1 // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! A dynamically-sized view into a contiguous sequence, `[T]`.
13 //! *[See also the slice primitive type](../../std/primitive.slice.html).*
15 //! Slices are a view into a block of memory represented as a pointer and a
20 //! let vec = vec![1, 2, 3];
21 //! let int_slice = &vec[..];
22 //! // coercing an array to a slice
23 //! let str_slice: &[&str] = &["one", "two", "three"];
26 //! Slices are either mutable or shared. The shared slice type is `&[T]`,
27 //! while the mutable slice type is `&mut [T]`, where `T` represents the element
28 //! type. For example, you can mutate the block of memory that a mutable slice
32 //! let x = &mut [1, 2, 3];
34 //! assert_eq!(x, &[1, 7, 3]);
37 //! Here are some of the things this module contains:
41 //! There are several structs that are useful for slices, such as [`Iter`], which
42 //! represents iteration over a slice.
44 //! ## Trait Implementations
46 //! There are several implementations of common traits for slices. Some examples
50 //! * [`Eq`], [`Ord`] - for slices whose element type are [`Eq`] or [`Ord`].
51 //! * [`Hash`] - for slices whose element type is [`Hash`].
55 //! The slices implement `IntoIterator`. The iterator yields references to the
59 //! let numbers = &[0, 1, 2];
60 //! for n in numbers {
61 //! println!("{} is a number!", n);
65 //! The mutable slice yields mutable references to the elements:
68 //! let mut scores = [7, 8, 9];
69 //! for score in &mut scores[..] {
74 //! This iterator yields mutable references to the slice's elements, so while
75 //! the element type of the slice is `i32`, the element type of the iterator is
78 //! * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default
80 //! * Further methods that return iterators are [`.split`], [`.splitn`],
81 //! [`.chunks`], [`.windows`] and more.
83 //! [`Clone`]: ../../std/clone/trait.Clone.html
84 //! [`Eq`]: ../../std/cmp/trait.Eq.html
85 //! [`Ord`]: ../../std/cmp/trait.Ord.html
86 //! [`Iter`]: struct.Iter.html
87 //! [`Hash`]: ../../std/hash/trait.Hash.html
88 //! [`.iter`]: ../../std/primitive.slice.html#method.iter
89 //! [`.iter_mut`]: ../../std/primitive.slice.html#method.iter_mut
90 //! [`.split`]: ../../std/primitive.slice.html#method.split
91 //! [`.splitn`]: ../../std/primitive.slice.html#method.splitn
92 //! [`.chunks`]: ../../std/primitive.slice.html#method.chunks
93 //! [`.windows`]: ../../std/primitive.slice.html#method.windows
94 #![stable(feature = "rust1", since = "1.0.0")]
96 // Many of the usings in this module are only used in the test configuration.
97 // It's cleaner to just turn off the unused_imports warning than to fix them.
98 #![cfg_attr(test, allow(unused_imports, dead_code))]
100 use core::cmp::Ordering::{self, Less};
101 use core::mem::size_of;
104 use core::{u8, u16, u32};
106 use borrow::{Borrow, BorrowMut, ToOwned};
110 #[stable(feature = "rust1", since = "1.0.0")]
111 pub use core::slice::{Chunks, Windows};
112 #[stable(feature = "rust1", since = "1.0.0")]
113 pub use core::slice::{Iter, IterMut};
114 #[stable(feature = "rust1", since = "1.0.0")]
115 pub use core::slice::{SplitMut, ChunksMut, Split};
116 #[stable(feature = "rust1", since = "1.0.0")]
117 pub use core::slice::{SplitN, RSplitN, SplitNMut, RSplitNMut};
118 #[stable(feature = "slice_rsplit", since = "1.27.0")]
119 pub use core::slice::{RSplit, RSplitMut};
120 #[stable(feature = "rust1", since = "1.0.0")]
121 pub use core::slice::{from_raw_parts, from_raw_parts_mut};
122 #[stable(feature = "from_ref", since = "1.28.0")]
123 pub use core::slice::{from_ref, from_mut};
124 #[stable(feature = "slice_get_slice", since = "1.28.0")]
125 pub use core::slice::SliceIndex;
126 #[unstable(feature = "exact_chunks", issue = "47115")]
127 pub use core::slice::{ExactChunks, ExactChunksMut};
129 ////////////////////////////////////////////////////////////////////////////////
130 // Basic slice extension methods
131 ////////////////////////////////////////////////////////////////////////////////
133 // HACK(japaric) needed for the implementation of `vec!` macro during testing
134 // NB see the hack module in this file for more details
136 pub use self::hack::into_vec;
138 // HACK(japaric) needed for the implementation of `Vec::clone` during testing
139 // NB see the hack module in this file for more details
141 pub use self::hack::to_vec;
143 // HACK(japaric): With cfg(test) `impl [T]` is not available, these three
144 // functions are actually methods that are in `impl [T]` but not in
145 // `core::slice::SliceExt` - we need to supply these functions for the
146 // `test_permutations` test
152 use string::ToString;
155 pub fn into_vec<T>(mut b: Box<[T]>) -> Vec<T> {
157 let xs = Vec::from_raw_parts(b.as_mut_ptr(), b.len(), b.len());
164 pub fn to_vec<T>(s: &[T]) -> Vec<T>
167 let mut vector = Vec::with_capacity(s.len());
168 vector.extend_from_slice(s);
173 #[lang = "slice_alloc"]
178 /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
180 /// When applicable, unstable sorting is preferred because it is generally faster than stable
181 /// sorting and it doesn't allocate auxiliary memory.
182 /// See [`sort_unstable`](#method.sort_unstable).
184 /// # Current implementation
186 /// The current algorithm is an adaptive, iterative merge sort inspired by
187 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
188 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
189 /// two or more sorted sequences concatenated one after another.
191 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
192 /// non-allocating insertion sort is used instead.
197 /// let mut v = [-5, 4, 1, -3, 2];
200 /// assert!(v == [-5, -3, 1, 2, 4]);
202 #[stable(feature = "rust1", since = "1.0.0")]
204 pub fn sort(&mut self)
207 merge_sort(self, |a, b| a.lt(b));
210 /// Sorts the slice with a comparator function.
212 /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
214 /// When applicable, unstable sorting is preferred because it is generally faster than stable
215 /// sorting and it doesn't allocate auxiliary memory.
216 /// See [`sort_unstable_by`](#method.sort_unstable_by).
218 /// # Current implementation
220 /// The current algorithm is an adaptive, iterative merge sort inspired by
221 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
222 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
223 /// two or more sorted sequences concatenated one after another.
225 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
226 /// non-allocating insertion sort is used instead.
231 /// let mut v = [5, 4, 1, 3, 2];
232 /// v.sort_by(|a, b| a.cmp(b));
233 /// assert!(v == [1, 2, 3, 4, 5]);
235 /// // reverse sorting
236 /// v.sort_by(|a, b| b.cmp(a));
237 /// assert!(v == [5, 4, 3, 2, 1]);
239 #[stable(feature = "rust1", since = "1.0.0")]
241 pub fn sort_by<F>(&mut self, mut compare: F)
242 where F: FnMut(&T, &T) -> Ordering
244 merge_sort(self, |a, b| compare(a, b) == Less);
247 /// Sorts the slice with a key extraction function.
249 /// This sort is stable (i.e. does not reorder equal elements) and `O(m n log(m n))`
250 /// worst-case, where the key function is `O(m)`.
252 /// When applicable, unstable sorting is preferred because it is generally faster than stable
253 /// sorting and it doesn't allocate auxiliary memory.
254 /// See [`sort_unstable_by_key`](#method.sort_unstable_by_key).
256 /// # Current implementation
258 /// The current algorithm is an adaptive, iterative merge sort inspired by
259 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
260 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
261 /// two or more sorted sequences concatenated one after another.
263 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
264 /// non-allocating insertion sort is used instead.
269 /// let mut v = [-5i32, 4, 1, -3, 2];
271 /// v.sort_by_key(|k| k.abs());
272 /// assert!(v == [1, 2, -3, 4, -5]);
274 #[stable(feature = "slice_sort_by_key", since = "1.7.0")]
276 pub fn sort_by_key<K, F>(&mut self, mut f: F)
277 where F: FnMut(&T) -> K, K: Ord
279 merge_sort(self, |a, b| f(a).lt(&f(b)));
282 /// Sorts the slice with a key extraction function.
284 /// During sorting, the key function is called only once per element.
286 /// This sort is stable (i.e. does not reorder equal elements) and `O(m n + n log n)`
287 /// worst-case, where the key function is `O(m)`.
289 /// For simple key functions (e.g. functions that are property accesses or
290 /// basic operations), [`sort_by_key`](#method.sort_by_key) is likely to be
293 /// # Current implementation
295 /// The current algorithm is based on [pattern-defeating quicksort][pdqsort] by Orson Peters,
296 /// which combines the fast average case of randomized quicksort with the fast worst case of
297 /// heapsort, while achieving linear time on slices with certain patterns. It uses some
298 /// randomization to avoid degenerate cases, but with a fixed seed to always provide
299 /// deterministic behavior.
301 /// In the worst case, the algorithm allocates temporary storage in a `Vec<(K, usize)>` the
302 /// length of the slice.
307 /// #![feature(slice_sort_by_cached_key)]
308 /// let mut v = [-5i32, 4, 32, -3, 2];
310 /// v.sort_by_cached_key(|k| k.to_string());
311 /// assert!(v == [-3, -5, 2, 32, 4]);
314 /// [pdqsort]: https://github.com/orlp/pdqsort
315 #[unstable(feature = "slice_sort_by_cached_key", issue = "34447")]
317 pub fn sort_by_cached_key<K, F>(&mut self, f: F)
318 where F: FnMut(&T) -> K, K: Ord
320 // Helper macro for indexing our vector by the smallest possible type, to reduce allocation.
321 macro_rules! sort_by_key {
322 ($t:ty, $slice:ident, $f:ident) => ({
323 let mut indices: Vec<_> =
324 $slice.iter().map($f).enumerate().map(|(i, k)| (k, i as $t)).collect();
325 // The elements of `indices` are unique, as they are indexed, so any sort will be
326 // stable with respect to the original slice. We use `sort_unstable` here because
327 // it requires less memory allocation.
328 indices.sort_unstable();
329 for i in 0..$slice.len() {
330 let mut index = indices[i].1;
331 while (index as usize) < i {
332 index = indices[index as usize].1;
334 indices[i].1 = index;
335 $slice.swap(i, index as usize);
340 let sz_u8 = mem::size_of::<(K, u8)>();
341 let sz_u16 = mem::size_of::<(K, u16)>();
342 let sz_u32 = mem::size_of::<(K, u32)>();
343 let sz_usize = mem::size_of::<(K, usize)>();
345 let len = self.len();
346 if len < 2 { return }
347 if sz_u8 < sz_u16 && len <= ( u8::MAX as usize) { return sort_by_key!( u8, self, f) }
348 if sz_u16 < sz_u32 && len <= (u16::MAX as usize) { return sort_by_key!(u16, self, f) }
349 if sz_u32 < sz_usize && len <= (u32::MAX as usize) { return sort_by_key!(u32, self, f) }
350 sort_by_key!(usize, self, f)
353 /// Copies `self` into a new `Vec`.
358 /// let s = [10, 40, 30];
359 /// let x = s.to_vec();
360 /// // Here, `s` and `x` can be modified independently.
362 #[rustc_conversion_suggestion]
363 #[stable(feature = "rust1", since = "1.0.0")]
365 pub fn to_vec(&self) -> Vec<T>
368 // NB see hack module in this file
372 /// Converts `self` into a vector without clones or allocation.
374 /// The resulting vector can be converted back into a box via
375 /// `Vec<T>`'s `into_boxed_slice` method.
380 /// let s: Box<[i32]> = Box::new([10, 40, 30]);
381 /// let x = s.into_vec();
382 /// // `s` cannot be used anymore because it has been converted into `x`.
384 /// assert_eq!(x, vec![10, 40, 30]);
386 #[stable(feature = "rust1", since = "1.0.0")]
388 pub fn into_vec(self: Box<Self>) -> Vec<T> {
389 // NB see hack module in this file
393 /// Creates a vector by repeating a slice `n` times.
397 /// This function will panic if the capacity would overflow.
404 /// #![feature(repeat_generic_slice)]
407 /// assert_eq!([1, 2].repeat(3), vec![1, 2, 1, 2, 1, 2]);
411 /// A panic upon overflow:
414 /// #![feature(repeat_generic_slice)]
416 /// // this will panic at runtime
417 /// b"0123456789abcdef".repeat(usize::max_value());
420 #[unstable(feature = "repeat_generic_slice",
421 reason = "it's on str, why not on slice?",
423 pub fn repeat(&self, n: usize) -> Vec<T> where T: Copy {
428 // If `n` is larger than zero, it can be split as
429 // `n = 2^expn + rem (2^expn > rem, expn >= 0, rem >= 0)`.
430 // `2^expn` is the number represented by the leftmost '1' bit of `n`,
431 // and `rem` is the remaining part of `n`.
433 // Using `Vec` to access `set_len()`.
434 let mut buf = Vec::with_capacity(self.len().checked_mul(n).expect("capacity overflow"));
436 // `2^expn` repetition is done by doubling `buf` `expn`-times.
440 // If `m > 0`, there are remaining bits up to the leftmost '1'.
442 // `buf.extend(buf)`:
444 ptr::copy_nonoverlapping(
446 (buf.as_mut_ptr() as *mut T).add(buf.len()),
449 // `buf` has capacity of `self.len() * n`.
450 let buf_len = buf.len();
451 buf.set_len(buf_len * 2);
458 // `rem` (`= n - 2^expn`) repetition is done by copying
459 // first `rem` repetitions from `buf` itself.
460 let rem_len = self.len() * n - buf.len(); // `self.len() * rem`
462 // `buf.extend(buf[0 .. rem_len])`:
464 // This is non-overlapping since `2^expn > rem`.
465 ptr::copy_nonoverlapping(
467 (buf.as_mut_ptr() as *mut T).add(buf.len()),
470 // `buf.len() + rem_len` equals to `buf.capacity()` (`= self.len() * n`).
471 let buf_cap = buf.capacity();
472 buf.set_len(buf_cap);
479 #[lang = "slice_u8_alloc"]
482 /// Returns a vector containing a copy of this slice where each byte
483 /// is mapped to its ASCII upper case equivalent.
485 /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
486 /// but non-ASCII letters are unchanged.
488 /// To uppercase the value in-place, use [`make_ascii_uppercase`].
490 /// [`make_ascii_uppercase`]: #method.make_ascii_uppercase
491 #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
493 pub fn to_ascii_uppercase(&self) -> Vec<u8> {
494 let mut me = self.to_vec();
495 me.make_ascii_uppercase();
499 /// Returns a vector containing a copy of this slice where each byte
500 /// is mapped to its ASCII lower case equivalent.
502 /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
503 /// but non-ASCII letters are unchanged.
505 /// To lowercase the value in-place, use [`make_ascii_lowercase`].
507 /// [`make_ascii_lowercase`]: #method.make_ascii_lowercase
508 #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
510 pub fn to_ascii_lowercase(&self) -> Vec<u8> {
511 let mut me = self.to_vec();
512 me.make_ascii_lowercase();
517 ////////////////////////////////////////////////////////////////////////////////
518 // Extension traits for slices over specific kinds of data
519 ////////////////////////////////////////////////////////////////////////////////
520 #[unstable(feature = "slice_concat_ext",
521 reason = "trait should not have to exist",
523 /// An extension trait for concatenating slices
525 /// While this trait is unstable, the methods are stable. `SliceConcatExt` is
526 /// included in the [standard library prelude], so you can use [`join()`] and
527 /// [`concat()`] as if they existed on `[T]` itself.
529 /// [standard library prelude]: ../../std/prelude/index.html
530 /// [`join()`]: #tymethod.join
531 /// [`concat()`]: #tymethod.concat
532 pub trait SliceConcatExt<T: ?Sized> {
533 #[unstable(feature = "slice_concat_ext",
534 reason = "trait should not have to exist",
536 /// The resulting type after concatenation
539 /// Flattens a slice of `T` into a single value `Self::Output`.
544 /// assert_eq!(["hello", "world"].concat(), "helloworld");
545 /// assert_eq!([[1, 2], [3, 4]].concat(), [1, 2, 3, 4]);
547 #[stable(feature = "rust1", since = "1.0.0")]
548 fn concat(&self) -> Self::Output;
550 /// Flattens a slice of `T` into a single value `Self::Output`, placing a
551 /// given separator between each.
556 /// assert_eq!(["hello", "world"].join(" "), "hello world");
557 /// assert_eq!([[1, 2], [3, 4]].join(&0), [1, 2, 0, 3, 4]);
559 #[stable(feature = "rename_connect_to_join", since = "1.3.0")]
560 fn join(&self, sep: &T) -> Self::Output;
562 #[stable(feature = "rust1", since = "1.0.0")]
563 #[rustc_deprecated(since = "1.3.0", reason = "renamed to join")]
564 fn connect(&self, sep: &T) -> Self::Output;
567 #[unstable(feature = "slice_concat_ext",
568 reason = "trait should not have to exist",
570 impl<T: Clone, V: Borrow<[T]>> SliceConcatExt<T> for [V] {
571 type Output = Vec<T>;
573 fn concat(&self) -> Vec<T> {
574 let size = self.iter().fold(0, |acc, v| acc + v.borrow().len());
575 let mut result = Vec::with_capacity(size);
577 result.extend_from_slice(v.borrow())
582 fn join(&self, sep: &T) -> Vec<T> {
583 let mut iter = self.iter();
584 let first = match iter.next() {
585 Some(first) => first,
586 None => return vec![],
588 let size = self.iter().fold(0, |acc, v| acc + v.borrow().len());
589 let mut result = Vec::with_capacity(size + self.len());
590 result.extend_from_slice(first.borrow());
593 result.push(sep.clone());
594 result.extend_from_slice(v.borrow())
599 fn connect(&self, sep: &T) -> Vec<T> {
604 ////////////////////////////////////////////////////////////////////////////////
605 // Standard trait implementations for slices
606 ////////////////////////////////////////////////////////////////////////////////
608 #[stable(feature = "rust1", since = "1.0.0")]
609 impl<T> Borrow<[T]> for Vec<T> {
610 fn borrow(&self) -> &[T] {
615 #[stable(feature = "rust1", since = "1.0.0")]
616 impl<T> BorrowMut<[T]> for Vec<T> {
617 fn borrow_mut(&mut self) -> &mut [T] {
622 #[stable(feature = "rust1", since = "1.0.0")]
623 impl<T: Clone> ToOwned for [T] {
626 fn to_owned(&self) -> Vec<T> {
631 fn to_owned(&self) -> Vec<T> {
635 fn clone_into(&self, target: &mut Vec<T>) {
636 // drop anything in target that will not be overwritten
637 target.truncate(self.len());
638 let len = target.len();
640 // reuse the contained values' allocations/resources.
641 target.clone_from_slice(&self[..len]);
643 // target.len <= self.len due to the truncate above, so the
644 // slice here is always in-bounds.
645 target.extend_from_slice(&self[len..]);
649 ////////////////////////////////////////////////////////////////////////////////
651 ////////////////////////////////////////////////////////////////////////////////
653 /// Inserts `v[0]` into pre-sorted sequence `v[1..]` so that whole `v[..]` becomes sorted.
655 /// This is the integral subroutine of insertion sort.
656 fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
657 where F: FnMut(&T, &T) -> bool
659 if v.len() >= 2 && is_less(&v[1], &v[0]) {
661 // There are three ways to implement insertion here:
663 // 1. Swap adjacent elements until the first one gets to its final destination.
664 // However, this way we copy data around more than is necessary. If elements are big
665 // structures (costly to copy), this method will be slow.
667 // 2. Iterate until the right place for the first element is found. Then shift the
668 // elements succeeding it to make room for it and finally place it into the
669 // remaining hole. This is a good method.
671 // 3. Copy the first element into a temporary variable. Iterate until the right place
672 // for it is found. As we go along, copy every traversed element into the slot
673 // preceding it. Finally, copy data from the temporary variable into the remaining
674 // hole. This method is very good. Benchmarks demonstrated slightly better
675 // performance than with the 2nd method.
677 // All methods were benchmarked, and the 3rd showed best results. So we chose that one.
678 let mut tmp = mem::ManuallyDrop::new(ptr::read(&v[0]));
680 // Intermediate state of the insertion process is always tracked by `hole`, which
681 // serves two purposes:
682 // 1. Protects integrity of `v` from panics in `is_less`.
683 // 2. Fills the remaining hole in `v` in the end.
687 // If `is_less` panics at any point during the process, `hole` will get dropped and
688 // fill the hole in `v` with `tmp`, thus ensuring that `v` still holds every object it
689 // initially held exactly once.
690 let mut hole = InsertionHole {
694 ptr::copy_nonoverlapping(&v[1], &mut v[0], 1);
696 for i in 2..v.len() {
697 if !is_less(&v[i], &*tmp) {
700 ptr::copy_nonoverlapping(&v[i], &mut v[i - 1], 1);
701 hole.dest = &mut v[i];
703 // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
707 // When dropped, copies from `src` into `dest`.
708 struct InsertionHole<T> {
713 impl<T> Drop for InsertionHole<T> {
715 unsafe { ptr::copy_nonoverlapping(self.src, self.dest, 1); }
720 /// Merges non-decreasing runs `v[..mid]` and `v[mid..]` using `buf` as temporary storage, and
721 /// stores the result into `v[..]`.
725 /// The two slices must be non-empty and `mid` must be in bounds. Buffer `buf` must be long enough
726 /// to hold a copy of the shorter slice. Also, `T` must not be a zero-sized type.
727 unsafe fn merge<T, F>(v: &mut [T], mid: usize, buf: *mut T, is_less: &mut F)
728 where F: FnMut(&T, &T) -> bool
731 let v = v.as_mut_ptr();
732 let v_mid = v.add(mid);
733 let v_end = v.add(len);
735 // The merge process first copies the shorter run into `buf`. Then it traces the newly copied
736 // run and the longer run forwards (or backwards), comparing their next unconsumed elements and
737 // copying the lesser (or greater) one into `v`.
739 // As soon as the shorter run is fully consumed, the process is done. If the longer run gets
740 // consumed first, then we must copy whatever is left of the shorter run into the remaining
743 // Intermediate state of the process is always tracked by `hole`, which serves two purposes:
744 // 1. Protects integrity of `v` from panics in `is_less`.
745 // 2. Fills the remaining hole in `v` if the longer run gets consumed first.
749 // If `is_less` panics at any point during the process, `hole` will get dropped and fill the
750 // hole in `v` with the unconsumed range in `buf`, thus ensuring that `v` still holds every
751 // object it initially held exactly once.
754 if mid <= len - mid {
755 // The left run is shorter.
756 ptr::copy_nonoverlapping(v, buf, mid);
763 // Initially, these pointers point to the beginnings of their arrays.
764 let left = &mut hole.start;
765 let mut right = v_mid;
766 let out = &mut hole.dest;
768 while *left < hole.end && right < v_end {
769 // Consume the lesser side.
770 // If equal, prefer the left run to maintain stability.
771 let to_copy = if is_less(&*right, &**left) {
772 get_and_increment(&mut right)
774 get_and_increment(left)
776 ptr::copy_nonoverlapping(to_copy, get_and_increment(out), 1);
779 // The right run is shorter.
780 ptr::copy_nonoverlapping(v_mid, buf, len - mid);
783 end: buf.add(len - mid),
787 // Initially, these pointers point past the ends of their arrays.
788 let left = &mut hole.dest;
789 let right = &mut hole.end;
792 while v < *left && buf < *right {
793 // Consume the greater side.
794 // If equal, prefer the right run to maintain stability.
795 let to_copy = if is_less(&*right.offset(-1), &*left.offset(-1)) {
796 decrement_and_get(left)
798 decrement_and_get(right)
800 ptr::copy_nonoverlapping(to_copy, decrement_and_get(&mut out), 1);
803 // Finally, `hole` gets dropped. If the shorter run was not fully consumed, whatever remains of
804 // it will now be copied into the hole in `v`.
806 unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T {
808 *ptr = ptr.offset(1);
812 unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T {
813 *ptr = ptr.offset(-1);
817 // When dropped, copies the range `start..end` into `dest..`.
818 struct MergeHole<T> {
824 impl<T> Drop for MergeHole<T> {
826 // `T` is not a zero-sized type, so it's okay to divide by its size.
827 let len = (self.end as usize - self.start as usize) / mem::size_of::<T>();
828 unsafe { ptr::copy_nonoverlapping(self.start, self.dest, len); }
833 /// This merge sort borrows some (but not all) ideas from TimSort, which is described in detail
834 /// [here](http://svn.python.org/projects/python/trunk/Objects/listsort.txt).
836 /// The algorithm identifies strictly descending and non-descending subsequences, which are called
837 /// natural runs. There is a stack of pending runs yet to be merged. Each newly found run is pushed
838 /// onto the stack, and then some pairs of adjacent runs are merged until these two invariants are
841 /// 1. for every `i` in `1..runs.len()`: `runs[i - 1].len > runs[i].len`
842 /// 2. for every `i` in `2..runs.len()`: `runs[i - 2].len > runs[i - 1].len + runs[i].len`
844 /// The invariants ensure that the total running time is `O(n log n)` worst-case.
845 fn merge_sort<T, F>(v: &mut [T], mut is_less: F)
846 where F: FnMut(&T, &T) -> bool
848 // Slices of up to this length get sorted using insertion sort.
849 const MAX_INSERTION: usize = 20;
850 // Very short runs are extended using insertion sort to span at least this many elements.
851 const MIN_RUN: usize = 10;
853 // Sorting has no meaningful behavior on zero-sized types.
854 if size_of::<T>() == 0 {
860 // Short arrays get sorted in-place via insertion sort to avoid allocations.
861 if len <= MAX_INSERTION {
863 for i in (0..len-1).rev() {
864 insert_head(&mut v[i..], &mut is_less);
870 // Allocate a buffer to use as scratch memory. We keep the length 0 so we can keep in it
871 // shallow copies of the contents of `v` without risking the dtors running on copies if
872 // `is_less` panics. When merging two sorted runs, this buffer holds a copy of the shorter run,
873 // which will always have length at most `len / 2`.
874 let mut buf = Vec::with_capacity(len / 2);
876 // In order to identify natural runs in `v`, we traverse it backwards. That might seem like a
877 // strange decision, but consider the fact that merges more often go in the opposite direction
878 // (forwards). According to benchmarks, merging forwards is slightly faster than merging
879 // backwards. To conclude, identifying runs by traversing backwards improves performance.
880 let mut runs = vec![];
883 // Find the next natural run, and reverse it if it's strictly descending.
884 let mut start = end - 1;
888 if is_less(v.get_unchecked(start + 1), v.get_unchecked(start)) {
889 while start > 0 && is_less(v.get_unchecked(start),
890 v.get_unchecked(start - 1)) {
893 v[start..end].reverse();
895 while start > 0 && !is_less(v.get_unchecked(start),
896 v.get_unchecked(start - 1)) {
903 // Insert some more elements into the run if it's too short. Insertion sort is faster than
904 // merge sort on short sequences, so this significantly improves performance.
905 while start > 0 && end - start < MIN_RUN {
907 insert_head(&mut v[start..end], &mut is_less);
910 // Push this run onto the stack.
917 // Merge some pairs of adjacent runs to satisfy the invariants.
918 while let Some(r) = collapse(&runs) {
919 let left = runs[r + 1];
922 merge(&mut v[left.start .. right.start + right.len], left.len, buf.as_mut_ptr(),
927 len: left.len + right.len,
933 // Finally, exactly one run must remain in the stack.
934 debug_assert!(runs.len() == 1 && runs[0].start == 0 && runs[0].len == len);
936 // Examines the stack of runs and identifies the next pair of runs to merge. More specifically,
937 // if `Some(r)` is returned, that means `runs[r]` and `runs[r + 1]` must be merged next. If the
938 // algorithm should continue building a new run instead, `None` is returned.
940 // TimSort is infamous for its buggy implementations, as described here:
941 // http://envisage-project.eu/timsort-specification-and-verification/
943 // The gist of the story is: we must enforce the invariants on the top four runs on the stack.
944 // Enforcing them on just top three is not sufficient to ensure that the invariants will still
945 // hold for *all* runs in the stack.
947 // This function correctly checks invariants for the top four runs. Additionally, if the top
948 // run starts at index 0, it will always demand a merge operation until the stack is fully
949 // collapsed, in order to complete the sort.
951 fn collapse(runs: &[Run]) -> Option<usize> {
953 if n >= 2 && (runs[n - 1].start == 0 ||
954 runs[n - 2].len <= runs[n - 1].len ||
955 (n >= 3 && runs[n - 3].len <= runs[n - 2].len + runs[n - 1].len) ||
956 (n >= 4 && runs[n - 4].len <= runs[n - 3].len + runs[n - 2].len)) {
957 if n >= 3 && runs[n - 3].len < runs[n - 1].len {
967 #[derive(Clone, Copy)]