn new_format<I, F>(iter: I, separator: &str, f: F) -> FormatWith<'_, I, F>
     where I: Iterator,
           F: FnMut(I::Item, &mut dyn FnMut(&dyn fmt::Display) -> fmt::Result) -> fmt::Result
 {
@@ -38,7 +38,7 @@ pub fn new_format<'a, I, F>(iter: I, separator: &'a str, f: F) -> FormatWith<'a,
     }
 }
 
-pub fn new_format_default<'a, I>(iter: I, separator: &'a str) -> Format<'a, I>
+pub fn new_format_default<I>(iter: I, separator: &str) -> Format<'_, I>
     where I: Iterator,
 {
     Format {
@@ -59,13 +59,12 @@ impl<'a, I, F> fmt::Display for FormatWith<'a, I, F>
 
         if let Some(fst) = iter.next() {
             format(fst, &mut |disp: &dyn fmt::Display| disp.fmt(f))?;
-            for elt in iter {
-                if self.sep.len() > 0 {
-
+            iter.try_for_each(|elt| {
+                if !self.sep.is_empty() {
                     f.write_str(self.sep)?;
                 }
-                format(elt, &mut |disp: &dyn fmt::Display| disp.fmt(f))?;
-            }
+                format(elt, &mut |disp: &dyn fmt::Display| disp.fmt(f))
+            })?;
         }
         Ok(())
     }
@@ -84,12 +83,12 @@ impl<'a, I> Format<'a, I>
 
         if let Some(fst) = iter.next() {
             cb(&fst, f)?;
-            for elt in iter {
-                if self.sep.len() > 0 {
+            iter.try_for_each(|elt| {
+                if !self.sep.is_empty() {
                     f.write_str(self.sep)?;
                 }
-                cb(&elt, f)?;
-            }
+                cb(&elt, f)
+            })?;
         }
         Ok(())
     }
diff --git a/src/free.rs b/src/free.rs
index a6bc1bd..bfc5ab4 100644
--- a/src/free.rs
+++ b/src/free.rs
@@ -3,13 +3,18 @@
 //! The benefit of free functions is that they accept any `IntoIterator` as
 //! argument, so the resulting code may be easier to read.
 
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 use std::fmt::Display;
 use std::iter::{self, Zip};
-#[cfg(feature = "use_std")]
-type VecIntoIter<T> = ::std::vec::IntoIter<T>;
+#[cfg(feature = "use_alloc")]
+type VecIntoIter<T> = alloc::vec::IntoIter<T>;
 
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
+use alloc::{
+    string::String,
+};
+
+#[cfg(feature = "use_alloc")]
 use crate::Itertools;
 
 pub use crate::adaptors::{
@@ -17,15 +22,17 @@ pub use crate::adaptors::{
     merge,
     put_back,
 };
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 pub use crate::put_back_n_impl::put_back_n;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 pub use crate::multipeek_impl::multipeek;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
+pub use crate::peek_nth::peek_nth;
+#[cfg(feature = "use_alloc")]
 pub use crate::kmerge_impl::kmerge;
 pub use crate::zip_eq_impl::zip_eq;
 pub use crate::merge_join::merge_join_by;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 pub use crate::rciter_impl::rciter;
 
 /// Iterate `iterable` with a running index.
@@ -206,7 +213,7 @@ pub fn min<I>(iterable: I) -> Option<I::Item>
 ///
 /// assert_eq!(join(&[1, 2, 3], ", "), "1, 2, 3");
 /// ```
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 pub fn join<I>(iterable: I, sep: &str) -> String
     where I: IntoIterator,
           I::Item: Display
@@ -226,7 +233,7 @@ pub fn join<I>(iterable: I, sep: &str) -> String
 ///
 /// assert_equal(sorted("rust".chars()), "rstu".chars());
 /// ```
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 pub fn sorted<I>(iterable: I) -> VecIntoIter<I::Item>
     where I: IntoIterator,
           I::Item: Ord
diff --git a/src/group_map.rs b/src/group_map.rs
index be9f842..4231de0 100644
--- a/src/group_map.rs
+++ b/src/group_map.rs
@@ -14,9 +14,19 @@ pub fn into_group_map<I, K, V>(iter: I) -> HashMap<K, Vec<V>>
 {
     let mut lookup = HashMap::new();
 
-    for (key, val) in iter {
-        lookup.entry(key).or_insert(Vec::new()).push(val);
-    }
+    iter.for_each(|(key, val)| {
+        lookup.entry(key).or_insert_with(Vec::new).push(val);
+    });
 
     lookup
-}
\ No newline at end of file
+}
+
+pub fn into_group_map_by<I, K, V>(iter: I, f: impl Fn(&V) -> K) -> HashMap<K, Vec<V>>
+    where
+        I: Iterator<Item=V>,
+        K: Hash + Eq,
+{
+    into_group_map(
+        iter.map(|v| (f(&v), v))
+    )
+}
diff --git a/src/groupbylazy.rs b/src/groupbylazy.rs
index bf6e3c7..5d4a0c9 100644
--- a/src/groupbylazy.rs
+++ b/src/groupbylazy.rs
@@ -1,5 +1,5 @@
 use std::cell::{Cell, RefCell};
-use std::vec;
+use alloc::vec::{self, Vec};
 
 /// A trait to unify FnMut for GroupBy with the chunk key in IntoChunks
 trait KeyFunction<A> {
diff --git a/src/grouping_map.rs b/src/grouping_map.rs
new file mode 100644
index 0000000..5232f5d
--- /dev/null
+++ b/src/grouping_map.rs
@@ -0,0 +1,536 @@
+#![cfg(feature = "use_std")]
+
+use crate::MinMaxResult;
+use std::collections::HashMap;
+use std::cmp::Ordering;
+use std::hash::Hash;
+use std::iter::Iterator;
+use std::ops::{Add, Mul};
+
+/// A wrapper to allow for an easy [`into_grouping_map_by`](../trait.Itertools.html#method.into_grouping_map_by)
+#[derive(Clone, Debug)]
+pub struct MapForGrouping<I, F>(I, F);
+
+impl<I, F> MapForGrouping<I, F> {
+    pub(crate) fn new(iter: I, key_mapper: F) -> Self {
+        Self(iter, key_mapper)
+    }
+}
+
+impl<K, V, I, F> Iterator for MapForGrouping<I, F>
+    where I: Iterator<Item = V>,
+          K: Hash + Eq,
+          F: FnMut(&V) -> K,
+{
+    type Item = (K, V);
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next().map(|val| ((self.1)(&val), val))
+    }
+}
+
+/// Creates a new `GroupingMap` from `iter`
+pub fn new<I, K, V>(iter: I) -> GroupingMap<I>
+    where I: Iterator<Item = (K, V)>,
+          K: Hash + Eq,
+{
+    GroupingMap { iter }
+}
+
+/// `GroupingMapBy` is an intermediate struct for efficient group-and-fold operations.
+/// 
+/// See [`GroupingMap`](./struct.GroupingMap.html) for more informations.
+#[must_use = "GroupingMapBy is lazy and do nothing unless consumed"]
+pub type GroupingMapBy<I, F> = GroupingMap<MapForGrouping<I, F>>;
+
+/// `GroupingMap` is an intermediate struct for efficient group-and-fold operations.
+/// It groups elements by their key and at the same time fold each group
+/// using some aggregating operation.
+/// 
+/// No method on this struct performs temporary allocations.
+#[derive(Clone, Debug)]
+#[must_use = "GroupingMap is lazy and do nothing unless consumed"]
+pub struct GroupingMap<I> {
+    iter: I,
+}
+
+impl<I, K, V> GroupingMap<I>
+    where I: Iterator<Item = (K, V)>,
+          K: Hash + Eq,
+{
+    /// This is the generic way to perform any operation on a `GroupingMap`.
+    /// It's suggested to use this method only to implement custom operations
+    /// when the already provided ones are not enough.
+    /// 
+    /// Groups elements from the `GroupingMap` source by key and applies `operation` to the elements
+    /// of each group sequentially, passing the previously accumulated value, a reference to the key
+    /// and the current element as arguments, and stores the results in an `HashMap`.
+    ///
+    /// The `operation` function is invoked on each element with the following parameters:
+    ///  - the current value of the accumulator of the group if there is currently one;
+    ///  - a reference to the key of the group this element belongs to;
+    ///  - the element from the source being aggregated;
+    /// 
+    /// If `operation` returns `Some(element)` then the accumulator is updated with `element`,
+    /// otherwise the previous accumulation is discarded.
+    ///
+    /// Return a `HashMap` associating the key of each group with the result of aggregation of
+    /// that group's elements. If the aggregation of the last element of a group discards the
+    /// accumulator then there won't be an entry associated to that group's key.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let data = vec![2, 8, 5, 7, 9, 0, 4, 10];
+    /// let lookup = data.into_iter()
+    ///     .into_grouping_map_by(|&n| n % 4)
+    ///     .aggregate(|acc, _key, val| {
+    ///         if val == 0 || val == 10 {
+    ///             None
+    ///         } else {
+    ///             Some(acc.unwrap_or(0) + val)
+    ///         }
+    ///     });
+    /// 
+    /// assert_eq!(lookup[&0], 4);        // 0 resets the accumulator so only 4 is summed
+    /// assert_eq!(lookup[&1], 5 + 9);
+    /// assert_eq!(lookup.get(&2), None); // 10 resets the accumulator and nothing is summed afterward
+    /// assert_eq!(lookup[&3], 7);
+    /// assert_eq!(lookup.len(), 3);      // The final keys are only 0, 1 and 2
+    /// ```
+    pub fn aggregate<FO, R>(self, mut operation: FO) -> HashMap<K, R>
+        where FO: FnMut(Option<R>, &K, V) -> Option<R>,
+    {
+        let mut destination_map = HashMap::new();
+
+        for (key, val) in self.iter {
+            let acc = destination_map.remove(&key);
+            if let Some(op_res) = operation(acc, &key, val) {
+                destination_map.insert(key, op_res);
+            }
+        }
+
+        destination_map
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and applies `operation` to the elements
+    /// of each group sequentially, passing the previously accumulated value, a reference to the key
+    /// and the current element as arguments, and stores the results in a new map.
+    ///
+    /// `init` is the value from which will be cloned the initial value of each accumulator.
+    ///
+    /// `operation` is a function that is invoked on each element with the following parameters:
+    ///  - the current value of the accumulator of the group;
+    ///  - a reference to the key of the group this element belongs to;
+    ///  - the element from the source being accumulated.
+    ///
+    /// Return a `HashMap` associating the key of each group with the result of folding that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = (1..=7)
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .fold(0, |acc, _key, val| acc + val);
+    /// 
+    /// assert_eq!(lookup[&0], 3 + 6);
+    /// assert_eq!(lookup[&1], 1 + 4 + 7);
+    /// assert_eq!(lookup[&2], 2 + 5);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn fold<FO, R>(self, init: R, mut operation: FO) -> HashMap<K, R>
+        where R: Clone,
+              FO: FnMut(R, &K, V) -> R,
+    {
+        self.aggregate(|acc, key, val| {
+            let acc = acc.unwrap_or_else(|| init.clone());
+            Some(operation(acc, key, val))
+        })
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and applies `operation` to the elements
+    /// of each group sequentially, passing the previously accumulated value, a reference to the key
+    /// and the current element as arguments, and stores the results in a new map.
+    ///
+    /// This is similar to [`fold`] but the initial value of the accumulator is the first element of the group.
+    ///
+    /// `operation` is a function that is invoked on each element with the following parameters:
+    ///  - the current value of the accumulator of the group;
+    ///  - a reference to the key of the group this element belongs to;
+    ///  - the element from the source being accumulated.
+    ///
+    /// Return a `HashMap` associating the key of each group with the result of folding that group's elements.
+    /// 
+    /// [`fold`]: #tymethod.fold
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = (1..=7)
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .fold_first(|acc, _key, val| acc + val);
+    /// 
+    /// assert_eq!(lookup[&0], 3 + 6);
+    /// assert_eq!(lookup[&1], 1 + 4 + 7);
+    /// assert_eq!(lookup[&2], 2 + 5);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn fold_first<FO>(self, mut operation: FO) -> HashMap<K, V>
+        where FO: FnMut(V, &K, V) -> V,
+    {
+        self.aggregate(|acc, key, val| {
+            Some(match acc {
+                Some(acc) => operation(acc, key, val),
+                None => val,
+            })
+        })
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and collects the elements of each group in
+    /// an instance of `C`. The iteration order is preserved when inserting elements. 
+    /// 
+    /// Return a `HashMap` associating the key of each group with the collection containing that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// use std::collections::HashSet;
+    /// 
+    /// let lookup = vec![0, 1, 2, 3, 4, 5, 6, 2, 3, 6].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .collect::<HashSet<_>>();
+    /// 
+    /// assert_eq!(lookup[&0], vec![0, 3, 6].into_iter().collect::<HashSet<_>>());
+    /// assert_eq!(lookup[&1], vec![1, 4].into_iter().collect::<HashSet<_>>());
+    /// assert_eq!(lookup[&2], vec![2, 5].into_iter().collect::<HashSet<_>>());
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn collect<C>(self) -> HashMap<K, C>
+        where C: Default + Extend<V>,
+    {
+        let mut destination_map = HashMap::new();
+
+        for (key, val) in self.iter {
+            destination_map.entry(key).or_insert_with(C::default).extend(Some(val));
+        }
+
+        destination_map
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and finds the maximum of each group.
+    /// 
+    /// If several elements are equally maximum, the last element is picked.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the maximum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .max();
+    /// 
+    /// assert_eq!(lookup[&0], 12);
+    /// assert_eq!(lookup[&1], 7);
+    /// assert_eq!(lookup[&2], 8);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn max(self) -> HashMap<K, V>
+        where V: Ord,
+    {
+        self.max_by(|_, v1, v2| V::cmp(v1, v2))
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and finds the maximum of each group
+    /// with respect to the specified comparison function.
+    /// 
+    /// If several elements are equally maximum, the last element is picked.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the maximum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .max_by(|_key, x, y| y.cmp(x));
+    /// 
+    /// assert_eq!(lookup[&0], 3);
+    /// assert_eq!(lookup[&1], 1);
+    /// assert_eq!(lookup[&2], 5);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn max_by<F>(self, mut compare: F) -> HashMap<K, V>
+        where F: FnMut(&K, &V, &V) -> Ordering,
+    {
+        self.fold_first(|acc, key, val| match compare(key, &acc, &val) {
+            Ordering::Less | Ordering::Equal => val,
+            Ordering::Greater => acc
+        })
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and finds the element of each group
+    /// that gives the maximum from the specified function.
+    /// 
+    /// If several elements are equally maximum, the last element is picked.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the maximum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .max_by_key(|_key, &val| val % 4);
+    /// 
+    /// assert_eq!(lookup[&0], 3);
+    /// assert_eq!(lookup[&1], 7);
+    /// assert_eq!(lookup[&2], 5);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn max_by_key<F, CK>(self, mut f: F) -> HashMap<K, V>
+        where F: FnMut(&K, &V) -> CK,
+              CK: Ord,
+    {
+        self.max_by(|key, v1, v2| f(key, &v1).cmp(&f(key, &v2)))
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and finds the minimum of each group.
+    /// 
+    /// If several elements are equally minimum, the first element is picked.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the minimum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .min();
+    /// 
+    /// assert_eq!(lookup[&0], 3);
+    /// assert_eq!(lookup[&1], 1);
+    /// assert_eq!(lookup[&2], 5);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn min(self) -> HashMap<K, V>
+        where V: Ord,
+    {
+        self.min_by(|_, v1, v2| V::cmp(v1, v2))
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and finds the minimum of each group
+    /// with respect to the specified comparison function.
+    /// 
+    /// If several elements are equally minimum, the first element is picked.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the minimum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .min_by(|_key, x, y| y.cmp(x));
+    /// 
+    /// assert_eq!(lookup[&0], 12);
+    /// assert_eq!(lookup[&1], 7);
+    /// assert_eq!(lookup[&2], 8);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn min_by<F>(self, mut compare: F) -> HashMap<K, V>
+        where F: FnMut(&K, &V, &V) -> Ordering,
+    {
+        self.fold_first(|acc, key, val| match compare(key, &acc, &val) {
+            Ordering::Less | Ordering::Equal => acc,
+            Ordering::Greater => val
+        })
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and finds the element of each group
+    /// that gives the minimum from the specified function.
+    /// 
+    /// If several elements are equally minimum, the first element is picked.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the minimum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .min_by_key(|_key, &val| val % 4);
+    /// 
+    /// assert_eq!(lookup[&0], 12);
+    /// assert_eq!(lookup[&1], 4);
+    /// assert_eq!(lookup[&2], 8);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn min_by_key<F, CK>(self, mut f: F) -> HashMap<K, V>
+        where F: FnMut(&K, &V) -> CK,
+              CK: Ord,
+    {
+        self.min_by(|key, v1, v2| f(key, &v1).cmp(&f(key, &v2)))
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and find the maximum and minimum of
+    /// each group.
+    /// 
+    /// If several elements are equally maximum, the last element is picked.
+    /// If several elements are equally minimum, the first element is picked.
+    /// 
+    /// See [.minmax()](../trait.Itertools.html#method.minmax) for the non-grouping version.
+    /// 
+    /// Differences from the non grouping version:
+    /// - It never produces a `MinMaxResult::NoElements`
+    /// - It doesn't have any speedup
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the minimum and maximum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::MinMaxResult::{OneElement, MinMax};
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .minmax();
+    /// 
+    /// assert_eq!(lookup[&0], MinMax(3, 12));
+    /// assert_eq!(lookup[&1], MinMax(1, 7));
+    /// assert_eq!(lookup[&2], OneElement(5));
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn minmax(self) -> HashMap<K, MinMaxResult<V>>
+        where V: Ord,
+    {
+        self.minmax_by(|_, v1, v2| V::cmp(v1, v2))
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and find the maximum and minimum of
+    /// each group with respect to the specified comparison function.
+    /// 
+    /// If several elements are equally maximum, the last element is picked.
+    /// If several elements are equally minimum, the first element is picked.
+    /// 
+    /// It has the same differences from the non-grouping version as `minmax`.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the minimum and maximum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::MinMaxResult::{OneElement, MinMax};
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .minmax_by(|_key, x, y| y.cmp(x));
+    /// 
+    /// assert_eq!(lookup[&0], MinMax(12, 3));
+    /// assert_eq!(lookup[&1], MinMax(7, 1));
+    /// assert_eq!(lookup[&2], OneElement(5));
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn minmax_by<F>(self, mut compare: F) -> HashMap<K, MinMaxResult<V>>
+        where F: FnMut(&K, &V, &V) -> Ordering,
+    {
+        self.aggregate(|acc, key, val| {
+            Some(match acc {
+                Some(MinMaxResult::OneElement(e)) => {
+                    if compare(key, &val, &e) == Ordering::Less {
+                        MinMaxResult::MinMax(val, e)
+                    } else {
+                        MinMaxResult::MinMax(e, val)
+                    }
+                }
+                Some(MinMaxResult::MinMax(min, max)) => {
+                    if compare(key, &val, &min) == Ordering::Less {
+                        MinMaxResult::MinMax(val, max)
+                    } else if compare(key, &val, &max) != Ordering::Less {
+                        MinMaxResult::MinMax(min, val)
+                    } else {
+                        MinMaxResult::MinMax(min, max)
+                    }
+                }
+                None => MinMaxResult::OneElement(val),
+                Some(MinMaxResult::NoElements) => unreachable!(),
+            })
+        })
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and find the elements of each group
+    /// that gives the minimum and maximum from the specified function.
+    /// 
+    /// If several elements are equally maximum, the last element is picked.
+    /// If several elements are equally minimum, the first element is picked.
+    /// 
+    /// It has the same differences from the non-grouping version as `minmax`.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the minimum and maximum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::MinMaxResult::{OneElement, MinMax};
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .minmax_by_key(|_key, &val| val % 4);
+    /// 
+    /// assert_eq!(lookup[&0], MinMax(12, 3));
+    /// assert_eq!(lookup[&1], MinMax(4, 7));
+    /// assert_eq!(lookup[&2], OneElement(5));
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn minmax_by_key<F, CK>(self, mut f: F) -> HashMap<K, MinMaxResult<V>>
+        where F: FnMut(&K, &V) -> CK,
+              CK: Ord,
+    {
+        self.minmax_by(|key, v1, v2| f(key, &v1).cmp(&f(key, &v2)))
+    }
+    
+    /// Groups elements from the `GroupingMap` source by key and sums them.
+    /// 
+    /// This is just a shorthand for `self.fold_first(|acc, _, val| acc + val)`.
+    /// It is more limited than `Iterator::sum` since it doesn't use the `Sum` trait.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the sum of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .sum();
+    /// 
+    /// assert_eq!(lookup[&0], 3 + 9 + 12);
+    /// assert_eq!(lookup[&1], 1 + 4 + 7);
+    /// assert_eq!(lookup[&2], 5 + 8);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn sum(self) -> HashMap<K, V>
+        where V: Add<V, Output = V>
+    {
+        self.fold_first(|acc, _, val| acc + val)
+    }
+
+    /// Groups elements from the `GroupingMap` source by key and multiply them.
+    /// 
+    /// This is just a shorthand for `self.fold_first(|acc, _, val| acc * val)`.
+    /// It is more limited than `Iterator::product` since it doesn't use the `Product` trait.
+    /// 
+    /// Returns a `HashMap` associating the key of each group with the product of that group's elements.
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    /// 
+    /// let lookup = vec![1, 3, 4, 5, 7, 8, 9, 12].into_iter()
+    ///     .into_grouping_map_by(|&n| n % 3)
+    ///     .product();
+    /// 
+    /// assert_eq!(lookup[&0], 3 * 9 * 12);
+    /// assert_eq!(lookup[&1], 1 * 4 * 7);
+    /// assert_eq!(lookup[&2], 5 * 8);
+    /// assert_eq!(lookup.len(), 3);
+    /// ```
+    pub fn product(self) -> HashMap<K, V>
+        where V: Mul<V, Output = V>,
+    {
+        self.fold_first(|acc, _, val| acc * val)
+    }
+}
diff --git a/src/intersperse.rs b/src/intersperse.rs
index 0579299..a0d79b0 100644
--- a/src/intersperse.rs
+++ b/src/intersperse.rs
@@ -1,7 +1,19 @@
 use std::iter::Fuse;
 use super::size_hint;
 
-#[derive(Clone)]
+pub trait IntersperseElement<Item> {
+    fn generate(&mut self) -> Item;
+}
+
+#[derive(Debug, Clone)]
+pub struct IntersperseElementSimple<Item>(Item);
+
+impl<Item: Clone> IntersperseElement<Item> for IntersperseElementSimple<Item> {
+    fn generate(&mut self) -> Item {
+        self.0.clone()
+    }
+}
+
 /// An iterator adaptor to insert a particular value
 /// between each element of the adapted iterator.
 ///
@@ -10,41 +22,64 @@ use super::size_hint;
 /// This iterator is *fused*.
 ///
 /// See [`.intersperse()`](../trait.Itertools.html#method.intersperse) for more information.
+pub type Intersperse<I> = IntersperseWith<I, IntersperseElementSimple<<I as Iterator>::Item>>;
+
+/// Create a new Intersperse iterator
+pub fn intersperse<I>(iter: I, elt: I::Item) -> Intersperse<I>
+    where I: Iterator,
+{
+    intersperse_with(iter, IntersperseElementSimple(elt))
+}
+
+impl<Item, F: FnMut()->Item> IntersperseElement<Item> for F {
+    fn generate(&mut self) -> Item {
+        self()
+    }
+}
+
+/// An iterator adaptor to insert a particular value created by a function
+/// between each element of the adapted iterator.
+///
+/// Iterator element type is `I::Item`
+///
+/// This iterator is *fused*.
+///
+/// See [`.intersperse_with()`](../trait.Itertools.html#method.intersperse_with) for more information.
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
-#[derive(Debug)]
-pub struct Intersperse<I>
-    where I: Iterator
+#[derive(Clone, Debug)]
+pub struct IntersperseWith<I, ElemF>
+    where I: Iterator,
 {
-    element: I::Item,
+    element: ElemF,
     iter: Fuse<I>,
     peek: Option<I::Item>,
 }
 
-/// Create a new Intersperse iterator
-pub fn intersperse<I>(iter: I, elt: I::Item) -> Intersperse<I>
-    where I: Iterator
+/// Create a new IntersperseWith iterator
+pub fn intersperse_with<I, ElemF>(iter: I, elt: ElemF) -> IntersperseWith<I, ElemF>
+    where I: Iterator,
 {
     let mut iter = iter.fuse();
-    Intersperse {
+    IntersperseWith {
         peek: iter.next(),
         iter,
         element: elt,
     }
 }
 
-impl<I> Iterator for Intersperse<I>
+impl<I, ElemF> Iterator for IntersperseWith<I, ElemF>
     where I: Iterator,
-          I::Item: Clone
+          ElemF: IntersperseElement<I::Item>
 {
     type Item = I::Item;
     #[inline]
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         if self.peek.is_some() {
             self.peek.take()
         } else {
             self.peek = self.iter.next();
             if self.peek.is_some() {
-                Some(self.element.clone())
+                Some(self.element.generate())
             } else {
                 None
             }
@@ -62,16 +97,16 @@ impl<I> Iterator for Intersperse<I>
         Self: Sized, F: FnMut(B, Self::Item) -> B,
     {
         let mut accum = init;
-        
+
         if let Some(x) = self.peek.take() {
             accum = f(accum, x);
         }
 
-        let element = &self.element;
+        let element = &mut self.element;
 
         self.iter.fold(accum,
             |accum, x| {
-                let accum = f(accum, element.clone());
+                let accum = f(accum, element.generate());
                 let accum = f(accum, x);
                 accum
         })
diff --git a/src/k_smallest.rs b/src/k_smallest.rs
new file mode 100644
index 0000000..d58ec70
--- /dev/null
+++ b/src/k_smallest.rs
@@ -0,0 +1,20 @@
+use alloc::collections::BinaryHeap;
+use core::cmp::Ord;
+
+pub(crate) fn k_smallest<T: Ord, I: Iterator<Item = T>>(mut iter: I, k: usize) -> BinaryHeap<T> {
+    if k == 0 { return BinaryHeap::new(); }
+
+    let mut heap = iter.by_ref().take(k).collect::<BinaryHeap<_>>();
+
+    for i in iter {
+        debug_assert_eq!(heap.len(), k);
+        // Equivalent to heap.push(min(i, heap.pop())) but more efficient.
+        // This should be done with a single `.peek_mut().unwrap()` but
+        //  `PeekMut` sifts-down unconditionally on Rust 1.46.0 and prior.
+        if *heap.peek().unwrap() > i {
+            *heap.peek_mut().unwrap() = i;
+        }
+    }
+
+    heap
+}
diff --git a/src/kmerge_impl.rs b/src/kmerge_impl.rs
index 8f68aeb..a1b3d8e 100644
--- a/src/kmerge_impl.rs
+++ b/src/kmerge_impl.rs
@@ -1,6 +1,7 @@
 use crate::size_hint;
 use crate::Itertools;
 
+use alloc::vec::Vec;
 use std::mem::replace;
 use std::fmt;
 
diff --git a/src/lazy_buffer.rs b/src/lazy_buffer.rs
index 01123d4..fa514ec 100644
--- a/src/lazy_buffer.rs
+++ b/src/lazy_buffer.rs
@@ -1,4 +1,5 @@
 use std::ops::Index;
+use alloc::vec::Vec;
 
 #[derive(Debug, Clone)]
 pub struct LazyBuffer<I: Iterator> {
@@ -23,10 +24,6 @@ where
         self.buffer.len()
     }
 
-    pub fn is_done(&self) -> bool {
-        self.done
-    }
-
     pub fn get_next(&mut self) -> bool {
         if self.done {
             return false;
@@ -43,6 +40,17 @@ where
             }
         }
     }
+
+    pub fn prefill(&mut self, len: usize) {
+        let buffer_len = self.buffer.len();
+
+        if !self.done && len > buffer_len {
+            let delta = len - buffer_len;
+
+            self.buffer.extend(self.it.by_ref().take(delta));
+            self.done = self.buffer.len() < len;
+        }
+    }
 }
 
 impl<I, J> Index<J> for LazyBuffer<I>
diff --git a/src/lib.rs b/src/lib.rs
index b8daefd..2ef7bd9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -50,6 +50,15 @@
 #[cfg(not(feature = "use_std"))]
 extern crate core as std;
 
+#[cfg(feature = "use_alloc")]
+extern crate alloc;
+
+#[cfg(feature = "use_alloc")]
+use alloc::{
+    string::String,
+    vec::Vec,
+};
+
 pub use either::Either;
 
 #[cfg(feature = "use_std")]
@@ -59,11 +68,11 @@ use std::cmp::Ordering;
 use std::fmt;
 #[cfg(feature = "use_std")]
 use std::hash::Hash;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 use std::fmt::Write;
-#[cfg(feature = "use_std")]
-type VecIntoIter<T> = ::std::vec::IntoIter<T>;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
+type VecIntoIter<T> = alloc::vec::IntoIter<T>;
+#[cfg(feature = "use_alloc")]
 use std::iter::FromIterator;
 
 #[macro_use]
@@ -78,13 +87,17 @@ pub mod structs {
     pub use crate::adaptors::{
         Dedup,
         DedupBy,
+        DedupWithCount,
+        DedupByWithCount,
         Interleave,
         InterleaveShortest,
+        FilterMapOk,
+        FilterOk,
         Product,
         PutBack,
         Batching,
         MapInto,
-        MapResults,
+        MapOk,
         Merge,
         MergeBy,
         TakeWhileRef,
@@ -95,39 +108,45 @@ pub mod structs {
         Update,
     };
     #[allow(deprecated)]
-    pub use crate::adaptors::Step;
-    #[cfg(feature = "use_std")]
+    pub use crate::adaptors::{MapResults, Step};
+    #[cfg(feature = "use_alloc")]
     pub use crate::adaptors::MultiProduct;
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     pub use crate::combinations::Combinations;
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     pub use crate::combinations_with_replacement::CombinationsWithReplacement;
     pub use crate::cons_tuples_impl::ConsTuples;
     pub use crate::exactly_one_err::ExactlyOneError;
     pub use crate::format::{Format, FormatWith};
     #[cfg(feature = "use_std")]
+    pub use crate::grouping_map::{GroupingMap, GroupingMapBy};
+    #[cfg(feature = "use_alloc")]
     pub use crate::groupbylazy::{IntoChunks, Chunk, Chunks, GroupBy, Group, Groups};
-    pub use crate::intersperse::Intersperse;
-    #[cfg(feature = "use_std")]
+    pub use crate::intersperse::{Intersperse, IntersperseWith};
+    #[cfg(feature = "use_alloc")]
     pub use crate::kmerge_impl::{KMerge, KMergeBy};
     pub use crate::merge_join::MergeJoinBy;
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     pub use crate::multipeek_impl::MultiPeek;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::peek_nth::PeekNth;
     pub use crate::pad_tail::PadUsing;
     pub use crate::peeking_take_while::PeekingTakeWhile;
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     pub use crate::permutations::Permutations;
     pub use crate::process_results_impl::ProcessResults;
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
+    pub use crate::powerset::Powerset;
+    #[cfg(feature = "use_alloc")]
     pub use crate::put_back_n_impl::PutBackN;
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     pub use crate::rciter_impl::RcIter;
     pub use crate::repeatn::RepeatN;
     #[allow(deprecated)]
     pub use crate::sources::{RepeatCall, Unfold, Iterate};
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     pub use crate::tee::Tee;
-    pub use crate::tuple_impl::{TupleBuffer, TupleWindows, Tuples};
+    pub use crate::tuple_impl::{TupleBuffer, TupleWindows, CircularTupleWindows, Tuples};
     #[cfg(feature = "use_std")]
     pub use crate::unique_impl::{Unique, UniqueBy};
     pub use crate::with_position::WithPosition;
@@ -147,7 +166,7 @@ pub use crate::concat_impl::concat;
 pub use crate::cons_tuples_impl::cons_tuples;
 pub use crate::diff::diff_with;
 pub use crate::diff::Diff;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 pub use crate::kmerge_impl::{kmerge_by};
 pub use crate::minmax::MinMaxResult;
 pub use crate::peeking_take_while::PeekingNext;
@@ -166,39 +185,47 @@ pub mod free;
 pub use crate::free::*;
 mod concat_impl;
 mod cons_tuples_impl;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod combinations;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod combinations_with_replacement;
 mod exactly_one_err;
 mod diff;
 mod format;
 #[cfg(feature = "use_std")]
+mod grouping_map;
+#[cfg(feature = "use_alloc")]
 mod group_map;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod groupbylazy;
 mod intersperse;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
+mod k_smallest;
+#[cfg(feature = "use_alloc")]
 mod kmerge_impl;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod lazy_buffer;
 mod merge_join;
 mod minmax;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod multipeek_impl;
 mod pad_tail;
+#[cfg(feature = "use_alloc")]
+mod peek_nth;
 mod peeking_take_while;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod permutations;
+#[cfg(feature = "use_alloc")]
+mod powerset;
 mod process_results_impl;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod put_back_n_impl;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod rciter_impl;
 mod repeatn;
 mod size_hint;
 mod sources;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 mod tee;
 mod tuple_impl;
 #[cfg(feature = "use_std")]
@@ -230,16 +257,16 @@ macro_rules! iproduct {
         $I
     );
     (@flatten $I:expr, $J:expr, $($K:expr,)*) => (
-        iproduct!(@flatten $crate::cons_tuples(iproduct!($I, $J)), $($K,)*)
+        $crate::iproduct!(@flatten $crate::cons_tuples($crate::iproduct!($I, $J)), $($K,)*)
     );
     ($I:expr) => (
         $crate::__std_iter::IntoIterator::into_iter($I)
     );
     ($I:expr, $J:expr) => (
-        $crate::Itertools::cartesian_product(iproduct!($I), iproduct!($J))
+        $crate::Itertools::cartesian_product($crate::iproduct!($I), $crate::iproduct!($J))
     );
     ($I:expr, $J:expr, $($K:expr),+) => (
-        iproduct!(@flatten iproduct!($I, $J), $($K,)+)
+        $crate::iproduct!(@flatten $crate::iproduct!($I, $J), $($K,)+)
     );
 }
 
@@ -290,7 +317,7 @@ macro_rules! izip {
 
     // The "b" identifier is a different identifier on each recursion level thanks to hygiene.
     ( @closure $p:pat => ( $($tup:tt)* ) , $_iter:expr $( , $tail:expr )* ) => {
-        izip!(@closure ($p, b) => ( $($tup)*, b ) $( , $tail )*)
+        $crate::izip!(@closure ($p, b) => ( $($tup)*, b ) $( , $tail )*)
     };
 
     // unary
@@ -300,18 +327,18 @@ macro_rules! izip {
 
     // binary
     ($first:expr, $second:expr $(,)*) => {
-        izip!($first)
+        $crate::izip!($first)
             .zip($second)
     };
 
     // n-ary where n > 2
     ( $first:expr $( , $rest:expr )* $(,)* ) => {
-        izip!($first)
+        $crate::izip!($first)
             $(
                 .zip($rest)
             )*
             .map(
-                izip!(@closure a => (a) $( , $rest )*)
+                $crate::izip!(@closure a => (a) $( , $rest )*)
             )
     };
 }
@@ -390,6 +417,27 @@ pub trait Itertools : Iterator {
         intersperse::intersperse(self, element)
     }
 
+    /// An iterator adaptor to insert a particular value created by a function
+    /// between each element of the adapted iterator.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut i = 10;
+    /// itertools::assert_equal((0..3).intersperse_with(|| { i -= 1; i }), vec![0, 9, 1, 8, 2]);
+    /// assert_eq!(i, 8);
+    /// ```
+    fn intersperse_with<F>(self, element: F) -> IntersperseWith<Self, F>
+        where Self: Sized,
+        F: FnMut() -> Self::Item
+    {
+        intersperse::intersperse_with(self, element)
+    }
+
     /// Create an iterator which iterates over both this and the specified
     /// iterator simultaneously, yielding pairs of two optional elements.
     ///
@@ -501,7 +549,7 @@ pub trait Itertools : Iterator {
     /// }
     /// assert_eq!(data_grouped, vec![(true, vec![1, 3]), (false, vec![-2, -2]), (true, vec![1, 0, 1, 2])]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn group_by<K, F>(self, key: F) -> GroupBy<K, Self, F>
         where Self: Sized,
               F: FnMut(&Self::Item) -> K,
@@ -537,7 +585,7 @@ pub trait Itertools : Iterator {
     ///     assert_eq!(4, chunk.sum());
     /// }
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn chunks(self, size: usize) -> IntoChunks<Self>
         where Self: Sized,
     {
@@ -555,6 +603,8 @@ pub trait Itertools : Iterator {
     /// ```
     /// use itertools::Itertools;
     /// let mut v = Vec::new();
+    ///
+    /// // pairwise iteration
     /// for (a, b) in (1..5).tuple_windows() {
     ///     v.push((a, b));
     /// }
@@ -584,6 +634,40 @@ pub trait Itertools : Iterator {
         tuple_impl::tuple_windows(self)
     }
 
+    /// Return an iterator over all windows, wrapping back to the first
+    /// elements when the window would otherwise exceed the length of the
+    /// iterator, producing tuples of a specific size (up to 4).
+    ///
+    /// `circular_tuple_windows` clones the iterator elements so that they can be
+    /// part of successive windows, this makes it most suited for iterators
+    /// of references and other values that are cheap to copy.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// let mut v = Vec::new();
+    /// for (a, b) in (1..5).circular_tuple_windows() {
+    ///     v.push((a, b));
+    /// }
+    /// assert_eq!(v, vec![(1, 2), (2, 3), (3, 4), (4, 1)]);
+    ///
+    /// let mut it = (1..5).circular_tuple_windows();
+    /// assert_eq!(Some((1, 2, 3)), it.next());
+    /// assert_eq!(Some((2, 3, 4)), it.next());
+    /// assert_eq!(Some((3, 4, 1)), it.next());
+    /// assert_eq!(Some((4, 1, 2)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// // this requires a type hint
+    /// let it = (1..5).circular_tuple_windows::<(_, _, _)>();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (2, 3, 4), (3, 4, 1), (4, 1, 2)]);
+    /// ```
+    fn circular_tuple_windows<T>(self) -> CircularTupleWindows<Self, T>
+        where Self: Sized + Clone + Iterator<Item = T::Item> + ExactSizeIterator,
+              T: tuple_impl::TupleCollect + Clone,
+              T::Item: Clone
+    {
+        tuple_impl::circular_tuple_windows(self)
+    }
     /// Return an iterator that groups the items in tuples of a specific size
     /// (up to 4).
     ///
@@ -639,7 +723,7 @@ pub trait Itertools : Iterator {
     /// itertools::assert_equal(t2, 0..4);
     /// itertools::assert_equal(t1, 1..4);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn tee(self) -> (Tee<Self>, Tee<Self>)
         where Self: Sized,
               Self::Item: Clone
@@ -663,7 +747,7 @@ pub trait Itertools : Iterator {
     /// let it = (0..8).step(3);
     /// itertools::assert_equal(it, vec![0, 3, 6]);
     /// ```
-    #[deprecated(note="Use std .step_by() instead", since="0.8")]
+    #[deprecated(note="Use std .step_by() instead", since="0.8.0")]
     #[allow(deprecated)]
     fn step(self, n: usize) -> Step<Self>
         where Self: Sized
@@ -685,6 +769,15 @@ pub trait Itertools : Iterator {
         adaptors::map_into(self)
     }
 
+    /// See [`.map_ok()`](#method.map_ok).
+    #[deprecated(note="Use .map_ok() instead", since="0.10.0")]
+    fn map_results<F, T, U, E>(self, f: F) -> MapOk<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(T) -> U,
+    {
+        self.map_ok(f)
+    }
+
     /// Return an iterator adaptor that applies the provided closure
     /// to every `Result::Ok` value. `Result::Err` values are
     /// unchanged.
@@ -693,14 +786,50 @@ pub trait Itertools : Iterator {
     /// use itertools::Itertools;
     ///
     /// let input = vec![Ok(41), Err(false), Ok(11)];
-    /// let it = input.into_iter().map_results(|i| i + 1);
+    /// let it = input.into_iter().map_ok(|i| i + 1);
     /// itertools::assert_equal(it, vec![Ok(42), Err(false), Ok(12)]);
     /// ```
-    fn map_results<F, T, U, E>(self, f: F) -> MapResults<Self, F>
+    fn map_ok<F, T, U, E>(self, f: F) -> MapOk<Self, F>
         where Self: Iterator<Item = Result<T, E>> + Sized,
               F: FnMut(T) -> U,
     {
-        adaptors::map_results(self, f)
+        adaptors::map_ok(self, f)
+    }
+
+    /// Return an iterator adaptor that filters every `Result::Ok`
+    /// value with the provided closure. `Result::Err` values are
+    /// unchanged.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![Ok(22), Err(false), Ok(11)];
+    /// let it = input.into_iter().filter_ok(|&i| i > 20);
+    /// itertools::assert_equal(it, vec![Ok(22), Err(false)]);
+    /// ```
+    fn filter_ok<F, T, E>(self, f: F) -> FilterOk<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(&T) -> bool,
+    {
+        adaptors::filter_ok(self, f)
+    }
+
+    /// Return an iterator adaptor that filters and transforms every
+    /// `Result::Ok` value with the provided closure. `Result::Err`
+    /// values are unchanged.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![Ok(22), Err(false), Ok(11)];
+    /// let it = input.into_iter().filter_map_ok(|i| if i > 20 { Some(i * 2) } else { None });
+    /// itertools::assert_equal(it, vec![Ok(44), Err(false)]);
+    /// ```
+    fn filter_map_ok<F, T, U, E>(self, f: F) -> FilterMapOk<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(T) -> Option<U>,
+    {
+        adaptors::filter_map_ok(self, f)
     }
 
     /// Return an iterator adaptor that merges the two base iterators in
@@ -768,17 +897,13 @@ pub trait Itertools : Iterator {
     /// use itertools::Itertools;
     /// use itertools::EitherOrBoth::{Left, Right, Both};
     ///
-    /// let ki = (0..10).step(3);
-    /// let ku = (0..10).step(5);
-    /// let ki_ku = ki.merge_join_by(ku, |i, j| i.cmp(j)).map(|either| {
-    ///     match either {
-    ///         Left(_) => "Ki",
-    ///         Right(_) => "Ku",
-    ///         Both(_, _) => "KiKu"
-    ///     }
-    /// });
+    /// let multiples_of_2 = (0..10).step(2);
+    /// let multiples_of_3 = (0..10).step(3);
     ///
-    /// itertools::assert_equal(ki_ku, vec!["KiKu", "Ki", "Ku", "Ki", "Ki"]);
+    /// itertools::assert_equal(
+    ///     multiples_of_2.merge_join_by(multiples_of_3, |i, j| i.cmp(j)),
+    ///     vec![Both(0, 0), Left(2), Right(3), Left(4), Both(6, 6), Left(8), Right(9)]
+    /// );
     /// ```
     #[inline]
     fn merge_join_by<J, F>(self, other: J, cmp_fn: F) -> MergeJoinBy<Self, J::IntoIter, F>
@@ -789,7 +914,6 @@ pub trait Itertools : Iterator {
         merge_join_by(self, other, cmp_fn)
     }
 
-
     /// Return an iterator adaptor that flattens an iterator of iterators by
     /// merging them in ascending order.
     ///
@@ -806,7 +930,7 @@ pub trait Itertools : Iterator {
     /// let it = vec![a, b, c].into_iter().kmerge();
     /// itertools::assert_equal(it, vec![0, 1, 2, 3, 4, 5]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn kmerge(self) -> KMerge<<Self::Item as IntoIterator>::IntoIter>
         where Self: Sized,
               Self::Item: IntoIterator,
@@ -835,7 +959,7 @@ pub trait Itertools : Iterator {
     /// assert_eq!(it.next(), Some(0.));
     /// assert_eq!(it.last(), Some(-7.));
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn kmerge_by<F>(self, first: F)
         -> KMergeBy<<Self::Item as IntoIterator>::IntoIter, F>
         where Self: Sized,
@@ -891,7 +1015,7 @@ pub trait Itertools : Iterator {
     /// assert_eq!(multi_prod.next(), Some(vec![1, 3, 5]));
     /// assert_eq!(multi_prod.next(), None);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn multi_cartesian_product(self) -> MultiProduct<<Self::Item as IntoIterator>::IntoIter>
         where Self: Iterator + Sized,
               Self::Item: IntoIterator,
@@ -970,7 +1094,7 @@ pub trait Itertools : Iterator {
     /// use itertools::Itertools;
     ///
     /// let data = vec![(0, 1.), (1, 1.), (0, 2.), (0, 3.), (1, 3.), (1, 2.), (2, 2.)];
-    /// itertools::assert_equal(data.into_iter().dedup_by(|x, y| x.1==y.1),
+    /// itertools::assert_equal(data.into_iter().dedup_by(|x, y| x.1 == y.1),
     ///                         vec![(0, 1.), (0, 2.), (0, 3.), (1, 2.)]);
     /// ```
     fn dedup_by<Cmp>(self, cmp: Cmp) -> DedupBy<Self, Cmp>
@@ -980,6 +1104,50 @@ pub trait Itertools : Iterator {
         adaptors::dedup_by(self, cmp)
     }
 
+    /// Remove duplicates from sections of consecutive identical elements, while keeping a count of
+    /// how many repeated elements were present.
+    /// If the iterator is sorted, all elements will be unique.
+    ///
+    /// Iterator element type is `(usize, Self::Item)`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![1., 1., 2., 3., 3., 2., 2.];
+    /// itertools::assert_equal(data.into_iter().dedup_with_count(),
+    ///                         vec![(2, 1.), (1, 2.), (2, 3.), (2, 2.)]);
+    /// ```
+    fn dedup_with_count(self) -> DedupWithCount<Self>
+        where Self: Sized,
+    {
+        adaptors::dedup_with_count(self)
+    }
+
+    /// Remove duplicates from sections of consecutive identical elements, while keeping a count of
+    /// how many repeated elements were present.
+    /// This will determine equality using a comparison function.
+    /// If the iterator is sorted, all elements will be unique.
+    ///
+    /// Iterator element type is `(usize, Self::Item)`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![(0, 1.), (1, 1.), (0, 2.), (0, 3.), (1, 3.), (1, 2.), (2, 2.)];
+    /// itertools::assert_equal(data.into_iter().dedup_by_with_count(|x, y| x.1 == y.1),
+    ///                         vec![(2, (0, 1.)), (1, (0, 2.)), (2, (0, 3.)), (2, (1, 2.))]);
+    /// ```
+    fn dedup_by_with_count<Cmp>(self, cmp: Cmp) -> DedupByWithCount<Self, Cmp>
+        where Self: Sized,
+              Cmp: FnMut(&Self::Item, &Self::Item) -> bool,
+    {
+        adaptors::dedup_by_with_count(self, cmp)
+    }
+
     /// Return an iterator adaptor that filters out elements that have
     /// already been produced once during the iteration. Duplicates
     /// are detected using hash and equality.
@@ -987,6 +1155,10 @@ pub trait Itertools : Iterator {
     /// Clones of visited elements are stored in a hash set in the
     /// iterator.
     ///
+    /// The iterator is stable, returning the non-duplicate items in the order
+    /// in which they occur in the adapted iterator. In a set of duplicate
+    /// items, the first item encountered is the item retained.
+    ///
     /// ```
     /// use itertools::Itertools;
     ///
@@ -1009,6 +1181,10 @@ pub trait Itertools : Iterator {
     /// with the keying function `f` by hash and equality.
     /// The keys are stored in a hash set in the iterator.
     ///
+    /// The iterator is stable, returning the non-duplicate items in the order
+    /// in which they occur in the adapted iterator. In a set of duplicate
+    /// items, the first item encountered is the item retained.
+    ///
     /// ```
     /// use itertools::Itertools;
     ///
@@ -1093,7 +1269,7 @@ pub trait Itertools : Iterator {
     /// elements from an iterator.
     ///
     /// Iterator element can be any homogeneous tuple of type `Self::Item` with
-    /// size up to 4.
+    /// size up to 12.
     ///
     /// ```
     /// use itertools::Itertools;
@@ -1159,7 +1335,7 @@ pub trait Itertools : Iterator {
     ///     vec![2, 2],
     /// ]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn combinations(self, k: usize) -> Combinations<Self>
         where Self: Sized,
               Self::Item: Clone
@@ -1186,7 +1362,7 @@ pub trait Itertools : Iterator {
     ///     vec![3, 3],
     /// ]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn combinations_with_replacement(self, k: usize) -> CombinationsWithReplacement<Self>
     where
         Self: Sized,
@@ -1232,7 +1408,7 @@ pub trait Itertools : Iterator {
     ///
     /// Note: The source iterator is collected lazily, and will not be
     /// re-iterated if the permutations adaptor is completed and re-iterated.
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn permutations(self, k: usize) -> Permutations<Self>
         where Self: Sized,
               Self::Item: Clone
@@ -1240,6 +1416,42 @@ pub trait Itertools : Iterator {
         permutations::permutations(self, k)
     }
 
+    /// Return an iterator that iterates through the powerset of the elements from an
+    /// iterator.
+    ///
+    /// Iterator element type is `Vec<Self::Item>`. The iterator produces a new `Vec`
+    /// per iteration, and clones the iterator elements.
+    ///
+    /// The powerset of a set contains all subsets including the empty set and the full
+    /// input set. A powerset has length _2^n_ where _n_ is the length of the input
+    /// set.
+    ///
+    /// Each `Vec` produced by this iterator represents a subset of the elements
+    /// produced by the source iterator.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let sets = (1..4).powerset().collect::<Vec<_>>();
+    /// itertools::assert_equal(sets, vec![
+    ///     vec![],
+    ///     vec![1],
+    ///     vec![2],
+    ///     vec![3],
+    ///     vec![1, 2],
+    ///     vec![1, 3],
+    ///     vec![2, 3],
+    ///     vec![1, 2, 3],
+    /// ]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn powerset(self) -> Powerset<Self>
+        where Self: Sized,
+              Self::Item: Clone,
+    {
+        powerset::powerset(self)
+    }
+
     /// Return an iterator adaptor that pads the sequence to a minimum length of
     /// `min` by filling missing elements using a closure `f`.
     ///
@@ -1328,7 +1540,7 @@ pub trait Itertools : Iterator {
 
     // non-adaptor methods
     /// Advances the iterator and returns the next items grouped in a tuple of
-    /// a specific size (up to 4).
+    /// a specific size (up to 12).
     ///
     /// If there are enough elements to be grouped in a tuple, then the tuple is
     /// returned inside `Some`, otherwise `None` is returned.
@@ -1348,7 +1560,7 @@ pub trait Itertools : Iterator {
     }
 
     /// Collects all items from the iterator into a tuple of a specific size
-    /// (up to 4).
+    /// (up to 12).
     ///
     /// If the number of elements inside the iterator is **exactly** equal to
     /// the tuple size, then the tuple is returned inside `Some`, otherwise
@@ -1494,7 +1706,7 @@ pub trait Itertools : Iterator {
     ///
     /// itertools::assert_equal(rx.iter(), vec![1, 3, 5, 7, 9]);
     /// ```
-    #[deprecated(note="Use .for_each() instead", since="0.8")]
+    #[deprecated(note="Use .for_each() instead", since="0.8.0")]
     fn foreach<F>(self, f: F)
         where F: FnMut(Self::Item),
               Self: Sized,
@@ -1524,7 +1736,7 @@ pub trait Itertools : Iterator {
 
     /// `.collect_vec()` is simply a type specialization of `.collect()`,
     /// for convenience.
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn collect_vec(self) -> Vec<Self::Item>
         where Self: Sized
     {
@@ -1551,7 +1763,7 @@ pub trait Itertools : Iterator {
     ///     Ok(())
     /// }
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn try_collect<T, U, E>(self) -> Result<U, E>
     where
         Self: Sized + Iterator<Item = Result<T, E>>,
@@ -1601,7 +1813,7 @@ pub trait Itertools : Iterator {
     /// assert_eq!(["a", "b", "c"].iter().join(", "), "a, b, c");
     /// assert_eq!([1, 2, 3].iter().join(", "), "1, 2, 3");
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn join(&mut self, sep: &str) -> String
         where Self::Item: std::fmt::Display
     {
@@ -1612,10 +1824,10 @@ pub trait Itertools : Iterator {
                 let (lower, _) = self.size_hint();
                 let mut result = String::with_capacity(sep.len() * lower);
                 write!(&mut result, "{}", first_elt).unwrap();
-                for elt in self {
+                self.for_each(|elt| {
                     result.push_str(sep);
                     write!(&mut result, "{}", elt).unwrap();
-                }
+                });
                 result
             }
         }
@@ -1681,6 +1893,15 @@ pub trait Itertools : Iterator {
         format::new_format(self, sep, format)
     }
 
+    /// See [`.fold_ok()`](#method.fold_ok).
+    #[deprecated(note="Use .fold_ok() instead", since="0.10.0")]
+    fn fold_results<A, E, B, F>(&mut self, start: B, f: F) -> Result<B, E>
+        where Self: Iterator<Item = Result<A, E>>,
+              F: FnMut(B, A) -> B
+    {
+        self.fold_ok(start, f)
+    }
+
     /// Fold `Result` values from an iterator.
     ///
     /// Only `Ok` values are folded. If no error is encountered, the folded
@@ -1713,17 +1934,17 @@ pub trait Itertools : Iterator {
     /// assert_eq!(
     ///     values.iter()
     ///           .map(Ok::<_, ()>)
-    ///           .fold_results(0, Add::add),
+    ///           .fold_ok(0, Add::add),
     ///     Ok(3)
     /// );
     /// assert!(
     ///     values.iter()
     ///           .map(|&x| if x >= 0 { Ok(x) } else { Err("Negative number") })
-    ///           .fold_results(0, Add::add)
+    ///           .fold_ok(0, Add::add)
     ///           .is_err()
     /// );
     /// ```
-    fn fold_results<A, E, B, F>(&mut self, mut start: B, mut f: F) -> Result<B, E>
+    fn fold_ok<A, E, B, F>(&mut self, mut start: B, mut f: F) -> Result<B, E>
         where Self: Iterator<Item = Result<A, E>>,
               F: FnMut(B, A) -> B
     {
@@ -1742,7 +1963,7 @@ pub trait Itertools : Iterator {
     /// value is returned inside `Some`. Otherwise, the operation terminates
     /// and returns `None`. No iterator elements are consumed after the `None`.
     ///
-    /// This is the `Option` equivalent to `fold_results`.
+    /// This is the `Option` equivalent to `fold_ok`.
     ///
     /// ```
     /// use std::ops::Add;
@@ -1933,19 +2154,26 @@ pub trait Itertools : Iterator {
     /// The big difference between the computations of `result2` and `result3` is that while
     /// `fold()` called the provided closure for every item of the callee iterator,
     /// `fold_while()` actually stopped iterating as soon as it encountered `Fold::Done(_)`.
-    #[deprecated(note="Use .try_fold() instead", since="0.8")]
     fn fold_while<B, F>(&mut self, init: B, mut f: F) -> FoldWhile<B>
         where Self: Sized,
               F: FnMut(B, Self::Item) -> FoldWhile<B>
     {
-        let mut acc = init;
-        while let Some(item) = self.next() {
-            match f(acc, item) {
-                FoldWhile::Continue(res) => acc = res,
-                res @ FoldWhile::Done(_) => return res,
+        use Result::{
+            Ok as Continue,
+            Err as Break,
+        };
+
+        let result = self.try_fold(init, #[inline(always)] |acc, v|
+            match f(acc, v) {
+              FoldWhile::Continue(acc) => Continue(acc),
+              FoldWhile::Done(acc) => Break(acc),
             }
+        );
+
+        match result {
+            Continue(acc) => FoldWhile::Continue(acc),
+            Break(acc) => FoldWhile::Done(acc),
         }
-        FoldWhile::Continue(acc)
     }
 
     /// Iterate over the entire iterator and add all the elements.
@@ -2005,6 +2233,101 @@ pub trait Itertools : Iterator {
             .map(|first| once(first).chain(self).product())
     }
 
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// `slice::sort_unstable()` method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort the letters of the text in ascending order
+    /// let text = "bdacfe";
+    /// itertools::assert_equal(text.chars().sorted_unstable(),
+    ///                         "abcdef".chars());
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_unstable(self) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              Self::Item: Ord
+    {
+        // Use .sort_unstable() directly since it is not quite identical with
+        // .sort_by(Ord::cmp)
+        let mut v = Vec::from_iter(self);
+        v.sort_unstable();
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// `slice::sort_unstable_by()` method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort people in descending order by age
+    /// let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];
+    ///
+    /// let oldest_people_first = people
+    ///     .into_iter()
+    ///     .sorted_unstable_by(|a, b| Ord::cmp(&b.1, &a.1))
+    ///     .map(|(person, _age)| person);
+    ///
+    /// itertools::assert_equal(oldest_people_first,
+    ///                         vec!["Jill", "Jack", "Jane", "John"]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_unstable_by<F>(self, cmp: F) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              F: FnMut(&Self::Item, &Self::Item) -> Ordering,
+    {
+        let mut v = Vec::from_iter(self);
+        v.sort_unstable_by(cmp);
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// `slice::sort_unstable_by_key()` method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort people in descending order by age
+    /// let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];
+    ///
+    /// let oldest_people_first = people
+    ///     .into_iter()
+    ///     .sorted_unstable_by_key(|x| -x.1)
+    ///     .map(|(person, _age)| person);
+    ///
+    /// itertools::assert_equal(oldest_people_first,
+    ///                         vec!["Jill", "Jack", "Jane", "John"]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_unstable_by_key<K, F>(self, f: F) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              K: Ord,
+              F: FnMut(&Self::Item) -> K,
+    {
+        let mut v = Vec::from_iter(self);
+        v.sort_unstable_by_key(f);
+        v.into_iter()
+    }
 
     /// Sort all iterator elements into a new iterator in ascending order.
     ///
@@ -2023,7 +2346,7 @@ pub trait Itertools : Iterator {
     /// itertools::assert_equal(text.chars().sorted(),
     ///                         "abcdef".chars());
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn sorted(self) -> VecIntoIter<Self::Item>
         where Self: Sized,
               Self::Item: Ord
@@ -2058,7 +2381,7 @@ pub trait Itertools : Iterator {
     /// itertools::assert_equal(oldest_people_first,
     ///                         vec!["Jill", "Jack", "Jane", "John"]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn sorted_by<F>(self, cmp: F) -> VecIntoIter<Self::Item>
         where Self: Sized,
               F: FnMut(&Self::Item, &Self::Item) -> Ordering,
@@ -2091,7 +2414,7 @@ pub trait Itertools : Iterator {
     /// itertools::assert_equal(oldest_people_first,
     ///                         vec!["Jill", "Jack", "Jane", "John"]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(feature = "use_alloc")]
     fn sorted_by_key<K, F>(self, f: F) -> VecIntoIter<Self::Item>
         where Self: Sized,
               K: Ord,
@@ -2102,6 +2425,43 @@ pub trait Itertools : Iterator {
         v.into_iter()
     }
 
+    /// Sort the k smallest elements into a new iterator, in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, and returns the result
+    /// as a new iterator that owns its elements.  If the input contains
+    /// less than k elements, the result is equivalent to `self.sorted()`.
+    ///
+    /// This is guaranteed to use `k * sizeof(Self::Item) + O(1)` memory
+    /// and `O(n log k)` time, with `n` the number of elements in the input.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// **Note:** This is functionally-equivalent to `self.sorted().take(k)`
+    /// but much more efficient.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // A random permutation of 0..15
+    /// let numbers = vec![6, 9, 1, 14, 0, 4, 8, 7, 11, 2, 10, 3, 13, 12, 5];
+    ///
+    /// let five_smallest = numbers
+    ///     .into_iter()
+    ///     .k_smallest(5);
+    ///
+    /// itertools::assert_equal(five_smallest, 0..5);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn k_smallest(self, k: usize) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              Self::Item: Ord
+    {
+        crate::k_smallest::k_smallest(self, k)
+            .into_sorted_vec()
+            .into_iter()
+    }
+
     /// Collect all iterator elements into one of two
     /// partitions. Unlike `Iterator::partition`, each partition may
     /// have a distinct type.
@@ -2162,6 +2522,75 @@ pub trait Itertools : Iterator {
         group_map::into_group_map(self)
     }
 
+    /// Return an `Iterator` on a HahMap. Keys mapped to `Vec`s of values. The key is specified in
+    /// in the closure.
+    /// Different of into_group_map_by because the key is still present. It is also more general.
+    /// you can also fold the group_map.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use std::collections::HashMap;
+    ///
+    /// let data = vec![(0, 10), (2, 12), (3, 13), (0, 20), (3, 33), (2, 42)];
+    /// let lookup: HashMap<u32,Vec<(u32, u32)>> = data.clone().into_iter().into_group_map_by(|a|
+    /// a.0);
+    ///
+    /// assert_eq!(lookup[&0], vec![(0,10),(0,20)]);
+    /// assert_eq!(lookup.get(&1), None);
+    /// assert_eq!(lookup[&2], vec![(2,12), (2,42)]);
+    /// assert_eq!(lookup[&3], vec![(3,13), (3,33)]);
+    ///
+    /// assert_eq!(
+    ///     data.into_iter()
+    ///     .into_group_map_by(|x| x.0)
+    ///     .into_iter()
+    ///     .map(|(key, values)| (key, values.into_iter().fold(0,|acc, (_,v)| acc + v )))
+    ///     .collect::<HashMap<u32,u32>>()[&0], 30)
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn into_group_map_by<K, V, F>(self, f: F) -> HashMap<K, Vec<V>>
+        where
+            Self: Iterator<Item=V> + Sized,
+            K: Hash + Eq,
+            F: Fn(&V) -> K,
+    {
+        group_map::into_group_map_by(self, f)
+    }
+
+    /// Constructs a `GroupingMap` to be used later with one of the efficient 
+    /// group-and-fold operations it allows to perform.
+    /// 
+    /// The input iterator must yield item in the form of `(K, V)` where the
+    /// value of type `K` will be used as key to identify the groups and the
+    /// value of type `V` as value for the folding operation.
+    /// 
+    /// See [`GroupingMap`](./structs/struct.GroupingMap.html) for more informations
+    /// on what operations are available.
+    #[cfg(feature = "use_std")]
+    fn into_grouping_map<K, V>(self) -> GroupingMap<Self>
+        where Self: Iterator<Item=(K, V)> + Sized,
+              K: Hash + Eq,
+    {
+        grouping_map::new(self)
+    }
+
+    /// Constructs a `GroupingMap` to be used later with one of the efficient 
+    /// group-and-fold operations it allows to perform.
+    /// 
+    /// The values from this iterator will be used as values for the folding operation
+    /// while the keys will be obtained from the values by calling `key_mapper`.
+    /// 
+    /// See [`GroupingMap`](./structs/struct.GroupingMap.html) for more informations
+    /// on what operations are available.
+    #[cfg(feature = "use_std")]
+    fn into_grouping_map_by<K, V, F>(self, key_mapper: F) -> GroupingMapBy<Self, F>
+        where Self: Iterator<Item=V> + Sized,
+              K: Hash + Eq,
+              F: FnMut(&V) -> K
+    {
+        grouping_map::new(grouping_map::MapForGrouping::new(self, key_mapper))
+    }
+
     /// Return the minimum and maximum elements in the iterator.
     ///
     /// The return type `MinMaxResult` is an enum of three variants:
@@ -2573,16 +3002,62 @@ pub trait Itertools : Iterator {
             Some(first) => {
                 match self.next() {
                     Some(second) => {
-                        Err(ExactlyOneError::new((Some(first), Some(second)), self))
+                        Err(ExactlyOneError::new(Some(Either::Left([first, second])), self))
                     }
                     None => {
                         Ok(first)
                     }
                 }
             }
-            None => Err(ExactlyOneError::new((None, None), self)),
+            None => Err(ExactlyOneError::new(None, self)),
         }
     }
+
+    /// An iterator adaptor that allows the user to peek at multiple `.next()`
+    /// values without advancing the base iterator.
+    ///
+    /// # Examples
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut iter = (0..10).multipeek();
+    /// assert_eq!(iter.peek(), Some(&0));
+    /// assert_eq!(iter.peek(), Some(&1));
+    /// assert_eq!(iter.peek(), Some(&2));
+    /// assert_eq!(iter.next(), Some(0));
+    /// assert_eq!(iter.peek(), Some(&1));
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn multipeek(self) -> MultiPeek<Self>
+    where
+        Self: Sized,
+    {
+        multipeek_impl::multipeek(self)
+    }
+
+    /// Collect the items in this iterator and return a `HashMap` which
+    /// contains each item that appears in the iterator and the number
+    /// of times it appears.
+    ///
+    /// # Examples
+    /// ```
+    /// # use itertools::Itertools;
+    /// let counts = [1, 1, 1, 3, 3, 5].into_iter().counts();
+    /// assert_eq!(counts[&1], 3);
+    /// assert_eq!(counts[&3], 2);
+    /// assert_eq!(counts[&5], 1);
+    /// assert_eq!(counts.get(&0), None);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn counts(self) -> HashMap<Self::Item, usize>
+    where
+        Self: Sized,
+        Self::Item: Eq + Hash,
+    {
+        let mut counts = HashMap::new();
+        self.for_each(|item| *counts.entry(item).or_default() += 1);
+        counts
+    }
 }
 
 impl<T: ?Sized> Itertools for T where T: Iterator { }
diff --git a/src/multipeek_impl.rs b/src/multipeek_impl.rs
index 0b64e1d..986e5b4 100644
--- a/src/multipeek_impl.rs
+++ b/src/multipeek_impl.rs
@@ -1,5 +1,5 @@
 use std::iter::Fuse;
-use std::collections::VecDeque;
+use alloc::collections::VecDeque;
 use crate::size_hint;
 use crate::PeekingNext;
 
@@ -80,13 +80,9 @@ impl<I> Iterator for MultiPeek<I>
 {
     type Item = I::Item;
 
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         self.index = 0;
-        if self.buf.is_empty() {
-            self.iter.next()
-        } else {
-            self.buf.pop_front()
-        }
+        self.buf.pop_front().or_else(|| self.iter.next())
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
diff --git a/src/pad_tail.rs b/src/pad_tail.rs
index 8d9d5ff..4ed83c3 100644
--- a/src/pad_tail.rs
+++ b/src/pad_tail.rs
@@ -36,7 +36,7 @@ impl<I, F> Iterator for PadUsing<I, F>
     type Item = I::Item;
 
     #[inline]
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         match self.iter.next() {
             None => {
                 if self.pos < self.min {
@@ -64,7 +64,7 @@ impl<I, F> DoubleEndedIterator for PadUsing<I, F>
     where I: DoubleEndedIterator + ExactSizeIterator,
           F: FnMut(usize) -> I::Item
 {
-    fn next_back(&mut self) -> Option<I::Item> {
+    fn next_back(&mut self) -> Option<Self::Item> {
         if self.min == 0 {
             self.iter.next_back()
         } else if self.iter.len() >= self.min {
diff --git a/src/peek_nth.rs b/src/peek_nth.rs
new file mode 100644
index 0000000..d22258b
--- /dev/null
+++ b/src/peek_nth.rs
@@ -0,0 +1,102 @@
+use crate::size_hint;
+use crate::PeekingNext;
+use alloc::collections::VecDeque;
+use std::iter::Fuse;
+
+/// See [`peek_nth()`](../fn.peek_nth.html) for more information.
+#[derive(Clone, Debug)]
+pub struct PeekNth<I>
+where
+    I: Iterator,
+{
+    iter: Fuse<I>,
+    buf: VecDeque<I::Item>,
+}
+
+/// A drop-in replacement for `std::iter::Peekable` which adds a `peek_nth`
+/// method allowing the user to `peek` at a value several iterations forward
+/// without advancing the base iterator.
+///
+/// This differs from `multipeek` in that subsequent calls to `peek` or
+/// `peek_nth` will always return the same value until `next` is called
+/// (making `reset_peek` unnecessary).
+pub fn peek_nth<I>(iterable: I) -> PeekNth<I::IntoIter>
+where
+    I: IntoIterator,
+{
+    PeekNth {
+        iter: iterable.into_iter().fuse(),
+        buf: VecDeque::new(),
+    }
+}
+
+impl<I> PeekNth<I>
+where
+    I: Iterator,
+{
+    /// Works exactly like the `peek` method in `std::iter::Peekable`
+    pub fn peek(&mut self) -> Option<&I::Item> {
+        self.peek_nth(0)
+    }
+
+    /// Returns a reference to the `nth` value without advancing the iterator.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```rust
+    /// use itertools::peek_nth;
+    ///
+    /// let xs = vec![1,2,3];
+    /// let mut iter = peek_nth(xs.iter());
+    ///
+    /// assert_eq!(iter.peek_nth(0), Some(&&1));
+    /// assert_eq!(iter.next(), Some(&1));
+    ///
+    /// // The iterator does not advance even if we call `peek_nth` multiple times
+    /// assert_eq!(iter.peek_nth(0), Some(&&2));
+    /// assert_eq!(iter.peek_nth(1), Some(&&3));
+    /// assert_eq!(iter.next(), Some(&2));
+    ///
+    /// // Calling `peek_nth` past the end of the iterator will return `None`
+    /// assert_eq!(iter.peek_nth(1), None);
+    /// ```
+    pub fn peek_nth(&mut self, n: usize) -> Option<&I::Item> {
+        let unbuffered_items = (n + 1).saturating_sub(self.buf.len());
+
+        self.buf.extend(self.iter.by_ref().take(unbuffered_items));
+
+        self.buf.get(n)
+    }
+}
+
+impl<I> Iterator for PeekNth<I>
+where
+    I: Iterator,
+{
+    type Item = I::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.buf.pop_front().or_else(|| self.iter.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        size_hint::add_scalar(self.iter.size_hint(), self.buf.len())
+    }
+}
+
+impl<I> ExactSizeIterator for PeekNth<I> where I: ExactSizeIterator {}
+
+impl<I> PeekingNext for PeekNth<I>
+where
+    I: Iterator,
+{
+    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
+    where
+        F: FnOnce(&Self::Item) -> bool,
+    {
+        self.peek().filter(|item| accept(item))?;
+        self.next()
+    }
+}
diff --git a/src/peeking_take_while.rs b/src/peeking_take_while.rs
index b404904..70ef988 100644
--- a/src/peeking_take_while.rs
+++ b/src/peeking_take_while.rs
@@ -1,6 +1,6 @@
 use std::iter::Peekable;
 use crate::PutBack;
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 use crate::PutBackN;
 
 /// An iterator that allows peeking at an element before deciding to accept it.
@@ -52,7 +52,7 @@ impl<I> PeekingNext for PutBack<I>
     }
 }
 
-#[cfg(feature = "use_std")]
+#[cfg(feature = "use_alloc")]
 impl<I> PeekingNext for PutBackN<I>
     where I: Iterator,
 {
@@ -104,8 +104,7 @@ impl<'a, I, F> Iterator for PeekingTakeWhile<'a, I, F>
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let (_, hi) = self.iter.size_hint();
-        (0, hi)
+        (0, self.iter.size_hint().1)
     }
 }
 
@@ -138,10 +137,10 @@ peeking_next_by_clone! { ['a] ::std::str::Bytes<'a> }
 peeking_next_by_clone! { ['a, T] ::std::option::Iter<'a, T> }
 peeking_next_by_clone! { ['a, T] ::std::result::Iter<'a, T> }
 peeking_next_by_clone! { [T] ::std::iter::Empty<T> }
-#[cfg(feature = "use_std")]
-peeking_next_by_clone! { ['a, T] ::std::collections::linked_list::Iter<'a, T> }
-#[cfg(feature = "use_std")]
-peeking_next_by_clone! { ['a, T] ::std::collections::vec_deque::Iter<'a, T> }
+#[cfg(feature = "use_alloc")]
+peeking_next_by_clone! { ['a, T] alloc::collections::linked_list::Iter<'a, T> }
+#[cfg(feature = "use_alloc")]
+peeking_next_by_clone! { ['a, T] alloc::collections::vec_deque::Iter<'a, T> }
 
 // cloning a Rev has no extra overhead; peekable and put backs are never DEI.
 peeking_next_by_clone! { [I: Clone + PeekingNext + DoubleEndedIterator]
diff --git a/src/permutations.rs b/src/permutations.rs
index fb4dd50..d96bbac 100644
--- a/src/permutations.rs
+++ b/src/permutations.rs
@@ -1,3 +1,4 @@
+use alloc::vec::Vec;
 use std::fmt;
 use std::iter::once;
 
@@ -104,21 +105,21 @@ where
 
         let &mut Permutations { ref vals, ref state } = self;
 
-        match state {
-            &PermutationState::StartUnknownLen { .. } => panic!("unexpected iterator state"),
-            &PermutationState::OngoingUnknownLen { k, min_n } => {
+        match *state {
+            PermutationState::StartUnknownLen { .. } => panic!("unexpected iterator state"),
+            PermutationState::OngoingUnknownLen { k, min_n } => {
                 let latest_idx = min_n - 1;
                 let indices = (0..(k - 1)).chain(once(latest_idx));
 
                 Some(indices.map(|i| vals[i].clone()).collect())
             }
-            &PermutationState::Complete(CompleteState::Start { .. }) => None,
-            &PermutationState::Complete(CompleteState::Ongoing { ref indices, ref cycles }) => {
+            PermutationState::Complete(CompleteState::Start { .. }) => None,
+            PermutationState::Complete(CompleteState::Ongoing { ref indices, ref cycles }) => {
                 let k = cycles.len();
 
                 Some(indices[0..k].iter().map(|&i| vals[i].clone()).collect())
             },
-            &PermutationState::Empty => None
+            PermutationState::Empty => None
         }
     }
 
@@ -174,11 +175,11 @@ where
     fn advance(&mut self) {
         let &mut Permutations { ref mut vals, ref mut state } = self;
 
-        *state = match state {
-            &mut PermutationState::StartUnknownLen { k } => {
+        *state = match *state {
+            PermutationState::StartUnknownLen { k } => {
                 PermutationState::OngoingUnknownLen { k, min_n: k }
             }
-            &mut PermutationState::OngoingUnknownLen { k, min_n } => {
+            PermutationState::OngoingUnknownLen { k, min_n } => {
                 if vals.get_next() {
                     PermutationState::OngoingUnknownLen { k, min_n: min_n + 1 }
                 } else {
@@ -194,20 +195,20 @@ where
                     PermutationState::Complete(complete_state)
                 }
             }
-            &mut PermutationState::Complete(ref mut state) => {
+            PermutationState::Complete(ref mut state) => {
                 state.advance();
 
                 return;
             }
-            &mut PermutationState::Empty => { return; }
+            PermutationState::Empty => { return; }
         };
     }
 }
 
 impl CompleteState {
     fn advance(&mut self) {
-        *self = match self {
-            &mut CompleteState::Start { n, k } => {
+        *self = match *self {
+            CompleteState::Start { n, k } => {
                 let indices = (0..n).collect();
                 let cycles = ((n - k)..n).rev().collect();
 
@@ -216,7 +217,7 @@ impl CompleteState {
                     indices
                 }
             },
-            &mut CompleteState::Ongoing { ref mut indices, ref mut cycles } => {
+            CompleteState::Ongoing { ref mut indices, ref mut cycles } => {
                 let n = indices.len();
                 let k = cycles.len();
 
@@ -243,8 +244,8 @@ impl CompleteState {
     fn remaining(&self) -> CompleteStateRemaining {
         use self::CompleteStateRemaining::{Known, Overflow};
 
-        match self {
-            &CompleteState::Start { n, k } => {
+        match *self {
+            CompleteState::Start { n, k } => {
                 if n < k {
                     return Known(0);
                 }
@@ -258,7 +259,7 @@ impl CompleteState {
                     None => Overflow
                 }
             }
-            &CompleteState::Ongoing { ref indices, ref cycles } => {
+            CompleteState::Ongoing { ref indices, ref cycles } => {
                 let mut count: usize = 0;
 
                 for (i, &c) in cycles.iter().enumerate() {
diff --git a/src/powerset.rs b/src/powerset.rs
new file mode 100644
index 0000000..ef17752
--- /dev/null
+++ b/src/powerset.rs
@@ -0,0 +1,83 @@
+use std::fmt;
+use std::usize;
+use alloc::vec::Vec;
+
+use super::combinations::{Combinations, combinations};
+use super::size_hint;
+
+/// An iterator to iterate through the powerset of the elements from an iterator.
+///
+/// See [`.powerset()`](../trait.Itertools.html#method.powerset) for more
+/// information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub struct Powerset<I: Iterator> {
+    combs: Combinations<I>,
+    // Iterator `position` (equal to count of yielded elements).
+    pos: usize,
+}
+
+impl<I> Clone for Powerset<I>
+    where I: Clone + Iterator,
+          I::Item: Clone,
+{
+    clone_fields!(combs, pos);
+}
+
+impl<I> fmt::Debug for Powerset<I>
+    where I: Iterator + fmt::Debug,
+          I::Item: fmt::Debug,
+{
+    debug_fmt_fields!(Powerset, combs, pos);
+}
+
+/// Create a new `Powerset` from a clonable iterator.
+pub fn powerset<I>(src: I) -> Powerset<I>
+    where I: Iterator,
+          I::Item: Clone,
+{
+    Powerset {
+        combs: combinations(src, 0),
+        pos: 0,
+    }
+}
+
+impl<I> Iterator for Powerset<I>
+    where
+        I: Iterator,
+        I::Item: Clone,
+{
+    type Item = Vec<I::Item>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(elt) = self.combs.next() {
+            self.pos = self.pos.saturating_add(1);
+            Some(elt)
+        } else if self.combs.k() < self.combs.n()
+            || self.combs.k() == 0
+        {
+            self.combs.reset(self.combs.k() + 1);
+            self.combs.next().map(|elt| {
+                self.pos = self.pos.saturating_add(1);
+                elt
+            })
+        } else {
+            None
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // Total bounds for source iterator.
+        let src_total = size_hint::add_scalar(self.combs.src().size_hint(), self.combs.n());
+
+        // Total bounds for self ( length(powerset(set) == 2 ^ length(set) )
+        let self_total = size_hint::pow_scalar_base(2, src_total);
+
+        if self.pos < usize::MAX {
+            // Subtract count of elements already yielded from total.
+            size_hint::sub_scalar(self_total, self.pos)
+        } else {
+            // Fallback: self.pos is saturated and no longer reliable.
+            (0, self_total.1)
+        }
+    }
+}
diff --git a/src/process_results_impl.rs b/src/process_results_impl.rs
index c819f50..d74925a 100644
--- a/src/process_results_impl.rs
+++ b/src/process_results_impl.rs
@@ -28,8 +28,7 @@ impl<'a, I, T, E> Iterator for ProcessResults<'a, I, E>
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let (_, hi) = self.iter.size_hint();
-        (0, hi)
+        (0, self.iter.size_hint().1)
     }
 }
 
diff --git a/src/put_back_n_impl.rs b/src/put_back_n_impl.rs
index dcb2894..60ea8e6 100644
--- a/src/put_back_n_impl.rs
+++ b/src/put_back_n_impl.rs
@@ -1,3 +1,5 @@
+use alloc::vec::Vec;
+
 use crate::size_hint;
 
 /// An iterator adaptor that allows putting multiple
@@ -47,12 +49,8 @@ impl<I: Iterator> PutBackN<I> {
 impl<I: Iterator> Iterator for PutBackN<I> {
     type Item = I::Item;
     #[inline]
-    fn next(&mut self) -> Option<I::Item> {
-        if self.top.is_empty() {
-            self.iter.next()
-        } else {
-            self.top.pop()
-        }
+    fn next(&mut self) -> Option<Self::Item> {
+        self.top.pop().or_else(|| self.iter.next())
     }
 
     #[inline]
diff --git a/src/rciter_impl.rs b/src/rciter_impl.rs
index 6ee90ad..9122dad 100644
--- a/src/rciter_impl.rs
+++ b/src/rciter_impl.rs
@@ -1,6 +1,6 @@
 
 use std::iter::IntoIterator;
-use std::rc::Rc;
+use alloc::rc::Rc;
 use std::cell::RefCell;
 
 /// A wrapper for `Rc<RefCell<I>>`, that implements the `Iterator` trait.
@@ -60,7 +60,7 @@ impl<A, I> Iterator for RcIter<I>
 {
     type Item = A;
     #[inline]
-    fn next(&mut self) -> Option<A> {
+    fn next(&mut self) -> Option<Self::Item> {
         self.rciter.borrow_mut().next()
     }
 
@@ -69,8 +69,7 @@ impl<A, I> Iterator for RcIter<I>
         // To work sanely with other API that assume they own an iterator,
         // so it can't change in other places, we can't guarantee as much
         // in our size_hint. Other clones may drain values under our feet.
-        let (_, hi) = self.rciter.borrow().size_hint();
-        (0, hi)
+        (0, self.rciter.borrow().size_hint().1)
     }
 }
 
@@ -78,7 +77,7 @@ impl<I> DoubleEndedIterator for RcIter<I>
     where I: DoubleEndedIterator
 {
     #[inline]
-    fn next_back(&mut self) -> Option<I::Item> {
+    fn next_back(&mut self) -> Option<Self::Item> {
         self.rciter.borrow_mut().next_back()
     }
 }
diff --git a/src/size_hint.rs b/src/size_hint.rs
index be54443..1168eca 100644
--- a/src/size_hint.rs
+++ b/src/size_hint.rs
@@ -3,6 +3,7 @@
 
 use std::usize;
 use std::cmp;
+use std::u32;
 
 /// **SizeHint** is the return type of **Iterator::size_hint()**.
 pub type SizeHint = (usize, Option<usize>);
@@ -10,7 +11,7 @@ pub type SizeHint = (usize, Option<usize>);
 /// Add **SizeHint** correctly.
 #[inline]
 pub fn add(a: SizeHint, b: SizeHint) -> SizeHint {
-    let min = a.0.checked_add(b.0).unwrap_or(usize::MAX);
+    let min = a.0.saturating_add(b.0);
     let max = match (a.1, b.1) {
         (Some(x), Some(y)) => x.checked_add(y),
         _ => None,
@@ -56,7 +57,7 @@ pub fn sub_scalar(sh: SizeHint, x: usize) -> SizeHint {
 /// ```
 #[inline]
 pub fn mul(a: SizeHint, b: SizeHint) -> SizeHint {
-    let low = a.0.checked_mul(b.0).unwrap_or(usize::MAX);
+    let low = a.0.saturating_mul(b.0);
     let hi = match (a.1, b.1) {
         (Some(x), Some(y)) => x.checked_mul(y),
         (Some(0), None) | (None, Some(0)) => Some(0),
@@ -74,6 +75,20 @@ pub fn mul_scalar(sh: SizeHint, x: usize) -> SizeHint {
     (low, hi)
 }
 
+/// Raise `base` correctly by a **`SizeHint`** exponent.
+#[inline]
+pub fn pow_scalar_base(base: usize, exp: SizeHint) -> SizeHint {
+    let exp_low = cmp::min(exp.0, u32::MAX as usize) as u32;
+    let low = base.saturating_pow(exp_low);
+
+    let hi = exp.1.and_then(|exp| {
+        let exp_hi = cmp::min(exp, u32::MAX as usize) as u32;
+        base.checked_pow(exp_hi)
+    });
+
+    (low, hi)
+}
+
 /// Return the maximum
 #[inline]
 pub fn max(a: SizeHint, b: SizeHint) -> SizeHint {
diff --git a/src/sources.rs b/src/sources.rs
index 17d9ff9..5bb6afe 100644
--- a/src/sources.rs
+++ b/src/sources.rs
@@ -7,7 +7,7 @@ use std::mem;
 
 /// See [`repeat_call`](../fn.repeat_call.html) for more information.
 #[derive(Clone)]
-#[deprecated(note="Use std repeat_with() instead", since="0.8")]
+#[deprecated(note="Use std repeat_with() instead", since="0.8.0")]
 pub struct RepeatCall<F> {
     f: F,
 }
@@ -39,7 +39,7 @@ impl<F> fmt::Debug for RepeatCall<F>
 ///     vec![1, 1, 1, 1, 1]
 /// );
 /// ```
-#[deprecated(note="Use std repeat_with() instead", since="0.8")]
+#[deprecated(note="Use std repeat_with() instead", since="0.8.0")]
 pub fn repeat_call<F, A>(function: F) -> RepeatCall<F>
     where F: FnMut() -> A
 {
@@ -52,7 +52,7 @@ impl<A, F> Iterator for RepeatCall<F>
     type Item = A;
 
     #[inline]
-    fn next(&mut self) -> Option<A> {
+    fn next(&mut self) -> Option<Self::Item> {
         Some((self.f)())
     }
 
@@ -76,22 +76,21 @@ impl<A, F> Iterator for RepeatCall<F>
 ///
 /// use itertools::unfold;
 ///
-/// let (mut x1, mut x2) = (1u32, 1u32);
-/// let mut fibonacci = unfold((), move |_| {
+/// let mut fibonacci = unfold((1u32, 1u32), |(x1, x2)| {
 ///     // Attempt to get the next Fibonacci number
-///     let next = x1.saturating_add(x2);
+///     let next = x1.saturating_add(*x2);
 ///
 ///     // Shift left: ret <- x1 <- x2 <- next
-///     let ret = x1;
-///     x1 = x2;
-///     x2 = next;
+///     let ret = *x1;
+///     *x1 = *x2;
+///     *x2 = next;
 ///
 ///     // If addition has saturated at the maximum, we are finished
-///     if ret == x1 && ret > 1 {
-///         return None;
+///     if ret == *x1 && ret > 1 {
+///         None
+///     } else {
+///         Some(ret)
 ///     }
-///
-///     Some(ret)
 /// });
 ///
 /// itertools::assert_equal(fibonacci.by_ref().take(8),
@@ -128,15 +127,9 @@ impl<A, St, F> Iterator for Unfold<St, F>
     type Item = A;
 
     #[inline]
-    fn next(&mut self) -> Option<A> {
+    fn next(&mut self) -> Option<Self::Item> {
         (self.f)(&mut self.state)
     }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        // no possible known bounds at this point
-        (0, None)
-    }
 }
 
 /// An iterator that infinitely applies function to value and yields results.
diff --git a/src/tee.rs b/src/tee.rs
index aa4be41..0b00302 100644
--- a/src/tee.rs
+++ b/src/tee.rs
@@ -1,8 +1,8 @@
 use super::size_hint;
 
 use std::cell::RefCell;
-use std::collections::VecDeque;
-use std::rc::Rc;
+use alloc::collections::VecDeque;
+use alloc::rc::Rc;
 
 /// Common buffer object for the two tee halves
 #[derive(Debug)]
@@ -39,7 +39,7 @@ impl<I> Iterator for Tee<I>
           I::Item: Clone
 {
     type Item = I::Item;
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         // .borrow_mut may fail here -- but only if the user has tied some kind of weird
         // knot where the iterator refers back to itself.
         let mut buffer = self.rcbuffer.borrow_mut();
diff --git a/src/tuple_impl.rs b/src/tuple_impl.rs
index f205f01..1d24b0b 100644
--- a/src/tuple_impl.rs
+++ b/src/tuple_impl.rs
@@ -1,6 +1,9 @@
 //! Some iterator that produces tuples
 
 use std::iter::Fuse;
+use std::iter::Take;
+use std::iter::Cycle;
+use std::marker::PhantomData;
 
 // `HomogeneousTuple` is a public facade for `TupleCollect`, allowing
 // tuple-related methods to be used by clients in generic contexts, while
@@ -54,12 +57,12 @@ impl<T> Iterator for TupleBuffer<T>
 
     fn size_hint(&self) -> (usize, Option<usize>) {
         let buffer = &self.buf.as_ref()[self.cur..];
-        let len = if buffer.len() == 0 {
+        let len = if buffer.is_empty() {
             0
         } else {
             buffer.iter()
                   .position(|x| x.is_none())
-                  .unwrap_or(buffer.len())
+                  .unwrap_or_else(|| buffer.len())
         };
         (len, Some(len))
     }
@@ -100,7 +103,7 @@ impl<I, T> Iterator for Tuples<I, T>
 {
     type Item = T;
 
-    fn next(&mut self) -> Option<T> {
+    fn next(&mut self) -> Option<Self::Item> {
         T::collect_from_iter(&mut self.iter, &mut self.buf)
     }
 }
@@ -170,7 +173,7 @@ impl<I, T> Iterator for TupleWindows<I, T>
 {
     type Item = T;
 
-    fn next(&mut self) -> Option<T> {
+    fn next(&mut self) -> Option<Self::Item> {
         if T::num_items() == 1 {
             return T::collect_from_iter_no_buf(&mut self.iter)
         }
@@ -184,6 +187,48 @@ impl<I, T> Iterator for TupleWindows<I, T>
     }
 }
 
+/// An iterator over all windows,wrapping back to the first elements when the
+/// window would otherwise exceed the length of the iterator, producing tuples
+/// of a specific size.
+///
+/// See [`.circular_tuple_windows()`](../trait.Itertools.html#method.circular_tuple_windows) for more
+/// information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+#[derive(Debug)]
+pub struct CircularTupleWindows<I, T: Clone>
+    where I: Iterator<Item = T::Item> + Clone,
+          T: TupleCollect + Clone
+{
+    iter: Take<TupleWindows<Cycle<I>, T>>,
+    phantom_data: PhantomData<T>
+}
+
+pub fn circular_tuple_windows<I, T>(iter: I) -> CircularTupleWindows<I, T>
+    where I: Iterator<Item = T::Item> + Clone + ExactSizeIterator,
+          T: TupleCollect + Clone,
+          T::Item: Clone
+{
+    let len = iter.len();
+    let iter = tuple_windows(iter.cycle()).take(len);
+
+    CircularTupleWindows {
+        iter,
+        phantom_data: PhantomData{}
+    }
+}
+
+impl<I, T> Iterator for CircularTupleWindows<I, T>
+    where I: Iterator<Item = T::Item> + Clone,
+          T: TupleCollect + Clone,
+          T::Item: Clone
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.iter.next()
+    }
+}
+
 pub trait TupleCollect: Sized {
     type Item;
     type Buffer: Default + AsRef<[Option<Self::Item>]> + AsMut<[Option<Self::Item>]>;
@@ -199,16 +244,32 @@ pub trait TupleCollect: Sized {
     fn left_shift_push(&mut self, item: Self::Item);
 }
 
+macro_rules! count_ident{
+    () => {0};
+    ($i0:ident, $($i:ident,)*) => {1 + count_ident!($($i,)*)};
+}
+macro_rules! ignore_ident{
+    ($id:ident, $($t:tt)*) => {$($t)*};
+}
+macro_rules! rev_for_each_ident{
+    ($m:ident, ) => {};
+    ($m:ident, $i0:ident, $($i:ident,)*) => {
+        rev_for_each_ident!($m, $($i,)*);
+        $m!($i0);
+    };
+}
+
 macro_rules! impl_tuple_collect {
-    () => ();
-    ($N:expr; $A:ident ; $($X:ident),* ; $($Y:ident),* ; $($Y_rev:ident),*) => (
-        impl<$A> TupleCollect for ($($X),*,) {
-            type Item = $A;
-            type Buffer = [Option<$A>; $N - 1];
+    ($dummy:ident,) => {}; // stop
+    ($dummy:ident, $($Y:ident,)*) => (
+        impl_tuple_collect!($($Y,)*);
+        impl<A> TupleCollect for ($(ignore_ident!($Y, A),)*) {
+            type Item = A;
+            type Buffer = [Option<A>; count_ident!($($Y,)*) - 1];
 
             #[allow(unused_assignments, unused_mut)]
             fn collect_from_iter<I>(iter: I, buf: &mut Self::Buffer) -> Option<Self>
-                where I: IntoIterator<Item = $A>
+                where I: IntoIterator<Item = A>
             {
                 let mut iter = iter.into_iter();
                 $(
@@ -236,44 +297,31 @@ macro_rules! impl_tuple_collect {
                 return None;
             }
 
-            #[allow(unused_assignments)]
             fn collect_from_iter_no_buf<I>(iter: I) -> Option<Self>
-                where I: IntoIterator<Item = $A>
+                where I: IntoIterator<Item = A>
             {
                 let mut iter = iter.into_iter();
-                loop {
-                    $(
-                        let $Y = if let Some($Y) = iter.next() {
-                            $Y
-                        } else {
-                            break;
-                        };
-                    )*
-                    return Some(($($Y),*,))
-                }
 
-                return None;
+                Some(($(
+                    { let $Y = iter.next()?; $Y },
+                )*))
             }
 
             fn num_items() -> usize {
-                $N
+                count_ident!($($Y,)*)
             }
 
-            fn left_shift_push(&mut self, item: $A) {
+            fn left_shift_push(&mut self, mut item: A) {
                 use std::mem::replace;
 
                 let &mut ($(ref mut $Y),*,) = self;
-                let tmp = item;
-                $(
-                    let tmp = replace($Y_rev, tmp);
-                )*
-                drop(tmp);
+                macro_rules! replace_item{($i:ident) => {
+                    item = replace($i, item);
+                }};
+                rev_for_each_ident!(replace_item, $($Y,)*);
+                drop(item);
             }
         }
     )
 }
-
-impl_tuple_collect!(1; A; A; a; a);
-impl_tuple_collect!(2; A; A, A; a, b; b, a);
-impl_tuple_collect!(3; A; A, A, A; a, b, c; c, b, a);
-impl_tuple_collect!(4; A; A, A, A, A; a, b, c, d; d, c, b, a);
+impl_tuple_collect!(dummy, a, b, c, d, e, f, g, h, i, j, k, l,);
diff --git a/src/unique_impl.rs b/src/unique_impl.rs
index cf8675c..14c14fc 100644
--- a/src/unique_impl.rs
+++ b/src/unique_impl.rs
@@ -54,7 +54,7 @@ impl<I, V, F> Iterator for UniqueBy<I, V, F>
 {
     type Item = I::Item;
 
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         while let Some(v) = self.iter.next() {
             let key = (self.f)(&v);
             if self.used.insert(key, ()).is_none() {
@@ -76,13 +76,29 @@ impl<I, V, F> Iterator for UniqueBy<I, V, F>
     }
 }
 
+impl<I, V, F> DoubleEndedIterator for UniqueBy<I, V, F>
+    where I: DoubleEndedIterator,
+          V: Eq + Hash,
+          F: FnMut(&I::Item) -> V
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        while let Some(v) = self.iter.next_back() {
+            let key = (self.f)(&v);
+            if self.used.insert(key, ()).is_none() {
+                return Some(v);
+            }
+        }
+        None
+    }
+}
+
 impl<I> Iterator for Unique<I>
     where I: Iterator,
           I::Item: Eq + Hash + Clone
 {
     type Item = I::Item;
 
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         while let Some(v) = self.iter.iter.next() {
             if let Entry::Vacant(entry) = self.iter.used.entry(v) {
                 let elt = entry.key().clone();
@@ -104,6 +120,22 @@ impl<I> Iterator for Unique<I>
     }
 }
 
+impl<I> DoubleEndedIterator for Unique<I>
+    where I: DoubleEndedIterator,
+          I::Item: Eq + Hash + Clone
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        while let Some(v) = self.iter.iter.next_back() {
+            if let Entry::Vacant(entry) = self.iter.used.entry(v) {
+                let elt = entry.key().clone();
+                entry.insert(());
+                return Some(elt);
+            }
+        }
+        None
+    }
+}
+
 /// An iterator adapter to filter out duplicate elements.
 ///
 /// See [`.unique()`](../trait.Itertools.html#method.unique) for more information.
diff --git a/src/ziptuple.rs b/src/ziptuple.rs
index 2dc3ea5..8f40193 100644
--- a/src/ziptuple.rs
+++ b/src/ziptuple.rs
@@ -98,6 +98,30 @@ macro_rules! impl_zip_iter {
                 $B: ExactSizeIterator,
             )*
         { }
+
+        #[allow(non_snake_case)]
+        impl<$($B),*> DoubleEndedIterator for Zip<($($B,)*)> where
+            $(
+                $B: DoubleEndedIterator + ExactSizeIterator,
+            )*
+        {
+            #[inline]
+            fn next_back(&mut self) -> Option<Self::Item> {
+                let ($(ref mut $B,)*) = self.t;
+                let size = *[$( $B.len(), )*].iter().min().unwrap();
+
+                $(
+                    if $B.len() != size {
+                        for _ in 0..$B.len() - size { $B.next_back(); }
+                    }
+                )*
+
+                match ($($B.next_back(),)*) {
+                    ($(Some($B),)*) => Some(($($B,)*)),
+                    _ => None,
+                }
+            }
+        }
     );
 }
 
@@ -109,3 +133,7 @@ impl_zip_iter!(A, B, C, D, E);
 impl_zip_iter!(A, B, C, D, E, F);
 impl_zip_iter!(A, B, C, D, E, F, G);
 impl_zip_iter!(A, B, C, D, E, F, G, H);
+impl_zip_iter!(A, B, C, D, E, F, G, H, I);
+impl_zip_iter!(A, B, C, D, E, F, G, H, I, J);
+impl_zip_iter!(A, B, C, D, E, F, G, H, I, J, K);
+impl_zip_iter!(A, B, C, D, E, F, G, H, I, J, K, L);
diff --git a/tests/macros_hygiene.rs b/tests/macros_hygiene.rs
new file mode 100644
index 0000000..d111124
--- /dev/null
+++ b/tests/macros_hygiene.rs
@@ -0,0 +1,13 @@
+#[test]
+fn iproduct_hygiene() {
+    let _ = itertools::iproduct!(0..6);
+    let _ = itertools::iproduct!(0..6, 0..9);
+    let _ = itertools::iproduct!(0..6, 0..9, 0..12);
+}
+
+#[test]
+fn izip_hygiene() {
+    let _ = itertools::izip!(0..6);
+    let _ = itertools::izip!(0..6, 0..9);
+    let _ = itertools::izip!(0..6, 0..9, 0..12);
+}
diff --git a/tests/quick.rs b/tests/quick.rs
index 683ba7a..e5bee17 100644
--- a/tests/quick.rs
+++ b/tests/quick.rs
@@ -5,9 +5,10 @@
 
 use quickcheck as qc;
 use std::default::Default;
+use std::num::Wrapping;
 use std::ops::Range;
 use std::cmp::{max, min, Ordering};
-use std::collections::HashSet;
+use std::collections::{HashMap, HashSet};
 use itertools::Itertools;
 use itertools::{
     multizip,
@@ -19,6 +20,7 @@ use itertools::free::{
     cloned,
     enumerate,
     multipeek,
+    peek_nth,
     put_back,
     put_back_n,
     rciter,
@@ -487,6 +489,15 @@ quickcheck! {
         exact_size(it)
     }
 
+    fn size_peek_nth(a: Iter<u16, Exact>, s: u8) -> bool {
+        let mut it = peek_nth(a);
+        // peek a few times
+        for n in 0..s {
+            it.peek_nth(n as usize);
+        }
+        exact_size(it)
+    }
+
     fn equal_merge(a: Vec<i16>, b: Vec<i16>) -> bool {
         let mut sa = a.clone();
         let mut sb = b.clone();
@@ -744,6 +755,29 @@ quickcheck! {
 }
 
 quickcheck! {
+    fn dedup_via_coalesce(a: Vec<i32>) -> bool {
+        let mut b = a.clone();
+        b.dedup();
+        itertools::equal(
+            &b,
+            a
+                .iter()
+                .coalesce(|x, y| {
+                    if x==y {
+                        Ok(x)
+                    } else {
+                        Err((x, y))
+                    }
+                })
+                .fold(vec![], |mut v, n| {
+                    v.push(n);
+                    v
+                })
+        )
+    }
+}
+
+quickcheck! {
     fn equal_dedup(a: Vec<i32>) -> bool {
         let mut b = a.clone();
         b.dedup();
@@ -875,6 +909,13 @@ quickcheck! {
 }
 
 quickcheck! {
+    fn size_powerset(it: Iter<u8, Exact>) -> bool {
+        // Powerset cardinality gets large very quickly, limit input to keep test fast.
+        correct_size_hint(it.take(12).powerset())
+    }
+}
+
+quickcheck! {
     fn size_unique(it: Iter<i8>) -> bool {
         correct_size_hint(it.unique())
     }
@@ -1155,3 +1196,388 @@ quickcheck! {
         }
     }
 }
+
+quickcheck! {
+    fn consistent_grouping_map_with_by(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+
+        let lookup_grouping_map = a.iter().copied().map(|i| (i % modulo, i)).into_grouping_map().collect::<Vec<_>>();
+        let lookup_grouping_map_by = a.iter().copied().into_grouping_map_by(|i| i % modulo).collect::<Vec<_>>();
+
+        assert_eq!(lookup_grouping_map, lookup_grouping_map_by);
+    }
+
+    fn correct_grouping_map_by_aggregate_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo < 2 { 2 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter()
+            .map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .aggregate(|acc, &key, val| {
+                assert!(val % modulo == key);
+                if val % (modulo - 1) == 0 {
+                    None
+                } else {
+                    Some(acc.unwrap_or(0) + val)
+                }
+            });
+        
+        let group_map_lookup = a.iter()
+            .map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .filter_map(|(key, vals)| {
+                vals.into_iter().fold(None, |acc, val| {
+                    if val % (modulo - 1) == 0 {
+                        None
+                    } else {
+                        Some(acc.unwrap_or(0) + val)
+                    }
+                }).map(|new_val| (key, new_val))
+            })
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for m in 0..modulo {
+            assert_eq!(
+                lookup.get(&m).copied(), 
+                a.iter()
+                    .map(|&b| b as u64)
+                    .filter(|&val| val % modulo == m)
+                    .fold(None, |acc, val| {
+                        if val % (modulo - 1) == 0 {
+                            None
+                        } else {
+                            Some(acc.unwrap_or(0) + val)
+                        }
+                    })
+            );
+        }
+    }
+
+    fn correct_grouping_map_by_fold_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter().map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .fold(0u64, |acc, &key, val| {
+                assert!(val % modulo == key);
+                acc + val
+            });
+
+        let group_map_lookup = a.iter()
+            .map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().fold(0u64, |acc, val| acc + val)))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &sum) in lookup.iter() {
+            assert_eq!(sum, a.iter().map(|&b| b as u64).filter(|&val| val % modulo == key).sum::<u64>());
+        }
+    }
+
+    fn correct_grouping_map_by_fold_first_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter().map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .fold_first(|acc, &key, val| {
+                assert!(val % modulo == key);
+                acc + val
+            });
+
+        // TODO: Swap `fold1` with stdlib's `fold_first` when it's stabilized
+        let group_map_lookup = a.iter()
+            .map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().fold1(|acc, val| acc + val).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &sum) in lookup.iter() {
+            assert_eq!(sum, a.iter().map(|&b| b as u64).filter(|&val| val % modulo == key).sum::<u64>());
+        }
+    }
+
+    fn correct_grouping_map_by_collect_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup_grouping_map = a.iter().copied().into_grouping_map_by(|i| i % modulo).collect::<Vec<_>>();
+        let lookup_group_map = a.iter().copied().map(|i| (i % modulo, i)).into_group_map();
+
+        assert_eq!(lookup_grouping_map, lookup_group_map);
+    }
+
+    fn correct_grouping_map_by_max_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).max();
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().max().unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &max) in lookup.iter() {
+            assert_eq!(Some(max), a.iter().copied().filter(|&val| val % modulo == key).max());
+        }
+    }
+
+    fn correct_grouping_map_by_max_by_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).max_by(|_, v1, v2| v1.cmp(v2));
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().max_by(|v1, v2| v1.cmp(v2)).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &max) in lookup.iter() {
+            assert_eq!(Some(max), a.iter().copied().filter(|&val| val % modulo == key).max_by(|v1, v2| v1.cmp(v2)));
+        }
+    }
+
+    fn correct_grouping_map_by_max_by_key_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).max_by_key(|_, &val| val);
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().max_by_key(|&val| val).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &max) in lookup.iter() {
+            assert_eq!(Some(max), a.iter().copied().filter(|&val| val % modulo == key).max_by_key(|&val| val));
+        }
+    }
+    
+    fn correct_grouping_map_by_min_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).min();
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().min().unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &min) in lookup.iter() {
+            assert_eq!(Some(min), a.iter().copied().filter(|&val| val % modulo == key).min());
+        }
+    }
+
+    fn correct_grouping_map_by_min_by_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).min_by(|_, v1, v2| v1.cmp(v2));
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().min_by(|v1, v2| v1.cmp(v2)).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &min) in lookup.iter() {
+            assert_eq!(Some(min), a.iter().copied().filter(|&val| val % modulo == key).min_by(|v1, v2| v1.cmp(v2)));
+        }
+    }
+
+    fn correct_grouping_map_by_min_by_key_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).min_by_key(|_, &val| val);
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().min_by_key(|&val| val).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &min) in lookup.iter() {
+            assert_eq!(Some(min), a.iter().copied().filter(|&val| val % modulo == key).min_by_key(|&val| val));
+        }
+    }
+    
+    fn correct_grouping_map_by_minmax_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).minmax();
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().minmax()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &minmax) in lookup.iter() {
+            assert_eq!(minmax, a.iter().copied().filter(|&val| val % modulo == key).minmax());
+        }
+    }
+
+    fn correct_grouping_map_by_minmax_by_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).minmax_by(|_, v1, v2| v1.cmp(v2));
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().minmax_by(|v1, v2| v1.cmp(v2))))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &minmax) in lookup.iter() {
+            assert_eq!(minmax, a.iter().copied().filter(|&val| val % modulo == key).minmax_by(|v1, v2| v1.cmp(v2)));
+        }
+    }
+
+    fn correct_grouping_map_by_minmax_by_key_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).minmax_by_key(|_, &val| val);
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().minmax_by_key(|&val| val)))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &minmax) in lookup.iter() {
+            assert_eq!(minmax, a.iter().copied().filter(|&val| val % modulo == key).minmax_by_key(|&val| val));
+        }
+    }
+
+    fn correct_grouping_map_by_sum_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter().map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .sum();
+
+        let group_map_lookup = a.iter().map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().sum()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &sum) in lookup.iter() {
+            assert_eq!(sum, a.iter().map(|&b| b as u64).filter(|&val| val % modulo == key).sum::<u64>());
+        }
+    }
+
+    fn correct_grouping_map_by_product_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = Wrapping(if modulo == 0 { 1 } else { modulo } as u64); // Avoid `% 0`
+        let lookup = a.iter().map(|&b| Wrapping(b as u64)) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .product();
+
+        let group_map_lookup = a.iter().map(|&b| Wrapping(b as u64))
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().product::<Wrapping<u64>>()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &prod) in lookup.iter() {
+            assert_eq!(
+                prod,
+                a.iter()
+                    .map(|&b| Wrapping(b as u64))
+                    .filter(|&val| val % modulo == key)
+                    .product::<Wrapping<u64>>()
+            );
+        }
+    }
+
+    // This should check that if multiple elements are equally minimum or maximum
+    // then `max`, `min` and `minmax` pick the first minimum and the last maximum.
+    // This is to be consistent with `std::iter::max` and `std::iter::min`.
+    fn correct_grouping_map_by_min_max_minmax_order_modulo_key() -> () {
+        use itertools::MinMaxResult;
+
+        let lookup = (0..=10)
+            .into_grouping_map_by(|_| 0)
+            .max_by(|_, _, _| Ordering::Equal);
+
+        assert_eq!(lookup[&0], 10);
+
+        let lookup = (0..=10)
+            .into_grouping_map_by(|_| 0)
+            .min_by(|_, _, _| Ordering::Equal);
+
+        assert_eq!(lookup[&0], 0);
+        
+        let lookup = (0..=10)
+            .into_grouping_map_by(|_| 0)
+            .minmax_by(|_, _, _| Ordering::Equal);
+
+        assert_eq!(lookup[&0], MinMaxResult::MinMax(0, 10));
+    }
+}
+
+quickcheck! {
+    #[test]
+    fn counts(nums: Vec<isize>) -> TestResult {
+        let counts = nums.iter().counts();
+        for (&item, &count) in counts.iter() {
+            if count <= 0 {
+                return TestResult::failed();
+            }
+            if count != nums.iter().filter(|&x| x == item).count() {
+                return TestResult::failed();
+            }
+        }
+        for item in nums.iter() {
+            if !counts.contains_key(item) {
+                return TestResult::failed();
+            }
+        }
+        TestResult::passed()
+    }
+}
+
+quickcheck! {
+    fn test_double_ended_zip_2(a: Vec<u8>, b: Vec<u8>) -> TestResult {
+        let mut x =
+          multizip((a.clone().into_iter(), b.clone().into_iter()))
+            .collect_vec();
+        x.reverse();
+
+        let y =
+          multizip((a.into_iter(), b.into_iter()))
+          .rfold(Vec::new(), |mut vec, e| { vec.push(e); vec });
+
+        TestResult::from_bool(itertools::equal(x, y))
+    }
+
+    fn test_double_ended_zip_3(a: Vec<u8>, b: Vec<u8>, c: Vec<u8>) -> TestResult {
+        let mut x =
+          multizip((a.clone().into_iter(), b.clone().into_iter(), c.clone().into_iter()))
+            .collect_vec();
+        x.reverse();
+
+        let y =
+          multizip((a.into_iter(), b.into_iter(), c.into_iter()))
+          .rfold(Vec::new(), |mut vec, e| { vec.push(e); vec });
+
+        TestResult::from_bool(itertools::equal(x, y))
+    }
+}
diff --git a/tests/specializations.rs b/tests/specializations.rs
index ef51bbe..bc337c2 100644
--- a/tests/specializations.rs
+++ b/tests/specializations.rs
@@ -1,6 +1,5 @@
-use itertools::{EitherOrBoth, Itertools};
+use itertools::Itertools;
 use std::fmt::Debug;
-use std::ops::BitXor;
 use quickcheck::quickcheck;
 
 struct Unspecialized<I>(I);
@@ -11,20 +10,14 @@ where
     type Item = I::Item;
 
     #[inline(always)]
-    fn next(&mut self) -> Option<I::Item> {
+    fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
-
-    #[inline(always)]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.0.size_hint()
-    }
 }
 
 fn check_specialized<'a, V, IterItem, Iter, F>(iterator: &Iter, mapper: F)
 where
     V: Eq + Debug,
-    IterItem: 'a,
     Iter: Iterator<Item = IterItem> + Clone + 'a,
     F: Fn(Box<dyn Iterator<Item = IterItem> + 'a>) -> V,
 {
@@ -34,27 +27,43 @@ where
     )
 }
 
-fn check_specialized_count_last_nth_sizeh<'a, IterItem, Iter>(
+fn test_specializations<IterItem, Iter>(
     it: &Iter,
-    known_expected_size: Option<usize>,
 ) where
-    IterItem: 'a + Eq + Debug,
-    Iter: Iterator<Item = IterItem> + Clone + 'a,
+    IterItem: Eq + Debug + Clone,
+    Iter: Iterator<Item = IterItem> + Clone,
 {
-    let size = it.clone().count();
-    if let Some(expected_size) = known_expected_size {
-        assert_eq!(size, expected_size);
-    }
     check_specialized(it, |i| i.count());
     check_specialized(it, |i| i.last());
+    check_specialized(it, |i| i.collect::<Vec<_>>());
+    check_specialized(it, |i| {
+        let mut parameters_from_fold = vec![];
+        let fold_result = i.fold(vec![], |mut acc, v: IterItem| {
+            parameters_from_fold.push((acc.clone(), v.clone()));
+            acc.push(v);
+            acc
+        });
+        (parameters_from_fold, fold_result)
+    });
+    check_specialized(it, |mut i| {
+        let mut parameters_from_all = vec![];
+        let first = i.next();
+        let all_result = i.all(|x| {
+            parameters_from_all.push(x.clone());
+            Some(x)==first
+        });
+        (parameters_from_all, all_result)
+    });
+    let size = it.clone().count();
     for n in 0..size + 2 {
         check_specialized(it, |mut i| i.nth(n));
     }
+    // size_hint is a bit harder to check
     let mut it_sh = it.clone();
     for n in 0..size + 2 {
         let len = it_sh.clone().count();
         let (min, max) = it_sh.size_hint();
-        assert_eq!((size - n.min(size)), len);
+        assert_eq!(size - n.min(size), len);
         assert!(min <= len);
         if let Some(max) = max {
             assert!(len <= max);
@@ -63,87 +72,29 @@ fn check_specialized_count_last_nth_sizeh<'a, IterItem, Iter>(
     }
 }
 
-fn check_specialized_fold_xor<'a, IterItem, Iter>(it: &Iter)
-where
-    IterItem: 'a
-        + BitXor
-        + Eq
-        + Debug
-        + BitXor<<IterItem as BitXor>::Output, Output = <IterItem as BitXor>::Output>
-        + Clone,
-    <IterItem as BitXor>::Output:
-        BitXor<Output = <IterItem as BitXor>::Output> + Eq + Debug + Clone,
-    Iter: Iterator<Item = IterItem> + Clone + 'a,
-{
-    check_specialized(it, |mut i| {
-        let first = i.next().map(|f| f.clone() ^ (f.clone() ^ f));
-        i.fold(first, |acc, v: IterItem| acc.map(move |a| v ^ a))
-    });
-}
-
-fn put_back_test(test_vec: Vec<i32>, known_expected_size: Option<usize>) {
-    {
-        // Lexical lifetimes support
-        let pb = itertools::put_back(test_vec.iter());
-        check_specialized_count_last_nth_sizeh(&pb, known_expected_size);
-        check_specialized_fold_xor(&pb);
-    }
-
-    let mut pb = itertools::put_back(test_vec.into_iter());
-    pb.put_back(1);
-    check_specialized_count_last_nth_sizeh(&pb, known_expected_size.map(|x| x + 1));
-    check_specialized_fold_xor(&pb)
-}
-
-#[test]
-fn put_back() {
-    put_back_test(vec![7, 4, 1], Some(3));
-}
-
 quickcheck! {
     fn put_back_qc(test_vec: Vec<i32>) -> () {
-        put_back_test(test_vec, None)
+        test_specializations(&itertools::put_back(test_vec.iter()));
+        let mut pb = itertools::put_back(test_vec.into_iter());
+        pb.put_back(1);
+        test_specializations(&pb);
     }
 }
 
-fn merge_join_by_test(i1: Vec<usize>, i2: Vec<usize>, known_expected_size: Option<usize>) {
-    let i1 = i1.into_iter();
-    let i2 = i2.into_iter();
-    let mjb = i1.clone().merge_join_by(i2.clone(), std::cmp::Ord::cmp);
-    check_specialized_count_last_nth_sizeh(&mjb, known_expected_size);
-    // Rust 1.24 compatibility:
-    fn eob_left_z(eob: EitherOrBoth<usize, usize>) -> usize {
-        eob.left().unwrap_or(0)
-    }
-    fn eob_right_z(eob: EitherOrBoth<usize, usize>) -> usize {
-        eob.left().unwrap_or(0)
-    }
-    fn eob_both_z(eob: EitherOrBoth<usize, usize>) -> usize {
-        let (a, b) = eob.both().unwrap_or((0, 0));
-        assert_eq!(a, b);
-        a
+quickcheck! {
+    fn merge_join_by_qc(i1: Vec<usize>, i2: Vec<usize>) -> () {
+        test_specializations(&i1.into_iter().merge_join_by(i2.into_iter(), std::cmp::Ord::cmp));
     }
-    check_specialized_fold_xor(&mjb.clone().map(eob_left_z));
-    check_specialized_fold_xor(&mjb.clone().map(eob_right_z));
-    check_specialized_fold_xor(&mjb.clone().map(eob_both_z));
-
-    // And the other way around
-    let mjb = i2.merge_join_by(i1, std::cmp::Ord::cmp);
-    check_specialized_count_last_nth_sizeh(&mjb, known_expected_size);
-    check_specialized_fold_xor(&mjb.clone().map(eob_left_z));
-    check_specialized_fold_xor(&mjb.clone().map(eob_right_z));
-    check_specialized_fold_xor(&mjb.clone().map(eob_both_z));
 }
 
-#[test]
-fn merge_join_by() {
-    let i1 = vec![1, 3, 5, 7, 8, 9];
-    let i2 = vec![0, 3, 4, 5];
-    merge_join_by_test(i1, i2, Some(8));
+quickcheck! {
+    fn map_into(v: Vec<u8>) -> () {
+        test_specializations(&v.into_iter().map_into::<u32>());
+    }
 }
 
 quickcheck! {
-    fn merge_join_by_qc(i1: Vec<usize>, i2: Vec<usize>) -> () {
-        merge_join_by_test(i1, i2, None)
+    fn map_ok(v: Vec<Result<u8, char>>) -> () {
+        test_specializations(&v.into_iter().map_ok(|u| u.checked_add(1)));
     }
 }
diff --git a/tests/test_std.rs b/tests/test_std.rs
index ba07784..d1ff815 100644
--- a/tests/test_std.rs
+++ b/tests/test_std.rs
@@ -1,8 +1,15 @@
+use paste;
 use permutohedron;
+use quickcheck as qc;
+use rand::{distributions::{Distribution, Standard}, Rng, SeedableRng, rngs::StdRng};
+use rand::{seq::SliceRandom, thread_rng};
+use std::{cmp::min, fmt::Debug, marker::PhantomData};
 use itertools as it;
 use crate::it::Itertools;
+use crate::it::ExactlyOneError;
 use crate::it::multizip;
 use crate::it::multipeek;
+use crate::it::peek_nth;
 use crate::it::free::rciter;
 use crate::it::free::put_back_n;
 use crate::it::FoldWhile;
@@ -58,6 +65,9 @@ fn unique_by() {
     let xs = ["aaa", "bbbbb", "aa", "ccc", "bbbb", "aaaaa", "cccc"];
     let ys = ["aaa", "bbbbb", "ccc"];
     it::assert_equal(ys.iter(), xs.iter().unique_by(|x| x[..2].to_string()));
+    it::assert_equal(ys.iter(), xs.iter().rev().unique_by(|x| x[..2].to_string()).rev());
+    let ys_rev = ["cccc", "aaaaa", "bbbb"];
+    it::assert_equal(ys_rev.iter(), xs.iter().unique_by(|x| x[..2].to_string()).rev());
 }
 
 #[test]
@@ -65,9 +75,16 @@ fn unique() {
     let xs = [0, 1, 2, 3, 2, 1, 3];
     let ys = [0, 1, 2, 3];
     it::assert_equal(ys.iter(), xs.iter().unique());
+    it::assert_equal(ys.iter(), xs.iter().rev().unique().rev());
+    let ys_rev = [3, 1, 2, 0];
+    it::assert_equal(ys_rev.iter(), xs.iter().unique().rev());
+
     let xs = [0, 1];
     let ys = [0, 1];
     it::assert_equal(ys.iter(), xs.iter().unique());
+    it::assert_equal(ys.iter(), xs.iter().rev().unique().rev());
+    let ys_rev = [1, 0];
+    it::assert_equal(ys_rev.iter(), xs.iter().unique().rev());
 }
 
 #[test]
@@ -99,6 +116,26 @@ fn dedup() {
 }
 
 #[test]
+fn coalesce() {
+    let data = vec![-1., -2., -3., 3., 1., 0., -1.];
+    let it = data.iter().cloned().coalesce(|x, y|
+        if (x >= 0.) == (y >= 0.) {
+            Ok(x + y)
+        } else {
+            Err((x, y))
+        }
+    );
+    itertools::assert_equal(it.clone(), vec![-6., 4., -1.]);
+    assert_eq!(
+        it.fold(vec![], |mut v, n| {
+            v.push(n);
+            v
+        }),
+        vec![-6., 4., -1.]
+    );
+}
+
+#[test]
 fn dedup_by() {
     let xs = [(0, 0), (0, 1), (1, 1), (2, 1), (0, 2), (3, 1), (0, 3), (1, 3)];
     let ys = [(0, 0), (0, 1), (0, 2), (3, 1), (0, 3)];
@@ -115,6 +152,33 @@ fn dedup_by() {
 }
 
 #[test]
+fn dedup_with_count() {
+    let xs: [i32; 8] = [0, 1, 1, 1, 2, 1, 3, 3];
+    let ys: [(usize, &i32); 5] = [(1, &0), (3, &1), (1, &2), (1, &1), (2, &3)];
+
+    it::assert_equal(ys.iter().cloned(), xs.iter().dedup_with_count());
+
+    let xs: [i32; 5] = [0, 0, 0, 0, 0];
+    let ys: [(usize, &i32); 1] = [(5, &0)];
+
+    it::assert_equal(ys.iter().cloned(), xs.iter().dedup_with_count());
+}
+
+
+#[test]
+fn dedup_by_with_count() {
+    let xs = [(0, 0), (0, 1), (1, 1), (2, 1), (0, 2), (3, 1), (0, 3), (1, 3)];
+    let ys = [(1, &(0, 0)), (3, &(0, 1)), (1, &(0, 2)), (1, &(3, 1)), (2, &(0, 3))];
+
+    it::assert_equal(ys.iter().cloned(), xs.iter().dedup_by_with_count(|x, y| x.1==y.1));
+
+    let xs = [(0, 1), (0, 2), (0, 3), (0, 4), (0, 5)];
+    let ys = [( 5, &(0, 1))];
+
+    it::assert_equal(ys.iter().cloned(), xs.iter().dedup_by_with_count(|x, y| x.0==y.0));
+}
+
+#[test]
 fn all_equal() {
     assert!("".chars().all_equal());
     assert!("A".chars().all_equal());
@@ -192,7 +256,7 @@ fn trait_pointers() {
         I: 'r + Iterator<Item=X>
     {
         type Item = X;
-        fn next(&mut self) -> Option<X>
+        fn next(&mut self) -> Option<Self::Item>
         {
             self.0.next()
         }
@@ -285,6 +349,26 @@ fn join() {
 }
 
 #[test]
+fn sorted_unstable_by() {
+    let sc = [3, 4, 1, 2].iter().cloned().sorted_by(|&a, &b| {
+        a.cmp(&b)
+    });
+    it::assert_equal(sc, vec![1, 2, 3, 4]);
+
+    let v = (0..5).sorted_unstable_by(|&a, &b| a.cmp(&b).reverse());
+    it::assert_equal(v, vec![4, 3, 2, 1, 0]);
+}
+
+#[test]
+fn sorted_unstable_by_key() {
+    let sc = [3, 4, 1, 2].iter().cloned().sorted_unstable_by_key(|&x| x);
+    it::assert_equal(sc, vec![1, 2, 3, 4]);
+
+    let v = (0..5).sorted_unstable_by_key(|&x| -x);
+    it::assert_equal(v, vec![4, 3, 2, 1, 0]);
+}
+
+#[test]
 fn sorted_by() {
     let sc = [3, 4, 1, 2].iter().cloned().sorted_by(|&a, &b| {
         a.cmp(&b)
@@ -295,6 +379,88 @@ fn sorted_by() {
     it::assert_equal(v, vec![4, 3, 2, 1, 0]);
 }
 
+qc::quickcheck! {
+    fn k_smallest_range(n: u64, m: u16, k: u16) -> () {
+        // u16 is used to constrain k and m to 0..2¹⁶,
+        //  otherwise the test could use too much memory.
+        let (k, m) = (k as u64, m as u64);
+
+        // Generate a random permutation of n..n+m
+        let i = {
+            let mut v: Vec<u64> = (n..n.saturating_add(m)).collect();
+            v.shuffle(&mut thread_rng());
+            v.into_iter()
+        };
+
+        // Check that taking the k smallest elements yields n..n+min(k, m)
+        it::assert_equal(
+            i.k_smallest(k as usize),
+            n..n.saturating_add(min(k, m))
+        );
+    }
+}
+
+#[derive(Clone, Debug)]
+struct RandIter<T: 'static + Clone + Send, R: 'static + Clone + Rng + SeedableRng + Send = StdRng> {
+    idx: usize,
+    len: usize,
+    rng: R,
+    _t: PhantomData<T>
+}
+
+impl<T: Clone + Send, R: Clone + Rng + SeedableRng + Send> Iterator for RandIter<T, R>
+where Standard: Distribution<T> {
+    type Item = T;
+    fn next(&mut self) -> Option<T> {
+        if self.idx == self.len {
+            None
+        } else {
+            self.idx += 1;
+            Some(self.rng.gen())
+        }
+    }
+}
+
+impl<T: Clone + Send, R: Clone + Rng + SeedableRng + Send> qc::Arbitrary for RandIter<T, R> {
+    fn arbitrary<G: qc::Gen>(g: &mut G) -> Self {
+        RandIter {
+            idx: 0,
+            len: g.size(),
+            rng: R::seed_from_u64(g.next_u64()),
+            _t : PhantomData{},
+        }
+    }
+}
+
+// Check that taking the k smallest is the same as
+//  sorting then taking the k first elements
+fn k_smallest_sort<I>(i: I, k: u16) -> ()
+where
+    I: Iterator + Clone,
+    I::Item: Ord + Debug,
+{
+    let j = i.clone();
+    let k = k as usize;
+    it::assert_equal(
+        i.k_smallest(k),
+        j.sorted().take(k)
+    )
+}
+
+macro_rules! generic_test {
+    ($f:ident, $($t:ty),+) => {
+        $(paste::item! {
+            qc::quickcheck! {
+                fn [< $f _ $t >](i: RandIter<$t>, k: u16) -> () {
+                    $f(i, k)
+                }
+            }
+        })+
+    };
+}
+
+generic_test!(k_smallest_sort, u8, u16, u32, u64, i8, i16, i32, i64);
+
 #[test]
 fn sorted_by_key() {
     let sc = [3, 4, 1, 2].iter().cloned().sorted_by_key(|&x| x);
@@ -328,7 +494,6 @@ fn test_multipeek() {
     assert_eq!(mp.next(), Some(5));
     assert_eq!(mp.next(), None);
     assert_eq!(mp.peek(), None);
-
 }
 
 #[test]
@@ -372,6 +537,70 @@ fn test_multipeek_peeking_next() {
 }
 
 #[test]
+fn test_peek_nth() {
+    let nums = vec![1u8,2,3,4,5];
+
+    let iter = peek_nth(nums.iter().map(|&x| x));
+    assert_eq!(nums, iter.collect::<Vec<_>>());
+
+    let mut iter = peek_nth(nums.iter().map(|&x| x));
+
+    assert_eq!(iter.peek_nth(0), Some(&1));
+    assert_eq!(iter.peek_nth(0), Some(&1));
+    assert_eq!(iter.next(), Some(1));
+
+    assert_eq!(iter.peek_nth(0), Some(&2));
+    assert_eq!(iter.peek_nth(1), Some(&3));
+    assert_eq!(iter.next(), Some(2));
+
+    assert_eq!(iter.peek_nth(0), Some(&3));
+    assert_eq!(iter.peek_nth(1), Some(&4));
+    assert_eq!(iter.peek_nth(2), Some(&5));
+    assert_eq!(iter.peek_nth(3), None);
+
+    assert_eq!(iter.next(), Some(3));
+    assert_eq!(iter.next(), Some(4));
+
+    assert_eq!(iter.peek_nth(0), Some(&5));
+    assert_eq!(iter.peek_nth(1), None);
+    assert_eq!(iter.next(), Some(5));
+    assert_eq!(iter.next(), None);
+
+    assert_eq!(iter.peek_nth(0), None);
+    assert_eq!(iter.peek_nth(1), None);
+}
+
+#[test]
+fn test_peek_nth_peeking_next() {
+    use it::PeekingNext;
+    let nums = vec![1u8,2,3,4,5,6,7];
+    let mut iter = peek_nth(nums.iter().map(|&x| x));
+
+    assert_eq!(iter.peeking_next(|&x| x != 0), Some(1));
+    assert_eq!(iter.next(), Some(2));
+
+    assert_eq!(iter.peek_nth(0), Some(&3));
+    assert_eq!(iter.peek_nth(1), Some(&4));
+    assert_eq!(iter.peeking_next(|&x| x == 3), Some(3));
+    assert_eq!(iter.peek(), Some(&4));
+
+    assert_eq!(iter.peeking_next(|&x| x != 4), None);
+    assert_eq!(iter.peeking_next(|&x| x == 4), Some(4));
+    assert_eq!(iter.peek_nth(0), Some(&5));
+    assert_eq!(iter.peek_nth(1), Some(&6));
+
+    assert_eq!(iter.peeking_next(|&x| x != 5), None);
+    assert_eq!(iter.peek(), Some(&5));
+
+    assert_eq!(iter.peeking_next(|&x| x == 5), Some(5));
+    assert_eq!(iter.peeking_next(|&x| x == 6), Some(6));
+    assert_eq!(iter.peek_nth(0), Some(&7));
+    assert_eq!(iter.peek_nth(1), None);
+    assert_eq!(iter.next(), Some(7));
+    assert_eq!(iter.peek(), None);
+}
+
+#[test]
 fn pad_using() {
     it::assert_equal((0..0).pad_using(1, |_| 1), 1..2);
 
@@ -642,6 +871,23 @@ fn combinations_with_replacement() {
 }
 
 #[test]
+fn powerset() {
+    it::assert_equal((0..0).powerset(), vec![vec![]]);
+    it::assert_equal((0..1).powerset(), vec![vec![], vec![0]]);
+    it::assert_equal((0..2).powerset(), vec![vec![], vec![0], vec![1], vec![0, 1]]);
+    it::assert_equal((0..3).powerset(), vec![
+        vec![],
+        vec![0], vec![1], vec![2],
+        vec![0, 1], vec![0, 2], vec![1, 2],
+        vec![0, 1, 2]
+    ]);
+
+    assert_eq!((0..4).powerset().count(), 1 << 4);
+    assert_eq!((0..8).powerset().count(), 1 << 8);
+    assert_eq!((0..16).powerset().count(), 1 << 16);
+}
+
+#[test]
 fn diff_mismatch() {
     let a = vec![1, 2, 3, 4];
     let b = vec![1.0, 5.0, 3.0, 4.0];
@@ -791,3 +1037,13 @@ fn tree_fold1() {
         assert_eq!(actual, expected);
     }
 }
+
+#[test]
+fn exactly_one_question_mark_syntax_works() {
+    exactly_one_question_mark_return().unwrap_err();
+}
+
+fn exactly_one_question_mark_return() -> Result<(), ExactlyOneError<std::slice::Iter<'static, ()>>> {
+    [].iter().exactly_one()?;
+    Ok(())
+}
diff --git a/tests/zip.rs b/tests/zip.rs
index b1af52c..5801b42 100644
--- a/tests/zip.rs
+++ b/tests/zip.rs
@@ -1,6 +1,7 @@
 use itertools::Itertools;
 use itertools::EitherOrBoth::{Both, Left, Right};
 use itertools::free::zip_eq;
+use itertools::multizip;
 
 #[test]
 fn zip_longest_fused() {
@@ -40,6 +41,20 @@ fn test_double_ended_zip_longest() {
     assert_eq!(it.next(), None);
 }
 
+#[test]
+fn test_double_ended_zip() {
+    let xs = [1, 2, 3, 4, 5, 6];
+    let ys = [1, 2, 3, 7];
+    let a = xs.iter().map(|&x| x);
+    let b = ys.iter().map(|&x| x);
+    let mut it = multizip((a, b));
+    assert_eq!(it.next_back(), Some((4, 7)));
+    assert_eq!(it.next_back(), Some((3, 3)));
+    assert_eq!(it.next_back(), Some((2, 2)));
+    assert_eq!(it.next_back(), Some((1, 1)));
+    assert_eq!(it.next_back(), None);
+}
+
 
 #[should_panic]
 #[test]
@@ -60,4 +75,3 @@ fn zip_eq_panic2()
 
     zip_eq(&a, &b).count();
 }
-