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Rollup merge of rust-lang#75026 - JulianKnodt:array_windows, r=Amanieu
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Add array_windows fn

This mimicks the functionality added by array_chunks, and implements a const-generic form of
`windows`. It makes egregious use of `unsafe`, but by necessity because the array must be
re-interpreted as a slice of arrays, and unlike array_chunks this cannot be done by casting the
original array once, since each time the index is advanced it needs to move one element, not
`N`.

I'm planning on adding more tests, but this should be good enough as a premise for the functionality.
Notably: should there be more functions overwritten for the iterator implementation/in general?

~~I've marked the issue as rust-lang#74985 as there is no corresponding exact issue for `array_windows`, but it's based of off `array_chunks`.~~

Edit: See Issue rust-lang#75027 created by @lcnr for tracking issue

~~Do not merge until I add more tests, please.~~

r? @lcnr
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tmandry authored Sep 16, 2020
2 parents 7bb106f + f240abc commit 23a6777
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Showing 7 changed files with 196 additions and 6 deletions.
1 change: 1 addition & 0 deletions library/alloc/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,7 @@
#![cfg_attr(test, feature(test))]
#![feature(allocator_api)]
#![feature(array_chunks)]
#![feature(array_windows)]
#![feature(allow_internal_unstable)]
#![feature(arbitrary_self_types)]
#![feature(box_patterns)]
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2 changes: 2 additions & 0 deletions library/alloc/src/slice.rs
Original file line number Diff line number Diff line change
Expand Up @@ -97,6 +97,8 @@ pub use core::slice::check_range;
pub use core::slice::ArrayChunks;
#[unstable(feature = "array_chunks", issue = "74985")]
pub use core::slice::ArrayChunksMut;
#[unstable(feature = "array_windows", issue = "75027")]
pub use core::slice::ArrayWindows;
#[stable(feature = "slice_get_slice", since = "1.28.0")]
pub use core::slice::SliceIndex;
#[stable(feature = "from_ref", since = "1.28.0")]
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100 changes: 100 additions & 0 deletions library/core/src/slice/iter.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1687,6 +1687,106 @@ unsafe impl<'a, T> TrustedRandomAccess for ChunksExactMut<'a, T> {
}
}

/// A windowed iterator over a slice in overlapping chunks (`N` elements at a
/// time), starting at the beginning of the slice
///
/// This struct is created by the [`array_windows`] method on [slices].
///
/// [`array_windows`]: ../../std/primitive.slice.html#method.array_windows
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug, Clone, Copy)]
#[unstable(feature = "array_windows", issue = "75027")]
pub struct ArrayWindows<'a, T: 'a, const N: usize> {
pub(crate) slice_head: *const T,
pub(crate) num: usize,
pub(crate) marker: marker::PhantomData<&'a [T; N]>,
}

#[unstable(feature = "array_windows", issue = "75027")]
impl<'a, T, const N: usize> Iterator for ArrayWindows<'a, T, N> {
type Item = &'a [T; N];

#[inline]
fn next(&mut self) -> Option<Self::Item> {
if self.num == 0 {
return None;
}
// SAFETY:
// This is safe because it's indexing into a slice guaranteed to be length > N.
let ret = unsafe { &*self.slice_head.cast::<[T; N]>() };
// SAFETY: Guaranteed that there are at least 1 item remaining otherwise
// earlier branch would've been hit
self.slice_head = unsafe { self.slice_head.add(1) };

self.num -= 1;
Some(ret)
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
(self.num, Some(self.num))
}

#[inline]
fn count(self) -> usize {
self.num
}

#[inline]
fn nth(&mut self, n: usize) -> Option<Self::Item> {
if self.num <= n {
self.num = 0;
return None;
}
// SAFETY:
// This is safe because it's indexing into a slice guaranteed to be length > N.
let ret = unsafe { &*self.slice_head.add(n).cast::<[T; N]>() };
// SAFETY: Guaranteed that there are at least n items remaining
self.slice_head = unsafe { self.slice_head.add(n + 1) };

self.num -= n + 1;
Some(ret)
}

#[inline]
fn last(mut self) -> Option<Self::Item> {
self.nth(self.num.checked_sub(1)?)
}
}

#[unstable(feature = "array_windows", issue = "75027")]
impl<'a, T, const N: usize> DoubleEndedIterator for ArrayWindows<'a, T, N> {
#[inline]
fn next_back(&mut self) -> Option<&'a [T; N]> {
if self.num == 0 {
return None;
}
// SAFETY: Guaranteed that there are n items remaining, n-1 for 0-indexing.
let ret = unsafe { &*self.slice_head.add(self.num - 1).cast::<[T; N]>() };
self.num -= 1;
Some(ret)
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<&'a [T; N]> {
if self.num <= n {
self.num = 0;
return None;
}
// SAFETY: Guaranteed that there are n items remaining, n-1 for 0-indexing.
let ret = unsafe { &*self.slice_head.add(self.num - (n + 1)).cast::<[T; N]>() };
self.num -= n + 1;
Some(ret)
}
}

#[unstable(feature = "array_windows", issue = "75027")]
impl<T, const N: usize> ExactSizeIterator for ArrayWindows<'_, T, N> {
fn is_empty(&self) -> bool {
self.num == 0
}
}

/// An iterator over a slice in (non-overlapping) chunks (`N` elements at a
/// time), starting at the beginning of the slice.
///
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37 changes: 37 additions & 0 deletions library/core/src/slice/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,9 @@ pub use iter::{RChunks, RChunksExact, RChunksExactMut, RChunksMut};
#[unstable(feature = "array_chunks", issue = "74985")]
pub use iter::{ArrayChunks, ArrayChunksMut};

#[unstable(feature = "array_windows", issue = "75027")]
pub use iter::ArrayWindows;

#[unstable(feature = "split_inclusive", issue = "72360")]
pub use iter::{SplitInclusive, SplitInclusiveMut};

Expand Down Expand Up @@ -1026,6 +1029,40 @@ impl<T> [T] {
}
}

/// Returns an iterator over overlapping windows of `N` elements of a slice,
/// starting at the beginning of the slice.
///
/// This is the const generic equivalent of [`windows`].
///
/// If `N` is smaller than the size of the array, it will return no windows.
///
/// # Panics
///
/// Panics if `N` is 0. This check will most probably get changed to a compile time
/// error before this method gets stabilized.
///
/// # Examples
///
/// ```
/// #![feature(array_windows)]
/// let slice = [0, 1, 2, 3];
/// let mut iter = slice.array_windows();
/// assert_eq!(iter.next().unwrap(), &[0, 1]);
/// assert_eq!(iter.next().unwrap(), &[1, 2]);
/// assert_eq!(iter.next().unwrap(), &[2, 3]);
/// assert!(iter.next().is_none());
/// ```
///
/// [`windows`]: #method.windows
#[unstable(feature = "array_windows", issue = "75027")]
#[inline]
pub fn array_windows<const N: usize>(&self) -> ArrayWindows<'_, T, N> {
assert_ne!(N, 0);

let num_windows = self.len().saturating_sub(N - 1);
ArrayWindows { slice_head: self.as_ptr(), num: num_windows, marker: marker::PhantomData }
}

/// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the end
/// of the slice.
///
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1 change: 1 addition & 0 deletions library/core/tests/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
#![feature(array_chunks)]
#![feature(array_methods)]
#![feature(array_map)]
#![feature(array_windows)]
#![feature(bool_to_option)]
#![feature(bound_cloned)]
#![feature(box_syntax)]
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49 changes: 49 additions & 0 deletions library/core/tests/slice.rs
Original file line number Diff line number Diff line change
Expand Up @@ -657,6 +657,55 @@ fn test_array_chunks_mut_zip() {
assert_eq!(v1, [13, 14, 19, 20, 4]);
}

#[test]
fn test_array_windows_infer() {
let v: &[i32] = &[0, 1, 0, 1];
assert_eq!(v.array_windows::<2>().count(), 3);
let c = v.array_windows();
for &[a, b] in c {
assert_eq!(a + b, 1);
}

let v2: &[i32] = &[0, 1, 2, 3, 4, 5, 6];
let total = v2.array_windows().map(|&[a, b, c]| a + b + c).sum::<i32>();
assert_eq!(total, 3 + 6 + 9 + 12 + 15);
}

#[test]
fn test_array_windows_count() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
let c = v.array_windows::<3>();
assert_eq!(c.count(), 4);

let v2: &[i32] = &[0, 1, 2, 3, 4];
let c2 = v2.array_windows::<6>();
assert_eq!(c2.count(), 0);

let v3: &[i32] = &[];
let c3 = v3.array_windows::<2>();
assert_eq!(c3.count(), 0);
}

#[test]
fn test_array_windows_nth() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
let snd = v.array_windows::<4>().nth(1);
assert_eq!(snd, Some(&[1, 2, 3, 4]));
let mut arr_windows = v.array_windows::<2>();
assert_ne!(arr_windows.nth(0), arr_windows.nth(0));
let last = v.array_windows::<3>().last();
assert_eq!(last, Some(&[3, 4, 5]));
}

#[test]
fn test_array_windows_nth_back() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
let snd = v.array_windows::<4>().nth_back(1);
assert_eq!(snd, Some(&[1, 2, 3, 4]));
let mut arr_windows = v.array_windows::<2>();
assert_ne!(arr_windows.nth_back(0), arr_windows.nth_back(0));
}

#[test]
fn test_rchunks_count() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
Expand Down
12 changes: 6 additions & 6 deletions src/test/ui/const-generics/type-dependent/issue-61936.rs
Original file line number Diff line number Diff line change
Expand Up @@ -5,21 +5,21 @@
#![cfg_attr(min, feature(min_const_generics))]

trait SliceExt<T: Clone> {
fn array_windows<'a, const N: usize>(&'a self) -> ArrayWindows<'a, T, N>;
fn array_windows_example<'a, const N: usize>(&'a self) -> ArrayWindowsExample<'a, T, N>;
}

impl <T: Clone> SliceExt<T> for [T] {
fn array_windows<'a, const N: usize>(&'a self) -> ArrayWindows<'a, T, N> {
ArrayWindows{ idx: 0, slice: &self }
fn array_windows_example<'a, const N: usize>(&'a self) -> ArrayWindowsExample<'a, T, N> {
ArrayWindowsExample{ idx: 0, slice: &self }
}
}

struct ArrayWindows<'a, T, const N: usize> {
struct ArrayWindowsExample<'a, T, const N: usize> {
slice: &'a [T],
idx: usize,
}

impl <'a, T: Clone, const N: usize> Iterator for ArrayWindows<'a, T, N> {
impl <'a, T: Clone, const N: usize> Iterator for ArrayWindowsExample<'a, T, N> {
type Item = [T; N];
fn next(&mut self) -> Option<Self::Item> {
// Note: this is unsound for some `T` and not meant as an example
Expand All @@ -45,7 +45,7 @@ const FOUR: usize = 4;
fn main() {
let v: Vec<usize> = vec![0; 100];

for array in v.as_slice().array_windows::<FOUR>() {
for array in v.as_slice().array_windows_example::<FOUR>() {
assert_eq!(array, [0, 0, 0, 0])
}
}

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