Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

1 files changed, 903 insertions, 0 deletions

diff --git a/vendor/rayon/src/slice/quicksort.rs b/vendor/rayon/src/slice/quicksort.rs
new file mode 100644
index 0000000..2bfc350
--- /dev/null
+++ b/vendor/rayon/src/slice/quicksort.rs
@@ -0,0 +1,903 @@
+//! Parallel quicksort.
+//!
+//! This implementation is copied verbatim from `std::slice::sort_unstable` and then parallelized.
+//! The only difference from the original is that calls to `recurse` are executed in parallel using
+//! `rayon_core::join`.
+
+use std::cmp;
+use std::marker::PhantomData;
+use std::mem::{self, MaybeUninit};
+use std::ptr;
+
+/// When dropped, copies from `src` into `dest`.
+#[must_use]
+struct CopyOnDrop<'a, T> {
+    src: *const T,
+    dest: *mut T,
+    /// `src` is often a local pointer here, make sure we have appropriate
+    /// PhantomData so that dropck can protect us.
+    marker: PhantomData<&'a mut T>,
+}
+
+impl<'a, T> CopyOnDrop<'a, T> {
+    /// Construct from a source pointer and a destination
+    /// Assumes dest lives longer than src, since there is no easy way to
+    /// copy down lifetime information from another pointer
+    unsafe fn new(src: &'a T, dest: *mut T) -> Self {
+        CopyOnDrop {
+            src,
+            dest,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<T> Drop for CopyOnDrop<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY:  This is a helper class.
+        //          Please refer to its usage for correctness.
+        //          Namely, one must be sure that `src` and `dst` does not overlap as required by `ptr::copy_nonoverlapping`.
+        unsafe {
+            ptr::copy_nonoverlapping(self.src, self.dest, 1);
+        }
+    }
+}
+
+/// Shifts the first element to the right until it encounters a greater or equal element.
+fn shift_head<T, F>(v: &mut [T], is_less: &F)
+where
+    F: Fn(&T, &T) -> bool,
+{
+    let len = v.len();
+    // SAFETY: The unsafe operations below involves indexing without a bounds check (by offsetting a
+    // pointer) and copying memory (`ptr::copy_nonoverlapping`).
+    //
+    // a. Indexing:
+    //  1. We checked the size of the array to >=2.
+    //  2. All the indexing that we will do is always between {0 <= index < len} at most.
+    //
+    // b. Memory copying
+    //  1. We are obtaining pointers to references which are guaranteed to be valid.
+    //  2. They cannot overlap because we obtain pointers to difference indices of the slice.
+    //     Namely, `i` and `i-1`.
+    //  3. If the slice is properly aligned, the elements are properly aligned.
+    //     It is the caller's responsibility to make sure the slice is properly aligned.
+    //
+    // See comments below for further detail.
+    unsafe {
+        // If the first two elements are out-of-order...
+        if len >= 2 && is_less(v.get_unchecked(1), v.get_unchecked(0)) {
+            // Read the first element into a stack-allocated variable. If a following comparison
+            // operation panics, `hole` will get dropped and automatically write the element back
+            // into the slice.
+            let tmp = mem::ManuallyDrop::new(ptr::read(v.get_unchecked(0)));
+            let v = v.as_mut_ptr();
+            let mut hole = CopyOnDrop::new(&*tmp, v.add(1));
+            ptr::copy_nonoverlapping(v.add(1), v.add(0), 1);
+
+            for i in 2..len {
+                if !is_less(&*v.add(i), &*tmp) {
+                    break;
+                }
+
+                // Move `i`-th element one place to the left, thus shifting the hole to the right.
+                ptr::copy_nonoverlapping(v.add(i), v.add(i - 1), 1);
+                hole.dest = v.add(i);
+            }
+            // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
+        }
+    }
+}
+
+/// Shifts the last element to the left until it encounters a smaller or equal element.
+fn shift_tail<T, F>(v: &mut [T], is_less: &F)
+where
+    F: Fn(&T, &T) -> bool,
+{
+    let len = v.len();
+    // SAFETY: The unsafe operations below involves indexing without a bound check (by offsetting a
+    // pointer) and copying memory (`ptr::copy_nonoverlapping`).
+    //
+    // a. Indexing:
+    //  1. We checked the size of the array to >= 2.
+    //  2. All the indexing that we will do is always between `0 <= index < len-1` at most.
+    //
+    // b. Memory copying
+    //  1. We are obtaining pointers to references which are guaranteed to be valid.
+    //  2. They cannot overlap because we obtain pointers to difference indices of the slice.
+    //     Namely, `i` and `i+1`.
+    //  3. If the slice is properly aligned, the elements are properly aligned.
+    //     It is the caller's responsibility to make sure the slice is properly aligned.
+    //
+    // See comments below for further detail.
+    unsafe {
+        // If the last two elements are out-of-order...
+        if len >= 2 && is_less(v.get_unchecked(len - 1), v.get_unchecked(len - 2)) {
+            // Read the last element into a stack-allocated variable. If a following comparison
+            // operation panics, `hole` will get dropped and automatically write the element back
+            // into the slice.
+            let tmp = mem::ManuallyDrop::new(ptr::read(v.get_unchecked(len - 1)));
+            let v = v.as_mut_ptr();
+            let mut hole = CopyOnDrop::new(&*tmp, v.add(len - 2));
+            ptr::copy_nonoverlapping(v.add(len - 2), v.add(len - 1), 1);
+
+            for i in (0..len - 2).rev() {
+                if !is_less(&*tmp, &*v.add(i)) {
+                    break;
+                }
+
+                // Move `i`-th element one place to the right, thus shifting the hole to the left.
+                ptr::copy_nonoverlapping(v.add(i), v.add(i + 1), 1);
+                hole.dest = v.add(i);
+            }
+            // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
+        }
+    }
+}
+
+/// Partially sorts a slice by shifting several out-of-order elements around.
+///
+/// Returns `true` if the slice is sorted at the end. This function is *O*(*n*) worst-case.
+#[cold]
+fn partial_insertion_sort<T, F>(v: &mut [T], is_less: &F) -> bool
+where
+    F: Fn(&T, &T) -> bool,
+{
+    // Maximum number of adjacent out-of-order pairs that will get shifted.
+    const MAX_STEPS: usize = 5;
+    // If the slice is shorter than this, don't shift any elements.
+    const SHORTEST_SHIFTING: usize = 50;
+
+    let len = v.len();
+    let mut i = 1;
+
+    for _ in 0..MAX_STEPS {
+        // SAFETY: We already explicitly did the bound checking with `i < len`.
+        // All our subsequent indexing is only in the range `0 <= index < len`
+        unsafe {
+            // Find the next pair of adjacent out-of-order elements.
+            while i < len && !is_less(v.get_unchecked(i), v.get_unchecked(i - 1)) {
+                i += 1;
+            }
+        }
+
+        // Are we done?
+        if i == len {
+            return true;
+        }
+
+        // Don't shift elements on short arrays, that has a performance cost.
+        if len < SHORTEST_SHIFTING {
+            return false;
+        }
+
+        // Swap the found pair of elements. This puts them in correct order.
+        v.swap(i - 1, i);
+
+        // Shift the smaller element to the left.
+        shift_tail(&mut v[..i], is_less);
+        // Shift the greater element to the right.
+        shift_head(&mut v[i..], is_less);
+    }
+
+    // Didn't manage to sort the slice in the limited number of steps.
+    false
+}
+
+/// Sorts a slice using insertion sort, which is *O*(*n*^2) worst-case.
+fn insertion_sort<T, F>(v: &mut [T], is_less: &F)
+where
+    F: Fn(&T, &T) -> bool,
+{
+    for i in 1..v.len() {
+        shift_tail(&mut v[..i + 1], is_less);
+    }
+}
+
+/// Sorts `v` using heapsort, which guarantees *O*(*n* \* log(*n*)) worst-case.
+#[cold]
+fn heapsort<T, F>(v: &mut [T], is_less: &F)
+where
+    F: Fn(&T, &T) -> bool,
+{
+    // This binary heap respects the invariant `parent >= child`.
+    let sift_down = |v: &mut [T], mut node| {
+        loop {
+            // Children of `node`.
+            let mut child = 2 * node + 1;
+            if child >= v.len() {
+                break;
+            }
+
+            // Choose the greater child.
+            if child + 1 < v.len() && is_less(&v[child], &v[child + 1]) {
+                child += 1;
+            }
+
+            // Stop if the invariant holds at `node`.
+            if !is_less(&v[node], &v[child]) {
+                break;
+            }
+
+            // Swap `node` with the greater child, move one step down, and continue sifting.
+            v.swap(node, child);
+            node = child;
+        }
+    };
+
+    // Build the heap in linear time.
+    for i in (0..v.len() / 2).rev() {
+        sift_down(v, i);
+    }
+
+    // Pop maximal elements from the heap.
+    for i in (1..v.len()).rev() {
+        v.swap(0, i);
+        sift_down(&mut v[..i], 0);
+    }
+}
+
+/// Partitions `v` into elements smaller than `pivot`, followed by elements greater than or equal
+/// to `pivot`.
+///
+/// Returns the number of elements smaller than `pivot`.
+///
+/// Partitioning is performed block-by-block in order to minimize the cost of branching operations.
+/// This idea is presented in the [BlockQuicksort][pdf] paper.
+///
+/// [pdf]: https://drops.dagstuhl.de/opus/volltexte/2016/6389/pdf/LIPIcs-ESA-2016-38.pdf
+fn partition_in_blocks<T, F>(v: &mut [T], pivot: &T, is_less: &F) -> usize
+where
+    F: Fn(&T, &T) -> bool,
+{
+    // Number of elements in a typical block.
+    const BLOCK: usize = 128;
+
+    // The partitioning algorithm repeats the following steps until completion:
+    //
+    // 1. Trace a block from the left side to identify elements greater than or equal to the pivot.
+    // 2. Trace a block from the right side to identify elements smaller than the pivot.
+    // 3. Exchange the identified elements between the left and right side.
+    //
+    // We keep the following variables for a block of elements:
+    //
+    // 1. `block` - Number of elements in the block.
+    // 2. `start` - Start pointer into the `offsets` array.
+    // 3. `end` - End pointer into the `offsets` array.
+    // 4. `offsets - Indices of out-of-order elements within the block.
+
+    // The current block on the left side (from `l` to `l.add(block_l)`).
+    let mut l = v.as_mut_ptr();
+    let mut block_l = BLOCK;
+    let mut start_l = ptr::null_mut();
+    let mut end_l = ptr::null_mut();
+    let mut offsets_l = [MaybeUninit::<u8>::uninit(); BLOCK];
+
+    // The current block on the right side (from `r.sub(block_r)` to `r`).
+    // SAFETY: The documentation for .add() specifically mention that `vec.as_ptr().add(vec.len())` is always safe`
+    let mut r = unsafe { l.add(v.len()) };
+    let mut block_r = BLOCK;
+    let mut start_r = ptr::null_mut();
+    let mut end_r = ptr::null_mut();
+    let mut offsets_r = [MaybeUninit::<u8>::uninit(); BLOCK];
+
+    // FIXME: When we get VLAs, try creating one array of length `min(v.len(), 2 * BLOCK)` rather
+    // than two fixed-size arrays of length `BLOCK`. VLAs might be more cache-efficient.
+
+    // Returns the number of elements between pointers `l` (inclusive) and `r` (exclusive).
+    fn width<T>(l: *mut T, r: *mut T) -> usize {
+        assert!(mem::size_of::<T>() > 0);
+        // FIXME: this should *likely* use `offset_from`, but more
+        // investigation is needed (including running tests in miri).
+        // TODO unstable: (r.addr() - l.addr()) / mem::size_of::<T>()
+        (r as usize - l as usize) / mem::size_of::<T>()
+    }
+
+    loop {
+        // We are done with partitioning block-by-block when `l` and `r` get very close. Then we do
+        // some patch-up work in order to partition the remaining elements in between.
+        let is_done = width(l, r) <= 2 * BLOCK;
+
+        if is_done {
+            // Number of remaining elements (still not compared to the pivot).
+            let mut rem = width(l, r);
+            if start_l < end_l || start_r < end_r {
+                rem -= BLOCK;
+            }
+
+            // Adjust block sizes so that the left and right block don't overlap, but get perfectly
+            // aligned to cover the whole remaining gap.
+            if start_l < end_l {
+                block_r = rem;
+            } else if start_r < end_r {
+                block_l = rem;
+            } else {
+                // There were the same number of elements to switch on both blocks during the last
+                // iteration, so there are no remaining elements on either block. Cover the remaining
+                // items with roughly equally-sized blocks.
+                block_l = rem / 2;
+                block_r = rem - block_l;
+            }
+            debug_assert!(block_l <= BLOCK && block_r <= BLOCK);
+            debug_assert!(width(l, r) == block_l + block_r);
+        }
+
+        if start_l == end_l {
+            // Trace `block_l` elements from the left side.
+            // TODO unstable: start_l = MaybeUninit::slice_as_mut_ptr(&mut offsets_l);
+            start_l = offsets_l.as_mut_ptr() as *mut u8;
+            end_l = start_l;
+            let mut elem = l;
+
+            for i in 0..block_l {
+                // SAFETY: The unsafety operations below involve the usage of the `offset`.
+                //         According to the conditions required by the function, we satisfy them because:
+                //         1. `offsets_l` is stack-allocated, and thus considered separate allocated object.
+                //         2. The function `is_less` returns a `bool`.
+                //            Casting a `bool` will never overflow `isize`.
+                //         3. We have guaranteed that `block_l` will be `<= BLOCK`.
+                //            Plus, `end_l` was initially set to the begin pointer of `offsets_` which was declared on the stack.
+                //            Thus, we know that even in the worst case (all invocations of `is_less` returns false) we will only be at most 1 byte pass the end.
+                //        Another unsafety operation here is dereferencing `elem`.
+                //        However, `elem` was initially the begin pointer to the slice which is always valid.
+                unsafe {
+                    // Branchless comparison.
+                    *end_l = i as u8;
+                    end_l = end_l.offset(!is_less(&*elem, pivot) as isize);
+                    elem = elem.offset(1);
+                }
+            }
+        }
+
+        if start_r == end_r {
+            // Trace `block_r` elements from the right side.
+            // TODO unstable: start_r = MaybeUninit::slice_as_mut_ptr(&mut offsets_r);
+            start_r = offsets_r.as_mut_ptr() as *mut u8;
+            end_r = start_r;
+            let mut elem = r;
+
+            for i in 0..block_r {
+                // SAFETY: The unsafety operations below involve the usage of the `offset`.
+                //         According to the conditions required by the function, we satisfy them because:
+                //         1. `offsets_r` is stack-allocated, and thus considered separate allocated object.
+                //         2. The function `is_less` returns a `bool`.
+                //            Casting a `bool` will never overflow `isize`.
+                //         3. We have guaranteed that `block_r` will be `<= BLOCK`.
+                //            Plus, `end_r` was initially set to the begin pointer of `offsets_` which was declared on the stack.
+                //            Thus, we know that even in the worst case (all invocations of `is_less` returns true) we will only be at most 1 byte pass the end.
+                //        Another unsafety operation here is dereferencing `elem`.
+                //        However, `elem` was initially `1 * sizeof(T)` past the end and we decrement it by `1 * sizeof(T)` before accessing it.
+                //        Plus, `block_r` was asserted to be less than `BLOCK` and `elem` will therefore at most be pointing to the beginning of the slice.
+                unsafe {
+                    // Branchless comparison.
+                    elem = elem.offset(-1);
+                    *end_r = i as u8;
+                    end_r = end_r.offset(is_less(&*elem, pivot) as isize);
+                }
+            }
+        }
+
+        // Number of out-of-order elements to swap between the left and right side.
+        let count = cmp::min(width(start_l, end_l), width(start_r, end_r));
+
+        if count > 0 {
+            macro_rules! left {
+                () => {
+                    l.offset(*start_l as isize)
+                };
+            }
+            macro_rules! right {
+                () => {
+                    r.offset(-(*start_r as isize) - 1)
+                };
+            }
+
+            // Instead of swapping one pair at the time, it is more efficient to perform a cyclic
+            // permutation. This is not strictly equivalent to swapping, but produces a similar
+            // result using fewer memory operations.
+
+            // SAFETY: The use of `ptr::read` is valid because there is at least one element in
+            // both `offsets_l` and `offsets_r`, so `left!` is a valid pointer to read from.
+            //
+            // The uses of `left!` involve calls to `offset` on `l`, which points to the
+            // beginning of `v`. All the offsets pointed-to by `start_l` are at most `block_l`, so
+            // these `offset` calls are safe as all reads are within the block. The same argument
+            // applies for the uses of `right!`.
+            //
+            // The calls to `start_l.offset` are valid because there are at most `count-1` of them,
+            // plus the final one at the end of the unsafe block, where `count` is the minimum number
+            // of collected offsets in `offsets_l` and `offsets_r`, so there is no risk of there not
+            // being enough elements. The same reasoning applies to the calls to `start_r.offset`.
+            //
+            // The calls to `copy_nonoverlapping` are safe because `left!` and `right!` are guaranteed
+            // not to overlap, and are valid because of the reasoning above.
+            unsafe {
+                let tmp = ptr::read(left!());
+                ptr::copy_nonoverlapping(right!(), left!(), 1);
+
+                for _ in 1..count {
+                    start_l = start_l.offset(1);
+                    ptr::copy_nonoverlapping(left!(), right!(), 1);
+                    start_r = start_r.offset(1);
+                    ptr::copy_nonoverlapping(right!(), left!(), 1);
+                }
+
+                ptr::copy_nonoverlapping(&tmp, right!(), 1);
+                mem::forget(tmp);
+                start_l = start_l.offset(1);
+                start_r = start_r.offset(1);
+            }
+        }
+
+        if start_l == end_l {
+            // All out-of-order elements in the left block were moved. Move to the next block.
+
+            // block-width-guarantee
+            // SAFETY: if `!is_done` then the slice width is guaranteed to be at least `2*BLOCK` wide. There
+            // are at most `BLOCK` elements in `offsets_l` because of its size, so the `offset` operation is
+            // safe. Otherwise, the debug assertions in the `is_done` case guarantee that
+            // `width(l, r) == block_l + block_r`, namely, that the block sizes have been adjusted to account
+            // for the smaller number of remaining elements.
+            l = unsafe { l.add(block_l) };
+        }
+
+        if start_r == end_r {
+            // All out-of-order elements in the right block were moved. Move to the previous block.
+
+            // SAFETY: Same argument as [block-width-guarantee]. Either this is a full block `2*BLOCK`-wide,
+            // or `block_r` has been adjusted for the last handful of elements.
+            r = unsafe { r.offset(-(block_r as isize)) };
+        }
+
+        if is_done {
+            break;
+        }
+    }
+
+    // All that remains now is at most one block (either the left or the right) with out-of-order
+    // elements that need to be moved. Such remaining elements can be simply shifted to the end
+    // within their block.
+
+    if start_l < end_l {
+        // The left block remains.
+        // Move its remaining out-of-order elements to the far right.
+        debug_assert_eq!(width(l, r), block_l);
+        while start_l < end_l {
+            // remaining-elements-safety
+            // SAFETY: while the loop condition holds there are still elements in `offsets_l`, so it
+            // is safe to point `end_l` to the previous element.
+            //
+            // The `ptr::swap` is safe if both its arguments are valid for reads and writes:
+            //  - Per the debug assert above, the distance between `l` and `r` is `block_l`
+            //    elements, so there can be at most `block_l` remaining offsets between `start_l`
+            //    and `end_l`. This means `r` will be moved at most `block_l` steps back, which
+            //    makes the `r.offset` calls valid (at that point `l == r`).
+            //  - `offsets_l` contains valid offsets into `v` collected during the partitioning of
+            //    the last block, so the `l.offset` calls are valid.
+            unsafe {
+                end_l = end_l.offset(-1);
+                ptr::swap(l.offset(*end_l as isize), r.offset(-1));
+                r = r.offset(-1);
+            }
+        }
+        width(v.as_mut_ptr(), r)
+    } else if start_r < end_r {
+        // The right block remains.
+        // Move its remaining out-of-order elements to the far left.
+        debug_assert_eq!(width(l, r), block_r);
+        while start_r < end_r {
+            // SAFETY: See the reasoning in [remaining-elements-safety].
+            unsafe {
+                end_r = end_r.offset(-1);
+                ptr::swap(l, r.offset(-(*end_r as isize) - 1));
+                l = l.offset(1);
+            }
+        }
+        width(v.as_mut_ptr(), l)
+    } else {
+        // Nothing else to do, we're done.
+        width(v.as_mut_ptr(), l)
+    }
+}
+
+/// Partitions `v` into elements smaller than `v[pivot]`, followed by elements greater than or
+/// equal to `v[pivot]`.
+///
+/// Returns a tuple of:
+///
+/// 1. Number of elements smaller than `v[pivot]`.
+/// 2. True if `v` was already partitioned.
+fn partition<T, F>(v: &mut [T], pivot: usize, is_less: &F) -> (usize, bool)
+where
+    F: Fn(&T, &T) -> bool,
+{
+    let (mid, was_partitioned) = {
+        // Place the pivot at the beginning of slice.
+        v.swap(0, pivot);
+        let (pivot, v) = v.split_at_mut(1);
+        let pivot = &mut pivot[0];
+
+        // Read the pivot into a stack-allocated variable for efficiency. If a following comparison
+        // operation panics, the pivot will be automatically written back into the slice.
+
+        // SAFETY: `pivot` is a reference to the first element of `v`, so `ptr::read` is safe.
+        let tmp = mem::ManuallyDrop::new(unsafe { ptr::read(pivot) });
+        let _pivot_guard = unsafe { CopyOnDrop::new(&*tmp, pivot) };
+        let pivot = &*tmp;
+
+        // Find the first pair of out-of-order elements.
+        let mut l = 0;
+        let mut r = v.len();
+
+        // SAFETY: The unsafety below involves indexing an array.
+        // For the first one: We already do the bounds checking here with `l < r`.
+        // For the second one: We initially have `l == 0` and `r == v.len()` and we checked that `l < r` at every indexing operation.
+        //                     From here we know that `r` must be at least `r == l` which was shown to be valid from the first one.
+        unsafe {
+            // Find the first element greater than or equal to the pivot.
+            while l < r && is_less(v.get_unchecked(l), pivot) {
+                l += 1;
+            }
+
+            // Find the last element smaller that the pivot.
+            while l < r && !is_less(v.get_unchecked(r - 1), pivot) {
+                r -= 1;
+            }
+        }
+
+        (
+            l + partition_in_blocks(&mut v[l..r], pivot, is_less),
+            l >= r,
+        )
+
+        // `_pivot_guard` goes out of scope and writes the pivot (which is a stack-allocated
+        // variable) back into the slice where it originally was. This step is critical in ensuring
+        // safety!
+    };
+
+    // Place the pivot between the two partitions.
+    v.swap(0, mid);
+
+    (mid, was_partitioned)
+}
+
+/// Partitions `v` into elements equal to `v[pivot]` followed by elements greater than `v[pivot]`.
+///
+/// Returns the number of elements equal to the pivot. It is assumed that `v` does not contain
+/// elements smaller than the pivot.
+fn partition_equal<T, F>(v: &mut [T], pivot: usize, is_less: &F) -> usize
+where
+    F: Fn(&T, &T) -> bool,
+{
+    // Place the pivot at the beginning of slice.
+    v.swap(0, pivot);
+    let (pivot, v) = v.split_at_mut(1);
+    let pivot = &mut pivot[0];
+
+    // Read the pivot into a stack-allocated variable for efficiency. If a following comparison
+    // operation panics, the pivot will be automatically written back into the slice.
+    // SAFETY: The pointer here is valid because it is obtained from a reference to a slice.
+    let tmp = mem::ManuallyDrop::new(unsafe { ptr::read(pivot) });
+    let _pivot_guard = unsafe { CopyOnDrop::new(&*tmp, pivot) };
+    let pivot = &*tmp;
+
+    // Now partition the slice.
+    let mut l = 0;
+    let mut r = v.len();
+    loop {
+        // SAFETY: The unsafety below involves indexing an array.
+        // For the first one: We already do the bounds checking here with `l < r`.
+        // For the second one: We initially have `l == 0` and `r == v.len()` and we checked that `l < r` at every indexing operation.
+        //                     From here we know that `r` must be at least `r == l` which was shown to be valid from the first one.
+        unsafe {
+            // Find the first element greater than the pivot.
+            while l < r && !is_less(pivot, v.get_unchecked(l)) {
+                l += 1;
+            }
+
+            // Find the last element equal to the pivot.
+            while l < r && is_less(pivot, v.get_unchecked(r - 1)) {
+                r -= 1;
+            }
+
+            // Are we done?
+            if l >= r {
+                break;
+            }
+
+            // Swap the found pair of out-of-order elements.
+            r -= 1;
+            let ptr = v.as_mut_ptr();
+            ptr::swap(ptr.add(l), ptr.add(r));
+            l += 1;
+        }
+    }
+
+    // We found `l` elements equal to the pivot. Add 1 to account for the pivot itself.
+    l + 1
+
+    // `_pivot_guard` goes out of scope and writes the pivot (which is a stack-allocated variable)
+    // back into the slice where it originally was. This step is critical in ensuring safety!
+}
+
+/// Scatters some elements around in an attempt to break patterns that might cause imbalanced
+/// partitions in quicksort.
+#[cold]
+fn break_patterns<T>(v: &mut [T]) {
+    let len = v.len();
+    if len >= 8 {
+        // Pseudorandom number generator from the "Xorshift RNGs" paper by George Marsaglia.
+        let mut random = len as u32;
+        let mut gen_u32 = || {
+            random ^= random << 13;
+            random ^= random >> 17;
+            random ^= random << 5;
+            random
+        };
+        let mut gen_usize = || {
+            if usize::BITS <= 32 {
+                gen_u32() as usize
+            } else {
+                (((gen_u32() as u64) << 32) | (gen_u32() as u64)) as usize
+            }
+        };
+
+        // Take random numbers modulo this number.
+        // The number fits into `usize` because `len` is not greater than `isize::MAX`.
+        let modulus = len.next_power_of_two();
+
+        // Some pivot candidates will be in the nearby of this index. Let's randomize them.
+        let pos = len / 4 * 2;
+
+        for i in 0..3 {
+            // Generate a random number modulo `len`. However, in order to avoid costly operations
+            // we first take it modulo a power of two, and then decrease by `len` until it fits
+            // into the range `[0, len - 1]`.
+            let mut other = gen_usize() & (modulus - 1);
+
+            // `other` is guaranteed to be less than `2 * len`.
+            if other >= len {
+                other -= len;
+            }
+
+            v.swap(pos - 1 + i, other);
+        }
+    }
+}
+
+/// Chooses a pivot in `v` and returns the index and `true` if the slice is likely already sorted.
+///
+/// Elements in `v` might be reordered in the process.
+fn choose_pivot<T, F>(v: &mut [T], is_less: &F) -> (usize, bool)
+where
+    F: Fn(&T, &T) -> bool,
+{
+    // Minimum length to choose the median-of-medians method.
+    // Shorter slices use the simple median-of-three method.
+    const SHORTEST_MEDIAN_OF_MEDIANS: usize = 50;
+    // Maximum number of swaps that can be performed in this function.
+    const MAX_SWAPS: usize = 4 * 3;
+
+    let len = v.len();
+
+    // Three indices near which we are going to choose a pivot.
+    #[allow(clippy::identity_op)]
+    let mut a = len / 4 * 1;
+    let mut b = len / 4 * 2;
+    let mut c = len / 4 * 3;
+
+    // Counts the total number of swaps we are about to perform while sorting indices.
+    let mut swaps = 0;
+
+    if len >= 8 {
+        // Swaps indices so that `v[a] <= v[b]`.
+        // SAFETY: `len >= 8` so there are at least two elements in the neighborhoods of
+        // `a`, `b` and `c`. This means the three calls to `sort_adjacent` result in
+        // corresponding calls to `sort3` with valid 3-item neighborhoods around each
+        // pointer, which in turn means the calls to `sort2` are done with valid
+        // references. Thus the `v.get_unchecked` calls are safe, as is the `ptr::swap`
+        // call.
+        let mut sort2 = |a: &mut usize, b: &mut usize| unsafe {
+            if is_less(v.get_unchecked(*b), v.get_unchecked(*a)) {
+                ptr::swap(a, b);
+                swaps += 1;
+            }
+        };
+
+        // Swaps indices so that `v[a] <= v[b] <= v[c]`.
+        let mut sort3 = |a: &mut usize, b: &mut usize, c: &mut usize| {
+            sort2(a, b);
+            sort2(b, c);
+            sort2(a, b);
+        };
+
+        if len >= SHORTEST_MEDIAN_OF_MEDIANS {
+            // Finds the median of `v[a - 1], v[a], v[a + 1]` and stores the index into `a`.
+            let mut sort_adjacent = |a: &mut usize| {
+                let tmp = *a;
+                sort3(&mut (tmp - 1), a, &mut (tmp + 1));
+            };
+
+            // Find medians in the neighborhoods of `a`, `b`, and `c`.
+            sort_adjacent(&mut a);
+            sort_adjacent(&mut b);
+            sort_adjacent(&mut c);
+        }
+
+        // Find the median among `a`, `b`, and `c`.
+        sort3(&mut a, &mut b, &mut c);
+    }
+
+    if swaps < MAX_SWAPS {
+        (b, swaps == 0)
+    } else {
+        // The maximum number of swaps was performed. Chances are the slice is descending or mostly
+        // descending, so reversing will probably help sort it faster.
+        v.reverse();
+        (len - 1 - b, true)
+    }
+}
+
+/// Sorts `v` recursively.
+///
+/// If the slice had a predecessor in the original array, it is specified as `pred`.
+///
+/// `limit` is the number of allowed imbalanced partitions before switching to `heapsort`. If zero,
+/// this function will immediately switch to heapsort.
+fn recurse<'a, T, F>(mut v: &'a mut [T], is_less: &F, mut pred: Option<&'a mut T>, mut limit: u32)
+where
+    T: Send,
+    F: Fn(&T, &T) -> bool + Sync,
+{
+    // Slices of up to this length get sorted using insertion sort.
+    const MAX_INSERTION: usize = 20;
+    // If both partitions are up to this length, we continue sequentially. This number is as small
+    // as possible but so that the overhead of Rayon's task scheduling is still negligible.
+    const MAX_SEQUENTIAL: usize = 2000;
+
+    // True if the last partitioning was reasonably balanced.
+    let mut was_balanced = true;
+    // True if the last partitioning didn't shuffle elements (the slice was already partitioned).
+    let mut was_partitioned = true;
+
+    loop {
+        let len = v.len();
+
+        // Very short slices get sorted using insertion sort.
+        if len <= MAX_INSERTION {
+            insertion_sort(v, is_less);
+            return;
+        }
+
+        // If too many bad pivot choices were made, simply fall back to heapsort in order to
+        // guarantee `O(n * log(n))` worst-case.
+        if limit == 0 {
+            heapsort(v, is_less);
+            return;
+        }
+
+        // If the last partitioning was imbalanced, try breaking patterns in the slice by shuffling
+        // some elements around. Hopefully we'll choose a better pivot this time.
+        if !was_balanced {
+            break_patterns(v);
+            limit -= 1;
+        }
+
+        // Choose a pivot and try guessing whether the slice is already sorted.
+        let (pivot, likely_sorted) = choose_pivot(v, is_less);
+
+        // If the last partitioning was decently balanced and didn't shuffle elements, and if pivot
+        // selection predicts the slice is likely already sorted...
+        if was_balanced && was_partitioned && likely_sorted {
+            // Try identifying several out-of-order elements and shifting them to correct
+            // positions. If the slice ends up being completely sorted, we're done.
+            if partial_insertion_sort(v, is_less) {
+                return;
+            }
+        }
+
+        // If the chosen pivot is equal to the predecessor, then it's the smallest element in the
+        // slice. Partition the slice into elements equal to and elements greater than the pivot.
+        // This case is usually hit when the slice contains many duplicate elements.
+        if let Some(ref p) = pred {
+            if !is_less(p, &v[pivot]) {
+                let mid = partition_equal(v, pivot, is_less);
+
+                // Continue sorting elements greater than the pivot.
+                v = &mut v[mid..];
+                continue;
+            }
+        }
+
+        // Partition the slice.
+        let (mid, was_p) = partition(v, pivot, is_less);
+        was_balanced = cmp::min(mid, len - mid) >= len / 8;
+        was_partitioned = was_p;
+
+        // Split the slice into `left`, `pivot`, and `right`.
+        let (left, right) = v.split_at_mut(mid);
+        let (pivot, right) = right.split_at_mut(1);
+        let pivot = &mut pivot[0];
+
+        if cmp::max(left.len(), right.len()) <= MAX_SEQUENTIAL {
+            // Recurse into the shorter side only in order to minimize the total number of recursive
+            // calls and consume less stack space. Then just continue with the longer side (this is
+            // akin to tail recursion).
+            if left.len() < right.len() {
+                recurse(left, is_less, pred, limit);
+                v = right;
+                pred = Some(pivot);
+            } else {
+                recurse(right, is_less, Some(pivot), limit);
+                v = left;
+            }
+        } else {
+            // Sort the left and right half in parallel.
+            rayon_core::join(
+                || recurse(left, is_less, pred, limit),
+                || recurse(right, is_less, Some(pivot), limit),
+            );
+            break;
+        }
+    }
+}
+
+/// Sorts `v` using pattern-defeating quicksort in parallel.
+///
+/// The algorithm is unstable, in-place, and *O*(*n* \* log(*n*)) worst-case.
+pub(super) fn par_quicksort<T, F>(v: &mut [T], is_less: F)
+where
+    T: Send,
+    F: Fn(&T, &T) -> bool + Sync,
+{
+    // Sorting has no meaningful behavior on zero-sized types.
+    if mem::size_of::<T>() == 0 {
+        return;
+    }
+
+    // Limit the number of imbalanced partitions to `floor(log2(len)) + 1`.
+    let limit = usize::BITS - v.len().leading_zeros();
+
+    recurse(v, &is_less, None, limit);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::heapsort;
+    use rand::distributions::Uniform;
+    use rand::{thread_rng, Rng};
+
+    #[test]
+    fn test_heapsort() {
+        let rng = &mut thread_rng();
+
+        for len in (0..25).chain(500..501) {
+            for &modulus in &[5, 10, 100] {
+                let dist = Uniform::new(0, modulus);
+                for _ in 0..100 {
+                    let v: Vec<i32> = rng.sample_iter(&dist).take(len).collect();
+
+                    // Test heapsort using `<` operator.
+                    let mut tmp = v.clone();
+                    heapsort(&mut tmp, &|a, b| a < b);
+                    assert!(tmp.windows(2).all(|w| w[0] <= w[1]));
+
+                    // Test heapsort using `>` operator.
+                    let mut tmp = v.clone();
+                    heapsort(&mut tmp, &|a, b| a > b);
+                    assert!(tmp.windows(2).all(|w| w[0] >= w[1]));
+                }
+            }
+        }
+
+        // Sort using a completely random comparison function.
+        // This will reorder the elements *somehow*, but won't panic.
+        let mut v: Vec<_> = (0..100).collect();
+        heapsort(&mut v, &|_, _| thread_rng().gen());
+        heapsort(&mut v, &|a, b| a < b);
+
+        for (i, &entry) in v.iter().enumerate() {
+            assert_eq!(entry, i);
+        }
+    }
+}