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Rollup merge of rust-lang#100026 - WaffleLapkin:array-chunks, r=scottmcm
Add `Iterator::array_chunks` (take N+1) A revival of rust-lang#92393. r? `@Mark-Simulacrum` cc `@rossmacarthur` `@scottmcm` `@the8472` I've tried to address most of the review comments on the previous attempt. The only thing I didn't address is `try_fold` implementation, I've left the "custom" one for now, not sure what exactly should it use.
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use crate::array; | ||
use crate::iter::{ByRefSized, FusedIterator, Iterator}; | ||
use crate::ops::{ControlFlow, NeverShortCircuit, Try}; | ||
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/// An iterator over `N` elements of the iterator at a time. | ||
/// | ||
/// The chunks do not overlap. If `N` does not divide the length of the | ||
/// iterator, then the last up to `N-1` elements will be omitted. | ||
/// | ||
/// This `struct` is created by the [`array_chunks`][Iterator::array_chunks] | ||
/// method on [`Iterator`]. See its documentation for more. | ||
#[derive(Debug, Clone)] | ||
#[must_use = "iterators are lazy and do nothing unless consumed"] | ||
#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")] | ||
pub struct ArrayChunks<I: Iterator, const N: usize> { | ||
iter: I, | ||
remainder: Option<array::IntoIter<I::Item, N>>, | ||
} | ||
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impl<I, const N: usize> ArrayChunks<I, N> | ||
where | ||
I: Iterator, | ||
{ | ||
#[track_caller] | ||
pub(in crate::iter) fn new(iter: I) -> Self { | ||
assert!(N != 0, "chunk size must be non-zero"); | ||
Self { iter, remainder: None } | ||
} | ||
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/// Returns an iterator over the remaining elements of the original iterator | ||
/// that are not going to be returned by this iterator. The returned | ||
/// iterator will yield at most `N-1` elements. | ||
#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")] | ||
#[inline] | ||
pub fn into_remainder(self) -> Option<array::IntoIter<I::Item, N>> { | ||
self.remainder | ||
} | ||
} | ||
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#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")] | ||
impl<I, const N: usize> Iterator for ArrayChunks<I, N> | ||
where | ||
I: Iterator, | ||
{ | ||
type Item = [I::Item; N]; | ||
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#[inline] | ||
fn next(&mut self) -> Option<Self::Item> { | ||
self.try_for_each(ControlFlow::Break).break_value() | ||
} | ||
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#[inline] | ||
fn size_hint(&self) -> (usize, Option<usize>) { | ||
let (lower, upper) = self.iter.size_hint(); | ||
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(lower / N, upper.map(|n| n / N)) | ||
} | ||
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#[inline] | ||
fn count(self) -> usize { | ||
self.iter.count() / N | ||
} | ||
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fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R | ||
where | ||
Self: Sized, | ||
F: FnMut(B, Self::Item) -> R, | ||
R: Try<Output = B>, | ||
{ | ||
let mut acc = init; | ||
loop { | ||
match self.iter.next_chunk() { | ||
Ok(chunk) => acc = f(acc, chunk)?, | ||
Err(remainder) => { | ||
// Make sure to not override `self.remainder` with an empty array | ||
// when `next` is called after `ArrayChunks` exhaustion. | ||
self.remainder.get_or_insert(remainder); | ||
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break try { acc }; | ||
} | ||
} | ||
} | ||
} | ||
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fn fold<B, F>(mut self, init: B, f: F) -> B | ||
where | ||
Self: Sized, | ||
F: FnMut(B, Self::Item) -> B, | ||
{ | ||
self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0 | ||
} | ||
} | ||
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#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")] | ||
impl<I, const N: usize> DoubleEndedIterator for ArrayChunks<I, N> | ||
where | ||
I: DoubleEndedIterator + ExactSizeIterator, | ||
{ | ||
#[inline] | ||
fn next_back(&mut self) -> Option<Self::Item> { | ||
self.try_rfold((), |(), x| ControlFlow::Break(x)).break_value() | ||
} | ||
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fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R | ||
where | ||
Self: Sized, | ||
F: FnMut(B, Self::Item) -> R, | ||
R: Try<Output = B>, | ||
{ | ||
// We are iterating from the back we need to first handle the remainder. | ||
self.next_back_remainder(); | ||
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let mut acc = init; | ||
let mut iter = ByRefSized(&mut self.iter).rev(); | ||
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// NB remainder is handled by `next_back_remainder`, so | ||
// `next_chunk` can't return `Err` with non-empty remainder | ||
// (assuming correct `I as ExactSizeIterator` impl). | ||
while let Ok(mut chunk) = iter.next_chunk() { | ||
// FIXME: do not do double reverse | ||
// (we could instead add `next_chunk_back` for example) | ||
chunk.reverse(); | ||
acc = f(acc, chunk)? | ||
} | ||
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try { acc } | ||
} | ||
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fn rfold<B, F>(mut self, init: B, f: F) -> B | ||
where | ||
Self: Sized, | ||
F: FnMut(B, Self::Item) -> B, | ||
{ | ||
self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).0 | ||
} | ||
} | ||
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impl<I, const N: usize> ArrayChunks<I, N> | ||
where | ||
I: DoubleEndedIterator + ExactSizeIterator, | ||
{ | ||
/// Updates `self.remainder` such that `self.iter.len` is divisible by `N`. | ||
fn next_back_remainder(&mut self) { | ||
// Make sure to not override `self.remainder` with an empty array | ||
// when `next_back` is called after `ArrayChunks` exhaustion. | ||
if self.remainder.is_some() { | ||
return; | ||
} | ||
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// We use the `ExactSizeIterator` implementation of the underlying | ||
// iterator to know how many remaining elements there are. | ||
let rem = self.iter.len() % N; | ||
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// Take the last `rem` elements out of `self.iter`. | ||
let mut remainder = | ||
// SAFETY: `unwrap_err` always succeeds because x % N < N for all x. | ||
unsafe { self.iter.by_ref().rev().take(rem).next_chunk().unwrap_err_unchecked() }; | ||
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// We used `.rev()` above, so we need to re-reverse the reminder | ||
remainder.as_mut_slice().reverse(); | ||
self.remainder = Some(remainder); | ||
} | ||
} | ||
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#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")] | ||
impl<I, const N: usize> FusedIterator for ArrayChunks<I, N> where I: FusedIterator {} | ||
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#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")] | ||
impl<I, const N: usize> ExactSizeIterator for ArrayChunks<I, N> | ||
where | ||
I: ExactSizeIterator, | ||
{ | ||
#[inline] | ||
fn len(&self) -> usize { | ||
self.iter.len() / N | ||
} | ||
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#[inline] | ||
fn is_empty(&self) -> bool { | ||
self.iter.len() < N | ||
} | ||
} |
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