1 files changed, 107 insertions, 0 deletions
diff --git a/src/combinations_with_replacement.rs b/src/combinations_with_replacement.rs
new file mode 100644
index 0000000..d511545
--- /dev/null
+++ b/src/combinations_with_replacement.rs
@@ -0,0 +1,107 @@
+use std::fmt;
+
+use super::lazy_buffer::LazyBuffer;
+
+/// An iterator to iterate through all the `n`-length combinations in an iterator, with replacement.
+///
+/// See [`.combinations_with_replacement()`](../trait.Itertools.html#method.combinations_with_replacement) for more information.
+#[derive(Clone)]
+pub struct CombinationsWithReplacement<I>
+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,
+}
+
+impl<I> fmt::Debug for CombinationsWithReplacement<I>
+where
+    I: Iterator + fmt::Debug,
+    I::Item: fmt::Debug + Clone,
+{
+    debug_fmt_fields!(Combinations, k, indices, max_index, pool, first);
+}
+
+impl<I> CombinationsWithReplacement<I>
+where
+    I: Iterator,
+    I::Item: Clone,
+{
+    /// Map the current mask over the pool to get an output combination
+    fn current(&self) -> Vec<I::Item> {
+        self.indices.iter().map(|i| self.pool[*i].clone()).collect()
+    }
+}
+
+/// Create a new `CombinationsWithReplacement` from a clonable iterator.
+pub fn combinations_with_replacement<I>(iter: I, k: usize) -> CombinationsWithReplacement<I>
+where
+    I: Iterator,
+    I::Item: Clone,
+{
+    let indices: Vec<usize> = vec![0; k];
+    let pool: LazyBuffer<I> = LazyBuffer::new(iter);
+
+    CombinationsWithReplacement {
+        k,
+        indices,
+        max_index: 0,
+        pool,
+        first: true,
+    }
+}
+
+impl<I> Iterator for CombinationsWithReplacement<I>
+where
+    I: Iterator,
+    I::Item: Clone,
+{
+    type Item = Vec<I::Item>;
+    fn next(&mut self) -> Option<Self::Item> {
+        // 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() {
+                None
+            // Otherwise, yield the initial state
+            } else {
+                self.first = false;
+                Some(self.current())
+            };
+        }
+
+        // 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;
+        }
+
+        // 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 {
+                increment = Some((i, indices_int + 1));
+                break;
+            }
+        }
+
+        match increment {
+            // If we can update the indices further
+            Some((increment_from, increment_value)) => {
+                // We need to update the rightmost non-max value
+                // and all those to the right
+                for indices_index in increment_from..self.indices.len() {
+                    self.indices[indices_index] = increment_value
+                }
+                Some(self.current())
+            }
+            // Otherwise, we're done
+            None => None,
+        }
+    }
+}