Add Iterator::array_windows()

library/core/src/iter/adapters/array_windows.rs

@@ -0,0 +1,102 @@
+use crate::iter::{ExactSizeIterator, Fuse, FusedIterator, Iterator, TrustedLen};
+
+use crate::array;
+
+/// An iterator over all contiguous windows of length `N`. The windows overlap.
+/// If the iterator is shorter than `N`, the iterator returns no values.
+///
+/// This `struct` is created by [`Iterator::array_windows`]. See its
+/// documentation for more.
+#[derive(Debug, Clone)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[unstable(feature = "iter_array_windows", reason = "recently added", issue = "none")]
+pub struct ArrayWindows<I, const N: usize>
+where
+    I: Iterator,
+    I::Item: Clone,
+{
+    iter: Fuse<I>,
+    last: Option<[I::Item; N]>,
+}
+
+impl<I, const N: usize> ArrayWindows<I, N>
+where
+    I: Iterator,
+    I::Item: Clone,
+{
+    #[inline]
+    pub(in crate::iter) fn new(iter: I) -> Self {
+        assert!(N != 0);
+        Self { iter: iter.fuse(), last: None }
+    }
+}
+
+#[unstable(feature = "iter_array_windows", reason = "recently added", issue = "none")]
+impl<I: Iterator, const N: usize> Iterator for ArrayWindows<I, N>
+where
+    I: Iterator,
+    I::Item: Clone,
+{
+    type Item = [I::Item; N];
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        let Self { iter, last } = self;
+
+        match last {
+            Some(last) => {
+                let item = iter.next()?;
+                last.rotate_left(1);
+                if let Some(end) = last.last_mut() {
+                    *end = item;
+                }
+                Some(last.clone())
+            }
+            None => {
+                let tmp = array::try_from_fn(|_| iter.next())?;
+                *last = Some(tmp.clone());
+                Some(tmp)
+            }
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (lower, upper) = self.iter.size_hint();
+        // Keep infinite iterator size hint lower bound as `usize::MAX`
+        if lower == usize::MAX {
+            (lower, upper)
+        } else {
+            (lower.saturating_sub(N - 1), upper.map(|n| n.saturating_sub(N - 1)))
+        }
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.iter.count().saturating_sub(N - 1)
+    }
+}
+
+#[unstable(feature = "iter_array_windows", reason = "recently added", issue = "none")]
+impl<I, const N: usize> FusedIterator for ArrayWindows<I, N>
+where
+    I: FusedIterator,
+    I::Item: Clone,
+{
+}
+
+#[unstable(feature = "iter_array_windows", reason = "recently added", issue = "none")]
+impl<I, const N: usize> ExactSizeIterator for ArrayWindows<I, N>
+where
+    I: ExactSizeIterator,
+    I::Item: Clone,
+{
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<I, const N: usize> TrustedLen for ArrayWindows<I, N>
+where
+    I: TrustedLen,
+    I::Item: Clone,
+{
+}

library/core/src/iter/adapters/mod.rs

@@ -1,6 +1,7 @@
 use crate::iter::{InPlaceIterable, Iterator};
 use crate::ops::{ControlFlow, Try};
 
+mod array_windows;
 mod chain;
 mod cloned;
 mod copied;
@@ -30,6 +31,9 @@ pub use self::{
     scan::Scan, skip::Skip, skip_while::SkipWhile, take::Take, take_while::TakeWhile, zip::Zip,
 };
 
+#[unstable(feature = "iter_array_windows", reason = "recently added", issue = "none")]
+pub use self::array_windows::ArrayWindows;
+
 #[stable(feature = "iter_cloned", since = "1.1.0")]
 pub use self::cloned::Cloned;
 

library/core/src/iter/mod.rs

@@ -393,6 +393,8 @@ pub use self::traits::{
 
 #[stable(feature = "iter_zip", since = "1.59.0")]
 pub use self::adapters::zip;
+#[unstable(feature = "iter_array_windows", reason = "recently added", issue = "none")]
+pub use self::adapters::ArrayWindows;
 #[stable(feature = "iter_cloned", since = "1.1.0")]
 pub use self::adapters::Cloned;
 #[stable(feature = "iter_copied", since = "1.36.0")]

library/core/src/iter/traits/iterator.rs

@@ -2,12 +2,13 @@ use crate::cmp::{self, Ordering};
 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 
 use super::super::TrustedRandomAccessNoCoerce;
+use super::super::{
+    ArrayWindows, Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take,
+    TakeWhile,
+};
 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
 use super::super::{FlatMap, Flatten};
 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
-use super::super::{
-    Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
-};
 
 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
 
@@ -3057,6 +3058,49 @@ pub trait Iterator {
         Cycle::new(self)
     }
 
+    /// Returns an iterator over all contiguous windows of length `N`. The
+    /// windows overlap. If the iterator is shorter than `N`, the iterator
+    /// returns no values.
+    ///
+    /// `array_windows` clones the iterator elements so that they can be part of
+    /// successive windows, this makes this it most suited for iterators of
+    /// references and other values that are cheap to clone.
+    ///
+    /// # Panics
+    ///
+    /// If called with `N = 0`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(iter_array_windows)]
+    ///
+    /// let mut iter = "rust".chars().array_windows();
+    /// assert_eq!(iter.next(), Some(['r', 'u']));
+    /// assert_eq!(iter.next(), Some(['u', 's']));
+    /// assert_eq!(iter.next(), Some(['s', 't']));
+    /// assert_eq!(iter.next(), None);
+    /// ```
+    ///
+    /// ```
+    /// #![feature(iter_array_windows)]
+    ///
+    /// let seq: &[i32] = &[0, 1, 1, 2, 3, 5, 8, 13];
+    /// for [x, y, z] in seq.iter().copied().array_windows() {
+    ///     assert_eq!(x + y, z);
+    /// }
+    /// ```
+    #[unstable(feature = "iter_array_windows", reason = "recently added", issue = "none")]
+    fn array_windows<const N: usize>(self) -> ArrayWindows<Self, N>
+    where
+        Self: Sized,
+        Self::Item: Clone,
+    {
+        ArrayWindows::new(self)
+    }
+
     /// Sums the elements of an iterator.
     ///
     /// Takes each element, adds them together, and returns the result.

library/core/tests/iter/adapters/array_windows.rs

@@ -0,0 +1,65 @@
+use core::iter;
+
+#[test]
+fn test_array_windows_infer() {
+    let s = [0, 1, 0, 1, 0, 1];
+    for [a, b] in s.iter().copied().array_windows() {
+        assert_eq!(a + b, 1);
+    }
+    for [a, b, c, d] in s.iter().copied().array_windows() {
+        assert_eq!(a + b + c + d, 2);
+    }
+}
+
+#[test]
+fn test_array_windows_size_hint() {
+    let iter = (0..6).array_windows::<1>();
+    assert_eq!(iter.size_hint(), (6, Some(6)));
+
+    let iter = (0..6).array_windows::<3>();
+    assert_eq!(iter.size_hint(), (4, Some(4)));
+
+    let iter = (0..6).array_windows::<5>();
+    assert_eq!(iter.size_hint(), (2, Some(2)));
+
+    let iter = (0..6).array_windows::<7>();
+    assert_eq!(iter.size_hint(), (0, Some(0)));
+
+    let iter = (1..).array_windows::<2>();
+    assert_eq!(iter.size_hint(), (usize::MAX, None));
+
+    let iter = (1..).filter(|x| x % 2 != 0).array_windows::<2>();
+    assert_eq!(iter.size_hint(), (0, None));
+}
+
+#[test]
+fn test_array_windows_count() {
+    let iter = (0..6).array_windows::<1>();
+    assert_eq!(iter.count(), 6);
+
+    let iter = (0..6).array_windows::<3>();
+    assert_eq!(iter.count(), 4);
+
+    let iter = (0..6).array_windows::<5>();
+    assert_eq!(iter.count(), 2);
+
+    let iter = (0..6).array_windows::<7>();
+    assert_eq!(iter.count(), 0);
+
+    let iter = (0..6).filter(|x| x % 2 == 0).array_windows::<2>();
+    assert_eq!(iter.count(), 2);
+
+    let iter = iter::empty::<i32>().array_windows::<2>();
+    assert_eq!(iter.count(), 0);
+
+    let iter = [(); usize::MAX].iter().array_windows::<2>();
+    assert_eq!(iter.count(), usize::MAX - 1);
+}
+
+#[test]
+fn test_array_windows_nth() {
+    let mut iter = (0..6).array_windows::<4>();
+    assert_eq!(iter.nth(1), Some([1, 2, 3, 4]));
+    assert_eq!(iter.nth(0), Some([2, 3, 4, 5]));
+    assert_eq!(iter.nth(1), None);
+}

library/core/tests/iter/adapters/mod.rs

@@ -1,3 +1,4 @@
+mod array_windows;
 mod chain;
 mod cloned;
 mod copied;

library/core/tests/lib.rs

@@ -60,6 +60,7 @@
 #![feature(slice_partition_dedup)]
 #![feature(int_log)]
 #![feature(iter_advance_by)]
+#![feature(iter_array_windows)]
 #![feature(iter_partition_in_place)]
 #![feature(iter_intersperse)]
 #![feature(iter_is_partitioned)]