alloc/collections/vec_deque/drain.rs
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use core::iter::FusedIterator;
use core::marker::PhantomData;
use core::mem::{self, SizedTypeProperties};
use core::ptr::NonNull;
use core::{fmt, ptr};
use super::VecDeque;
use crate::alloc::{Allocator, Global};
/// 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
#[stable(feature = "drain", since = "1.6.0")]
pub struct Drain<
'a,
T: 'a,
#[unstable(feature = "allocator_api", issue = "32838")] 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 remaining after dropping the drain
new_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 new_len = orig_len - drain_len;
Drain {
deque: NonNull::from(deque),
drain_len,
idx: drain_start,
new_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))
}
}
}
#[stable(feature = "collection_debug", since = "1.17.0")]
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.new_len)
.field(&self.remaining)
.finish()
}
}
#[stable(feature = "drain", since = "1.6.0")]
unsafe impl<T: Sync, A: Allocator + Sync> Sync for Drain<'_, T, A> {}
#[stable(feature = "drain", since = "1.6.0")]
unsafe impl<T: Send, A: Allocator + Send> Send for Drain<'_, T, A> {}
#[stable(feature = "drain", since = "1.6.0")]
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>);
let guard = DropGuard(self);
if mem::needs_drop::<T>() && 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<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
#[inline]
fn drop(&mut self) {
if mem::needs_drop::<T>() && 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_len = self.0.drain_len;
let new_len = self.0.new_len;
if T::IS_ZST {
// no need to copy around any memory if T is a ZST
source_deque.len = new_len;
return;
}
let head_len = source_deque.len; // #elements in front of the drain
let tail_len = new_len - head_len; // #elements behind the drain
// Next, we will fill the hole left by the drain with as few writes as possible.
// The code below handles the following control flow and reduces the amount of
// branches under the assumption that `head_len == 0 || tail_len == 0`, i.e.
// draining at the front or at the back of the dequeue is especially common.
//
// H = "head index" = `deque.head`
// h = elements in front of the drain
// d = elements in the drain
// t = elements behind the drain
//
// Note that the buffer may wrap at any point and the wrapping is handled by
// `wrap_copy` and `to_physical_idx`.
//
// Case 1: if `head_len == 0 && tail_len == 0`
// Everything was drained, reset the head index back to 0.
// H
// [ . . . . . d d d d . . . . . ]
// H
// [ . . . . . . . . . . . . . . ]
//
// Case 2: else if `tail_len == 0`
// Don't move data or the head index.
// H
// [ . . . h h h h d d d d . . . ]
// H
// [ . . . h h h h . . . . . . . ]
//
// Case 3: else if `head_len == 0`
// Don't move data, but move the head index.
// H
// [ . . . d d d d t t t t . . . ]
// H
// [ . . . . . . . t t t t . . . ]
//
// Case 4: else if `tail_len <= head_len`
// Move data, but not the head index.
// H
// [ . . h h h h d d d d t t . . ]
// H
// [ . . h h h h t t . . . . . . ]
//
// Case 5: else
// Move data and the head index.
// H
// [ . . h h d d d d t t t t . . ]
// H
// [ . . . . . . h h t t t t . . ]
// When draining at the front (`.drain(..n)`) or at the back (`.drain(n..)`),
// we don't need to copy any data. The number of elements copied would be 0.
if head_len != 0 && tail_len != 0 {
join_head_and_tail_wrapping(source_deque, drain_len, head_len, tail_len);
// Marking this function as cold helps LLVM to eliminate it entirely if
// this branch is never taken.
// We use `#[cold]` instead of `#[inline(never)]`, because inlining this
// function into the general case (`.drain(n..m)`) is fine.
// See `tests/codegen/vecdeque-drain.rs` for a test.
#[cold]
fn join_head_and_tail_wrapping<T, A: Allocator>(
source_deque: &mut VecDeque<T, A>,
drain_len: usize,
head_len: usize,
tail_len: usize,
) {
// Pick whether to move the head or the tail here.
let (src, dst, len);
if head_len < tail_len {
src = source_deque.head;
dst = source_deque.to_physical_idx(drain_len);
len = head_len;
} else {
src = source_deque.to_physical_idx(head_len + drain_len);
dst = source_deque.to_physical_idx(head_len);
len = tail_len;
};
unsafe {
source_deque.wrap_copy(src, dst, len);
}
}
}
if new_len == 0 {
// Special case: If the entire dequeue was drained, reset the head back to 0,
// like `.clear()` does.
source_deque.head = 0;
} else if head_len < tail_len {
// If we moved the head above, then we need to adjust the head index here.
source_deque.head = source_deque.to_physical_idx(drain_len);
}
source_deque.len = new_len;
}
}
}
}
#[stable(feature = "drain", since = "1.6.0")]
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))
}
}
#[stable(feature = "drain", since = "1.6.0")]
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) })
}
}
#[stable(feature = "drain", since = "1.6.0")]
impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {}
#[stable(feature = "fused", since = "1.26.0")]
impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}