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Rollup merge of rust-lang#94667 - frank-king:feature/iter_map_windows…
…, r=Mark-Simulacrum Add `Iterator::map_windows` Tracking issue: rust-lang#87155. This is inherited from the old PR rust-lang#82413. Unlike rust-lang#82413, this PR implements the `MapWindows` to be lazy: only when pulling from the outer iterator, `.next()` of the inner iterator will be called. ## Implementaion Steps - [x] Implement `MapWindows` to keep the iterators' [*Laziness*](https://doc.rust-lang.org/std/iter/index.html#laziness) contract. - [x] Fix the known bug of memory access error. - [ ] Full specialization of iterator-related traits for `MapWindows`. - [x] `Iterator::size_hint`, - [x] ~`Iterator::count`~, - [x] `ExactSizeIterator` (when `I: ExactSizeIterator`), - [x] ~`TrustedLen` (when `I: TrustedLen`)~, - [x] `FusedIterator`, - [x] ~`Iterator::advance_by`~, - [x] ~`Iterator::nth`~, - [ ] ... - [ ] More tests and docs. ## Unresolved Questions: - [ ] Is there any more iterator-related traits should be specialized? - [ ] Is the double-space buffer worth? - [ ] Should there be `rmap_windows` or something else? - [ ] Taking GAT for consideration, should the mapper function be `FnMut(&[I::Item; N]) -> R` or something like `FnMut(ArrayView<'_, I::Item, N>) -> R`? Where `ArrayView` is mentioned in rust-lang/generic-associated-types-initiative#2. - It can save memory, only the same size as the array window is needed, - It is more efficient, which requires less data copies, - It is possibly compatible with the GATified version of `LendingIterator::windows`. - But it prevents the array pattern matching like `iter.map_windows(|_arr: [_; N]| ())`, unless we extend the array pattern to allow matching the `ArrayView`.
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use crate::{ | ||
fmt, | ||
iter::{ExactSizeIterator, FusedIterator}, | ||
mem::{self, MaybeUninit}, | ||
ptr, | ||
}; | ||
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/// An iterator over the mapped windows of another iterator. | ||
/// | ||
/// This `struct` is created by the [`Iterator::map_windows`]. See its | ||
/// documentation for more information. | ||
#[must_use = "iterators are lazy and do nothing unless consumed"] | ||
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")] | ||
pub struct MapWindows<I: Iterator, F, const N: usize> { | ||
f: F, | ||
inner: MapWindowsInner<I, N>, | ||
} | ||
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struct MapWindowsInner<I: Iterator, const N: usize> { | ||
// We fuse the inner iterator because there shouldn't be "holes" in | ||
// the sliding window. Once the iterator returns a `None`, we make | ||
// our `MapWindows` iterator return `None` forever. | ||
iter: Option<I>, | ||
// Since iterators are assumed lazy, i.e. it only yields an item when | ||
// `Iterator::next()` is called, and `MapWindows` is not an exception. | ||
// | ||
// Before the first iteration, we keep the buffer `None`. When the user | ||
// first call `next` or other methods that makes the iterator advance, | ||
// we collect the first `N` items yielded from the inner iterator and | ||
// put it into the buffer. | ||
// | ||
// When the inner iterator has returned a `None` (i.e. fused), we take | ||
// away this `buffer` and leave it `None` to reclaim its resources. | ||
// | ||
// FIXME: should we shrink the size of `buffer` using niche optimization? | ||
buffer: Option<Buffer<I::Item, N>>, | ||
} | ||
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// `Buffer` uses two times of space to reduce moves among the iterations. | ||
// `Buffer<T, N>` is semantically `[MaybeUninit<T>; 2 * N]`. However, due | ||
// to limitations of const generics, we use this different type. Note that | ||
// it has the same underlying memory layout. | ||
struct Buffer<T, const N: usize> { | ||
// Invariant: `self.buffer[self.start..self.start N]` is initialized, | ||
// with all other elements being uninitialized. This also | ||
// implies that `self.start <= N`. | ||
buffer: [[MaybeUninit<T>; N]; 2], | ||
start: usize, | ||
} | ||
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impl<I: Iterator, F, const N: usize> MapWindows<I, F, N> { | ||
pub(in crate::iter) fn new(iter: I, f: F) -> Self { | ||
assert!(N != 0, "array in `Iterator::map_windows` must contain more than 0 elements"); | ||
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// Only ZST arrays' length can be so large. | ||
if mem::size_of::<I::Item>() == 0 { | ||
assert!( | ||
N.checked_mul(2).is_some(), | ||
"array size of `Iterator::map_windows` is too large" | ||
); | ||
} | ||
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Self { inner: MapWindowsInner::new(iter), f } | ||
} | ||
} | ||
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impl<I: Iterator, const N: usize> MapWindowsInner<I, N> { | ||
#[inline] | ||
fn new(iter: I) -> Self { | ||
Self { iter: Some(iter), buffer: None } | ||
} | ||
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fn next_window(&mut self) -> Option<&[I::Item; N]> { | ||
let iter = self.iter.as_mut()?; | ||
match self.buffer { | ||
// It is the first time to advance. We collect | ||
// the first `N` items from `self.iter` to initialize `self.buffer`. | ||
None => self.buffer = Buffer::try_from_iter(iter), | ||
Some(ref mut buffer) => match iter.next() { | ||
None => { | ||
// Fuse the inner iterator since it yields a `None`. | ||
self.iter.take(); | ||
self.buffer.take(); | ||
} | ||
// Advance the iterator. We first call `next` before changing our buffer | ||
// at all. This means that if `next` panics, our invariant is upheld and | ||
// our `Drop` impl drops the correct elements. | ||
Some(item) => buffer.push(item), | ||
}, | ||
} | ||
self.buffer.as_ref().map(Buffer::as_array_ref) | ||
} | ||
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fn size_hint(&self) -> (usize, Option<usize>) { | ||
let Some(ref iter) = self.iter else { return (0, Some(0)) }; | ||
let (lo, hi) = iter.size_hint(); | ||
if self.buffer.is_some() { | ||
// If the first `N` items are already yielded by the inner iterator, | ||
// the size hint is then equal to the that of the inner iterator's. | ||
(lo, hi) | ||
} else { | ||
// If the first `N` items are not yet yielded by the inner iterator, | ||
// the first `N` elements should be counted as one window, so both bounds | ||
// should subtract `N - 1`. | ||
(lo.saturating_sub(N - 1), hi.map(|hi| hi.saturating_sub(N - 1))) | ||
} | ||
} | ||
} | ||
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impl<T, const N: usize> Buffer<T, N> { | ||
fn try_from_iter(iter: &mut impl Iterator<Item = T>) -> Option<Self> { | ||
let first_half = crate::array::iter_next_chunk(iter).ok()?; | ||
let buffer = [MaybeUninit::new(first_half).transpose(), MaybeUninit::uninit_array()]; | ||
Some(Self { buffer, start: 0 }) | ||
} | ||
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#[inline] | ||
fn buffer_ptr(&self) -> *const MaybeUninit<T> { | ||
self.buffer.as_ptr().cast() | ||
} | ||
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#[inline] | ||
fn buffer_mut_ptr(&mut self) -> *mut MaybeUninit<T> { | ||
self.buffer.as_mut_ptr().cast() | ||
} | ||
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#[inline] | ||
fn as_array_ref(&self) -> &[T; N] { | ||
debug_assert!(self.start N <= 2 * N); | ||
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// SAFETY: our invariant guarantees these elements are initialized. | ||
unsafe { &*self.buffer_ptr().add(self.start).cast() } | ||
} | ||
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#[inline] | ||
fn as_uninit_array_mut(&mut self) -> &mut MaybeUninit<[T; N]> { | ||
debug_assert!(self.start N <= 2 * N); | ||
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// SAFETY: our invariant guarantees these elements are in bounds. | ||
unsafe { &mut *self.buffer_mut_ptr().add(self.start).cast() } | ||
} | ||
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/// Pushes a new item `next` to the back, and pops the front-most one. | ||
/// | ||
/// All the elements will be shifted to the front end when pushing reaches | ||
/// the back end. | ||
fn push(&mut self, next: T) { | ||
let buffer_mut_ptr = self.buffer_mut_ptr(); | ||
debug_assert!(self.start N <= 2 * N); | ||
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let to_drop = if self.start == N { | ||
// We have reached the end of our buffer and have to copy | ||
// everything to the start. Example layout for N = 3. | ||
// | ||
// 0 1 2 3 4 5 0 1 2 3 4 5 | ||
// ┌───┬───┬───┬───┬───┬───┐ ┌───┬───┬───┬───┬───┬───┐ | ||
// │ - │ - │ - │ a │ b │ c │ -> │ b │ c │ n │ - │ - │ - │ | ||
// └───┴───┴───┴───┴───┴───┘ └───┴───┴───┴───┴───┴───┘ | ||
// ↑ ↑ | ||
// start start | ||
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// SAFETY: the two pointers are valid for reads/writes of N -1 | ||
// elements because our array's size is semantically 2 * N. The | ||
// regions also don't overlap for the same reason. | ||
// | ||
// We leave the old elements in place. As soon as `start` is set | ||
// to 0, we treat them as uninitialized and treat their copies | ||
// as initialized. | ||
let to_drop = unsafe { | ||
ptr::copy_nonoverlapping(buffer_mut_ptr.add(self.start 1), buffer_mut_ptr, N - 1); | ||
(*buffer_mut_ptr.add(N - 1)).write(next); | ||
buffer_mut_ptr.add(self.start) | ||
}; | ||
self.start = 0; | ||
to_drop | ||
} else { | ||
// SAFETY: `self.start` is < N as guaranteed by the invariant | ||
// plus the check above. Even if the drop at the end panics, | ||
// the invariant is upheld. | ||
// | ||
// Example layout for N = 3: | ||
// | ||
// 0 1 2 3 4 5 0 1 2 3 4 5 | ||
// ┌───┬───┬───┬───┬───┬───┐ ┌───┬───┬───┬───┬───┬───┐ | ||
// │ - │ a │ b │ c │ - │ - │ -> │ - │ - │ b │ c │ n │ - │ | ||
// └───┴───┴───┴───┴───┴───┘ └───┴───┴───┴───┴───┴───┘ | ||
// ↑ ↑ | ||
// start start | ||
// | ||
let to_drop = unsafe { | ||
(*buffer_mut_ptr.add(self.start N)).write(next); | ||
buffer_mut_ptr.add(self.start) | ||
}; | ||
self.start = 1; | ||
to_drop | ||
}; | ||
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// SAFETY: the index is valid and this is element `a` in the | ||
// diagram above and has not been dropped yet. | ||
unsafe { ptr::drop_in_place(to_drop.cast::<T>()) }; | ||
} | ||
} | ||
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impl<T: Clone, const N: usize> Clone for Buffer<T, N> { | ||
fn clone(&self) -> Self { | ||
let mut buffer = Buffer { | ||
buffer: [MaybeUninit::uninit_array(), MaybeUninit::uninit_array()], | ||
start: self.start, | ||
}; | ||
buffer.as_uninit_array_mut().write(self.as_array_ref().clone()); | ||
buffer | ||
} | ||
} | ||
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impl<I, const N: usize> Clone for MapWindowsInner<I, N> | ||
where | ||
I: Iterator Clone, | ||
I::Item: Clone, | ||
{ | ||
fn clone(&self) -> Self { | ||
Self { iter: self.iter.clone(), buffer: self.buffer.clone() } | ||
} | ||
} | ||
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impl<T, const N: usize> Drop for Buffer<T, N> { | ||
fn drop(&mut self) { | ||
// SAFETY: our invariant guarantees that N elements starting from | ||
// `self.start` are initialized. We drop them here. | ||
unsafe { | ||
let initialized_part: *mut [T] = crate::ptr::slice_from_raw_parts_mut( | ||
self.buffer_mut_ptr().add(self.start).cast(), | ||
N, | ||
); | ||
ptr::drop_in_place(initialized_part); | ||
} | ||
} | ||
} | ||
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#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")] | ||
impl<I, F, R, const N: usize> Iterator for MapWindows<I, F, N> | ||
where | ||
I: Iterator, | ||
F: FnMut(&[I::Item; N]) -> R, | ||
{ | ||
type Item = R; | ||
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fn next(&mut self) -> Option<Self::Item> { | ||
let window = self.inner.next_window()?; | ||
let out = (self.f)(window); | ||
Some(out) | ||
} | ||
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fn size_hint(&self) -> (usize, Option<usize>) { | ||
self.inner.size_hint() | ||
} | ||
} | ||
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// Note that even if the inner iterator not fused, the `MapWindows` is still fused, | ||
// because we don't allow "holes" in the mapping window. | ||
#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")] | ||
impl<I, F, R, const N: usize> FusedIterator for MapWindows<I, F, N> | ||
where | ||
I: Iterator, | ||
F: FnMut(&[I::Item; N]) -> R, | ||
{ | ||
} | ||
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#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")] | ||
impl<I, F, R, const N: usize> ExactSizeIterator for MapWindows<I, F, N> | ||
where | ||
I: ExactSizeIterator, | ||
F: FnMut(&[I::Item; N]) -> R, | ||
{ | ||
} | ||
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#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")] | ||
impl<I: Iterator fmt::Debug, F, const N: usize> fmt::Debug for MapWindows<I, F, N> { | ||
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | ||
f.debug_struct("MapWindows").field("iter", &self.inner.iter).finish() | ||
} | ||
} | ||
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#[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")] | ||
impl<I, F, const N: usize> Clone for MapWindows<I, F, N> | ||
where | ||
I: Iterator Clone, | ||
F: Clone, | ||
I::Item: Clone, | ||
{ | ||
fn clone(&self) -> Self { | ||
Self { f: self.f.clone(), inner: self.inner.clone() } | ||
} | ||
} |
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