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use core::fmt;
use core::iter::FusedIterator;
use core::marker::PhantomData;
use core::mem;
use crate::alloc::{Allocator, Global, SizedTypeProperties};
use crate::ptr::{self, NonNull};
use super::VecDeque;
/// A draining iterator over the elements of a `VecDeque`.
///
/// This `struct` is created by the [`drain`] method on [`VecDeque`]. See its
/// documentation for more.
///
/// [`drain`]: VecDeque::drain
pub struct Drain<'a, T: 'a, A: Allocator = Global> {
// We can't just use a &mut VecDeque<T, A>, as that would make Drain invariant over T
// and we want it to be covariant instead
deque: NonNull<VecDeque<T, A>>,
// drain_start is stored in deque.len
drain_len: usize,
// index into the logical array, not the physical one (always lies in [0..deque.len))
idx: usize,
// number of elements after the drain range
tail_len: usize,
remaining: usize,
// Needed to make Drain covariant over T
_marker: PhantomData<&'a T>,
}
impl<'a, T, A: Allocator> Drain<'a, T, A> {
pub(super) unsafe fn new(
deque: &'a mut VecDeque<T, A>,
drain_start: usize,
drain_len: usize,
) -> Self {
let orig_len = mem::replace(&mut deque.len, drain_start);
let tail_len = orig_len - drain_start - drain_len;
Drain {
deque: NonNull::from(deque),
drain_len,
idx: drain_start,
tail_len,
remaining: drain_len,
_marker: PhantomData,
}
}
// Only returns pointers to the slices, as that's all we need
// to drop them. May only be called if `self.remaining != 0`.
unsafe fn as_slices(&self) -> (*mut [T], *mut [T]) {
unsafe {
let deque = self.deque.as_ref();
// We know that `self.idx + self.remaining <= deque.len <= usize::MAX`, so this won't overflow.
let logical_remaining_range = self.idx..self.idx + self.remaining;
// SAFETY: `logical_remaining_range` represents the
// range into the logical buffer of elements that
// haven't been drained yet, so they're all initialized,
// and `slice::range(start..end, end) == start..end`,
// so the preconditions for `slice_ranges` are met.
let (a_range, b_range) =
deque.slice_ranges(logical_remaining_range.clone(), logical_remaining_range.end);
(deque.buffer_range(a_range), deque.buffer_range(b_range))
}
}
}
impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_tuple("Drain")
.field(&self.drain_len)
.field(&self.idx)
.field(&self.tail_len)
.field(&self.remaining)
.finish()
}
}
unsafe impl<T: Sync, A: Allocator + Sync> Sync for Drain<'_, T, A> {}
unsafe impl<T: Send, A: Allocator + Send> Send for Drain<'_, T, A> {}
impl<T, A: Allocator> Drop for Drain<'_, T, A> {
fn drop(&mut self) {
struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
impl<T, A: Allocator> Drop for DropGuard<'_, '_, T, A> {
fn drop(&mut self) {
if self.0.remaining != 0 {
unsafe {
// SAFETY: We just checked that `self.remaining != 0`.
let (front, back) = self.0.as_slices();
ptr::drop_in_place(front);
ptr::drop_in_place(back);
}
}
let source_deque = unsafe { self.0.deque.as_mut() };
let drain_start = source_deque.len();
let drain_len = self.0.drain_len;
let drain_end = drain_start + drain_len;
let orig_len = self.0.tail_len + drain_end;
if T::IS_ZST {
// no need to copy around any memory if T is a ZST
source_deque.len = orig_len - drain_len;
return;
}
let head_len = drain_start;
let tail_len = self.0.tail_len;
match (head_len, tail_len) {
(0, 0) => {
source_deque.head = 0;
source_deque.len = 0;
}
(0, _) => {
source_deque.head = source_deque.to_physical_idx(drain_len);
source_deque.len = orig_len - drain_len;
}
(_, 0) => {
source_deque.len = orig_len - drain_len;
}
_ => unsafe {
if head_len <= tail_len {
source_deque.wrap_copy(
source_deque.head,
source_deque.to_physical_idx(drain_len),
head_len,
);
source_deque.head = source_deque.to_physical_idx(drain_len);
source_deque.len = orig_len - drain_len;
} else {
source_deque.wrap_copy(
source_deque.to_physical_idx(head_len + drain_len),
source_deque.to_physical_idx(head_len),
tail_len,
);
source_deque.len = orig_len - drain_len;
}
},
}
}
}
let guard = DropGuard(self);
if guard.0.remaining != 0 {
unsafe {
// SAFETY: We just checked that `self.remaining != 0`.
let (front, back) = guard.0.as_slices();
// since idx is a logical index, we don't need to worry about wrapping.
guard.0.idx += front.len();
guard.0.remaining -= front.len();
ptr::drop_in_place(front);
guard.0.remaining = 0;
ptr::drop_in_place(back);
}
}
// Dropping `guard` handles moving the remaining elements into place.
}
}
impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
type Item = T;
#[inline]
fn next(&mut self) -> Option<T> {
if self.remaining == 0 {
return None;
}
let wrapped_idx = unsafe { self.deque.as_ref().to_physical_idx(self.idx) };
self.idx += 1;
self.remaining -= 1;
Some(unsafe { self.deque.as_mut().buffer_read(wrapped_idx) })
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let len = self.remaining;
(len, Some(len))
}
}
impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
#[inline]
fn next_back(&mut self) -> Option<T> {
if self.remaining == 0 {
return None;
}
self.remaining -= 1;
let wrapped_idx = unsafe {
self.deque
.as_ref()
.to_physical_idx(self.idx + self.remaining)
};
Some(unsafe { self.deque.as_mut().buffer_read(wrapped_idx) })
}
}
impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {}
impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}