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Revert "Auto merge of rust-lang#52553 - Pazzaz:vecdeque-append, r=Sim…
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…onSapin"

This partially reverts commit d5b6b95,
reversing changes made to 6b1ff19.

Fixes rust-lang#53529.
Cc: rust-lang#53564.
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@MaloJaffre
MaloJaffre committed Aug 22, 2018
1 parent 786ccc3 commit 241b9b4
Showing 1 changed file with 2 additions and 159 deletions.
161 changes: 2 additions & 159 deletions src/liballoc/collections/vec_deque.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -202,23 +202,6 @@ impl<T> VecDeque<T> {
len);
}

/// Returns a pair of slices which contain the contents of the buffer not used by the VecDeque.
#[inline]
unsafe fn unused_as_mut_slices<'a>(&'a mut self) -> (&'a mut [T], &'a mut [T]) {
let head = self.head;
let tail = self.tail;
let buf = self.buffer_as_mut_slice();
if head != tail {
// In buf, head..tail contains the VecDeque and tail..head is unused.
// So calling `ring_slices` with tail and head swapped returns unused slices.
RingSlices::ring_slices(buf, tail, head)
} else {
// Swapping doesn't help when head == tail.
let (before, after) = buf.split_at_mut(head);
(after, before)
}
}

/// Copies a potentially wrapping block of memory len long from src to dest.
/// (abs(dst - src) + len) must be no larger than cap() (There must be at
/// most one continuous overlapping region between src and dest).
Expand Down Expand Up @@ -1851,148 +1834,8 @@ impl<T> VecDeque<T> {
#[inline]
#[stable(feature = "append", since = "1.4.0")]
pub fn append(&mut self, other: &mut Self) {
// Copies all values from `src_slice` to the start of `dst_slice`.
unsafe fn copy_whole_slice<T>(src_slice: &[T], dst_slice: &mut [T]) {
let len = src_slice.len();
ptr::copy_nonoverlapping(src_slice.as_ptr(), dst_slice[..len].as_mut_ptr(), len);
}

let src_total = other.len();

// Guarantees there is space in `self` for `other`.
self.reserve(src_total);

self.head = {
let original_head = self.head;

// The goal is to copy all values from `other` into `self`. To avoid any
// mismatch, all valid values in `other` are retrieved...
let (src_high, src_low) = other.as_slices();
// and unoccupied parts of self are retrieved.
let (dst_high, dst_low) = unsafe { self.unused_as_mut_slices() };

// Then all that is needed is to copy all values from
// src (src_high and src_low) to dst (dst_high and dst_low).
//
// other [o o o . . . . . o o o o]
// [5 6 7] [1 2 3 4]
// src_low src_high
//
// self [. . . . . . o o o o . .]
// [3 4 5 6 7 .] [1 2]
// dst_low dst_high
//
// Values are not copied one by one but as slices in `copy_whole_slice`.
// What slices are used depends on various properties of src and dst.
// There are 6 cases in total:
// 1. `src` is contiguous and fits in dst_high
// 2. `src` is contiguous and does not fit in dst_high
// 3. `src` is discontiguous and fits in dst_high
// 4. `src` is discontiguous and does not fit in dst_high
// + src_high is smaller than dst_high
// 5. `src` is discontiguous and does not fit in dst_high
// + dst_high is smaller than src_high
// 6. `src` is discontiguous and does not fit in dst_high
// + dst_high is the same size as src_high
let src_contiguous = src_low.is_empty();
let dst_high_fits_src = dst_high.len() >= src_total;
match (src_contiguous, dst_high_fits_src) {
(true, true) => {
// 1.
// other [. . . o o o . . . . . .]
// [] [1 1 1]
//
// self [. o o o o o . . . . . .]
// [.] [1 1 1 . . .]

unsafe {
copy_whole_slice(src_high, dst_high);
}
original_head + src_total
}
(true, false) => {
// 2.
// other [. . . o o o o o . . . .]
// [] [1 1 2 2 2]
//
// self [. . . . . . . o o o . .]
// [2 2 2 . . . .] [1 1]

let (src_1, src_2) = src_high.split_at(dst_high.len());
unsafe {
copy_whole_slice(src_1, dst_high);
copy_whole_slice(src_2, dst_low);
}
src_total - dst_high.len()
}
(false, true) => {
// 3.
// other [o o . . . . . . . o o o]
// [2 2] [1 1 1]
//
// self [. o o . . . . . . . . .]
// [.] [1 1 1 2 2 . . . .]

let (dst_1, dst_2) = dst_high.split_at_mut(src_high.len());
unsafe {
copy_whole_slice(src_high, dst_1);
copy_whole_slice(src_low, dst_2);
}
original_head + src_total
}
(false, false) => {
if src_high.len() < dst_high.len() {
// 4.
// other [o o o . . . . . . o o o]
// [2 3 3] [1 1 1]
//
// self [. . . . . . o o . . . .]
// [3 3 . . . .] [1 1 1 2]

let (dst_1, dst_2) = dst_high.split_at_mut(src_high.len());
let (src_2, src_3) = src_low.split_at(dst_2.len());
unsafe {
copy_whole_slice(src_high, dst_1);
copy_whole_slice(src_2, dst_2);
copy_whole_slice(src_3, dst_low);
}
src_3.len()
} else if src_high.len() > dst_high.len() {
// 5.
// other [o o o . . . . . o o o o]
// [3 3 3] [1 1 2 2]
//
// self [. . . . . . o o o o . .]
// [2 2 3 3 3 .] [1 1]

let (src_1, src_2) = src_high.split_at(dst_high.len());
let (dst_2, dst_3) = dst_low.split_at_mut(src_2.len());
unsafe {
copy_whole_slice(src_1, dst_high);
copy_whole_slice(src_2, dst_2);
copy_whole_slice(src_low, dst_3);
}
dst_2.len() + src_low.len()
} else {
// 6.
// other [o o . . . . . . . o o o]
// [2 2] [1 1 1]
//
// self [. . . . . . . o o . . .]
// [2 2 . . . . .] [1 1 1]

unsafe {
copy_whole_slice(src_high, dst_high);
copy_whole_slice(src_low, dst_low);
}
src_low.len()
}
}
}
};

// Some values now exist in both `other` and `self` but are made inaccessible in `other`.
other.tail = other.head;
// naive impl
self.extend(other.drain(..));
}

/// Retains only the elements specified by the predicate.
Expand Down

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