1 files changed, 102 insertions, 0 deletions
diff --git a/src/peek_nth.rs b/src/peek_nth.rs
new file mode 100644
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--- /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()
+    }
+}