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 #[stable(feature = "chunks_exact", since = "1.31.0")]
127 pub use core::slice::{ChunksExact, ChunksExactMut};
128 #[stable(feature = "rchunks", since = "1.31.0")]
129 pub use core::slice::{RChunks, RChunksMut, RChunksExact, RChunksExactMut};
131 ////////////////////////////////////////////////////////////////////////////////
132 // Basic slice extension methods
133 ////////////////////////////////////////////////////////////////////////////////
135 // HACK(japaric) needed for the implementation of `vec!` macro during testing
136 // NB see the hack module in this file for more details
138 pub use self::hack::into_vec;
140 // HACK(japaric) needed for the implementation of `Vec::clone` during testing
141 // NB see the hack module in this file for more details
143 pub use self::hack::to_vec;
145 // HACK(japaric): With cfg(test) `impl [T]` is not available, these three
146 // functions are actually methods that are in `impl [T]` but not in
147 // `core::slice::SliceExt` - we need to supply these functions for the
148 // `test_permutations` test
154 use string::ToString;
157 pub fn into_vec<T>(mut b: Box<[T]>) -> Vec<T> {
159 let xs = Vec::from_raw_parts(b.as_mut_ptr(), b.len(), b.len());
166 pub fn to_vec<T>(s: &[T]) -> Vec<T>
169 let mut vector = Vec::with_capacity(s.len());
170 vector.extend_from_slice(s);
175 #[lang = "slice_alloc"]
180 /// This sort is stable (i.e., does not reorder equal elements) and `O(n log n)` worst-case.
182 /// When applicable, unstable sorting is preferred because it is generally faster than stable
183 /// sorting and it doesn't allocate auxiliary memory.
184 /// See [`sort_unstable`](#method.sort_unstable).
186 /// # Current implementation
188 /// The current algorithm is an adaptive, iterative merge sort inspired by
189 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
190 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
191 /// two or more sorted sequences concatenated one after another.
193 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
194 /// non-allocating insertion sort is used instead.
199 /// let mut v = [-5, 4, 1, -3, 2];
202 /// assert!(v == [-5, -3, 1, 2, 4]);
204 #[stable(feature = "rust1", since = "1.0.0")]
206 pub fn sort(&mut self)
209 merge_sort(self, |a, b| a.lt(b));
212 /// Sorts the slice with a comparator function.
214 /// This sort is stable (i.e., does not reorder equal elements) and `O(n log n)` worst-case.
216 /// The comparator function must define a total ordering for the elements in the slice. If
217 /// the ordering is not total, the order of the elements is unspecified. An order is a
218 /// total order if it is (for all a, b and c):
220 /// * total and antisymmetric: exactly one of a < b, a == b or a > b is true; and
221 /// * transitive, a < b and b < c implies a < c. The same must hold for both == and >.
223 /// For example, while [`f64`] doesn't implement [`Ord`] because `NaN != NaN`, we can use
224 /// `partial_cmp` as our sort function when we know the slice doesn't contain a `NaN`.
227 /// let mut floats = [5f64, 4.0, 1.0, 3.0, 2.0];
228 /// floats.sort_by(|a, b| a.partial_cmp(b).unwrap());
229 /// assert_eq!(floats, [1.0, 2.0, 3.0, 4.0, 5.0]);
232 /// When applicable, unstable sorting is preferred because it is generally faster than stable
233 /// sorting and it doesn't allocate auxiliary memory.
234 /// See [`sort_unstable_by`](#method.sort_unstable_by).
236 /// # Current implementation
238 /// The current algorithm is an adaptive, iterative merge sort inspired by
239 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
240 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
241 /// two or more sorted sequences concatenated one after another.
243 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
244 /// non-allocating insertion sort is used instead.
249 /// let mut v = [5, 4, 1, 3, 2];
250 /// v.sort_by(|a, b| a.cmp(b));
251 /// assert!(v == [1, 2, 3, 4, 5]);
253 /// // reverse sorting
254 /// v.sort_by(|a, b| b.cmp(a));
255 /// assert!(v == [5, 4, 3, 2, 1]);
257 #[stable(feature = "rust1", since = "1.0.0")]
259 pub fn sort_by<F>(&mut self, mut compare: F)
260 where F: FnMut(&T, &T) -> Ordering
262 merge_sort(self, |a, b| compare(a, b) == Less);
265 /// Sorts the slice with a key extraction function.
267 /// This sort is stable (i.e., does not reorder equal elements) and `O(m n log(m n))`
268 /// worst-case, where the key function is `O(m)`.
270 /// When applicable, unstable sorting is preferred because it is generally faster than stable
271 /// sorting and it doesn't allocate auxiliary memory.
272 /// See [`sort_unstable_by_key`](#method.sort_unstable_by_key).
274 /// # Current implementation
276 /// The current algorithm is an adaptive, iterative merge sort inspired by
277 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
278 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
279 /// two or more sorted sequences concatenated one after another.
281 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
282 /// non-allocating insertion sort is used instead.
287 /// let mut v = [-5i32, 4, 1, -3, 2];
289 /// v.sort_by_key(|k| k.abs());
290 /// assert!(v == [1, 2, -3, 4, -5]);
292 #[stable(feature = "slice_sort_by_key", since = "1.7.0")]
294 pub fn sort_by_key<K, F>(&mut self, mut f: F)
295 where F: FnMut(&T) -> K, K: Ord
297 merge_sort(self, |a, b| f(a).lt(&f(b)));
300 /// Sorts the slice with a key extraction function.
302 /// During sorting, the key function is called only once per element.
304 /// This sort is stable (i.e., does not reorder equal elements) and `O(m n + n log n)`
305 /// worst-case, where the key function is `O(m)`.
307 /// For simple key functions (e.g., functions that are property accesses or
308 /// basic operations), [`sort_by_key`](#method.sort_by_key) is likely to be
311 /// # Current implementation
313 /// The current algorithm is based on [pattern-defeating quicksort][pdqsort] by Orson Peters,
314 /// which combines the fast average case of randomized quicksort with the fast worst case of
315 /// heapsort, while achieving linear time on slices with certain patterns. It uses some
316 /// randomization to avoid degenerate cases, but with a fixed seed to always provide
317 /// deterministic behavior.
319 /// In the worst case, the algorithm allocates temporary storage in a `Vec<(K, usize)>` the
320 /// length of the slice.
325 /// #![feature(slice_sort_by_cached_key)]
326 /// let mut v = [-5i32, 4, 32, -3, 2];
328 /// v.sort_by_cached_key(|k| k.to_string());
329 /// assert!(v == [-3, -5, 2, 32, 4]);
332 /// [pdqsort]: https://github.com/orlp/pdqsort
333 #[unstable(feature = "slice_sort_by_cached_key", issue = "34447")]
335 pub fn sort_by_cached_key<K, F>(&mut self, f: F)
336 where F: FnMut(&T) -> K, K: Ord
338 // Helper macro for indexing our vector by the smallest possible type, to reduce allocation.
339 macro_rules! sort_by_key {
340 ($t:ty, $slice:ident, $f:ident) => ({
341 let mut indices: Vec<_> =
342 $slice.iter().map($f).enumerate().map(|(i, k)| (k, i as $t)).collect();
343 // The elements of `indices` are unique, as they are indexed, so any sort will be
344 // stable with respect to the original slice. We use `sort_unstable` here because
345 // it requires less memory allocation.
346 indices.sort_unstable();
347 for i in 0..$slice.len() {
348 let mut index = indices[i].1;
349 while (index as usize) < i {
350 index = indices[index as usize].1;
352 indices[i].1 = index;
353 $slice.swap(i, index as usize);
358 let sz_u8 = mem::size_of::<(K, u8)>();
359 let sz_u16 = mem::size_of::<(K, u16)>();
360 let sz_u32 = mem::size_of::<(K, u32)>();
361 let sz_usize = mem::size_of::<(K, usize)>();
363 let len = self.len();
364 if len < 2 { return }
365 if sz_u8 < sz_u16 && len <= ( u8::MAX as usize) { return sort_by_key!( u8, self, f) }
366 if sz_u16 < sz_u32 && len <= (u16::MAX as usize) { return sort_by_key!(u16, self, f) }
367 if sz_u32 < sz_usize && len <= (u32::MAX as usize) { return sort_by_key!(u32, self, f) }
368 sort_by_key!(usize, self, f)
371 /// Copies `self` into a new `Vec`.
376 /// let s = [10, 40, 30];
377 /// let x = s.to_vec();
378 /// // Here, `s` and `x` can be modified independently.
380 #[rustc_conversion_suggestion]
381 #[stable(feature = "rust1", since = "1.0.0")]
383 pub fn to_vec(&self) -> Vec<T>
386 // NB see hack module in this file
390 /// Converts `self` into a vector without clones or allocation.
392 /// The resulting vector can be converted back into a box via
393 /// `Vec<T>`'s `into_boxed_slice` method.
398 /// let s: Box<[i32]> = Box::new([10, 40, 30]);
399 /// let x = s.into_vec();
400 /// // `s` cannot be used anymore because it has been converted into `x`.
402 /// assert_eq!(x, vec![10, 40, 30]);
404 #[stable(feature = "rust1", since = "1.0.0")]
406 pub fn into_vec(self: Box<Self>) -> Vec<T> {
407 // NB see hack module in this file
411 /// Creates a vector by repeating a slice `n` times.
415 /// This function will panic if the capacity would overflow.
422 /// #![feature(repeat_generic_slice)]
425 /// assert_eq!([1, 2].repeat(3), vec![1, 2, 1, 2, 1, 2]);
429 /// A panic upon overflow:
432 /// #![feature(repeat_generic_slice)]
434 /// // this will panic at runtime
435 /// b"0123456789abcdef".repeat(usize::max_value());
438 #[unstable(feature = "repeat_generic_slice",
439 reason = "it's on str, why not on slice?",
441 pub fn repeat(&self, n: usize) -> Vec<T> where T: Copy {
446 // If `n` is larger than zero, it can be split as
447 // `n = 2^expn + rem (2^expn > rem, expn >= 0, rem >= 0)`.
448 // `2^expn` is the number represented by the leftmost '1' bit of `n`,
449 // and `rem` is the remaining part of `n`.
451 // Using `Vec` to access `set_len()`.
452 let mut buf = Vec::with_capacity(self.len().checked_mul(n).expect("capacity overflow"));
454 // `2^expn` repetition is done by doubling `buf` `expn`-times.
458 // If `m > 0`, there are remaining bits up to the leftmost '1'.
460 // `buf.extend(buf)`:
462 ptr::copy_nonoverlapping(
464 (buf.as_mut_ptr() as *mut T).add(buf.len()),
467 // `buf` has capacity of `self.len() * n`.
468 let buf_len = buf.len();
469 buf.set_len(buf_len * 2);
476 // `rem` (`= n - 2^expn`) repetition is done by copying
477 // first `rem` repetitions from `buf` itself.
478 let rem_len = self.len() * n - buf.len(); // `self.len() * rem`
480 // `buf.extend(buf[0 .. rem_len])`:
482 // This is non-overlapping since `2^expn > rem`.
483 ptr::copy_nonoverlapping(
485 (buf.as_mut_ptr() as *mut T).add(buf.len()),
488 // `buf.len() + rem_len` equals to `buf.capacity()` (`= self.len() * n`).
489 let buf_cap = buf.capacity();
490 buf.set_len(buf_cap);
497 #[lang = "slice_u8_alloc"]
500 /// Returns a vector containing a copy of this slice where each byte
501 /// is mapped to its ASCII upper case equivalent.
503 /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
504 /// but non-ASCII letters are unchanged.
506 /// To uppercase the value in-place, use [`make_ascii_uppercase`].
508 /// [`make_ascii_uppercase`]: #method.make_ascii_uppercase
509 #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
511 pub fn to_ascii_uppercase(&self) -> Vec<u8> {
512 let mut me = self.to_vec();
513 me.make_ascii_uppercase();
517 /// Returns a vector containing a copy of this slice where each byte
518 /// is mapped to its ASCII lower case equivalent.
520 /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
521 /// but non-ASCII letters are unchanged.
523 /// To lowercase the value in-place, use [`make_ascii_lowercase`].
525 /// [`make_ascii_lowercase`]: #method.make_ascii_lowercase
526 #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
528 pub fn to_ascii_lowercase(&self) -> Vec<u8> {
529 let mut me = self.to_vec();
530 me.make_ascii_lowercase();
535 ////////////////////////////////////////////////////////////////////////////////
536 // Extension traits for slices over specific kinds of data
537 ////////////////////////////////////////////////////////////////////////////////
538 #[unstable(feature = "slice_concat_ext",
539 reason = "trait should not have to exist",
541 /// An extension trait for concatenating slices
543 /// While this trait is unstable, the methods are stable. `SliceConcatExt` is
544 /// included in the [standard library prelude], so you can use [`join()`] and
545 /// [`concat()`] as if they existed on `[T]` itself.
547 /// [standard library prelude]: ../../std/prelude/index.html
548 /// [`join()`]: #tymethod.join
549 /// [`concat()`]: #tymethod.concat
550 pub trait SliceConcatExt<T: ?Sized> {
551 #[unstable(feature = "slice_concat_ext",
552 reason = "trait should not have to exist",
554 /// The resulting type after concatenation
557 /// Flattens a slice of `T` into a single value `Self::Output`.
562 /// assert_eq!(["hello", "world"].concat(), "helloworld");
563 /// assert_eq!([[1, 2], [3, 4]].concat(), [1, 2, 3, 4]);
565 #[stable(feature = "rust1", since = "1.0.0")]
566 fn concat(&self) -> Self::Output;
568 /// Flattens a slice of `T` into a single value `Self::Output`, placing a
569 /// given separator between each.
574 /// assert_eq!(["hello", "world"].join(" "), "hello world");
575 /// assert_eq!([[1, 2], [3, 4]].join(&0), [1, 2, 0, 3, 4]);
577 #[stable(feature = "rename_connect_to_join", since = "1.3.0")]
578 fn join(&self, sep: &T) -> Self::Output;
580 #[stable(feature = "rust1", since = "1.0.0")]
581 #[rustc_deprecated(since = "1.3.0", reason = "renamed to join")]
582 fn connect(&self, sep: &T) -> Self::Output;
585 #[unstable(feature = "slice_concat_ext",
586 reason = "trait should not have to exist",
588 impl<T: Clone, V: Borrow<[T]>> SliceConcatExt<T> for [V] {
589 type Output = Vec<T>;
591 fn concat(&self) -> Vec<T> {
592 let size = self.iter().fold(0, |acc, v| acc + v.borrow().len());
593 let mut result = Vec::with_capacity(size);
595 result.extend_from_slice(v.borrow())
600 fn join(&self, sep: &T) -> Vec<T> {
601 let mut iter = self.iter();
602 let first = match iter.next() {
603 Some(first) => first,
604 None => return vec![],
606 let size = self.iter().fold(0, |acc, v| acc + v.borrow().len());
607 let mut result = Vec::with_capacity(size + self.len());
608 result.extend_from_slice(first.borrow());
611 result.push(sep.clone());
612 result.extend_from_slice(v.borrow())
617 fn connect(&self, sep: &T) -> Vec<T> {
622 ////////////////////////////////////////////////////////////////////////////////
623 // Standard trait implementations for slices
624 ////////////////////////////////////////////////////////////////////////////////
626 #[stable(feature = "rust1", since = "1.0.0")]
627 impl<T> Borrow<[T]> for Vec<T> {
628 fn borrow(&self) -> &[T] {
633 #[stable(feature = "rust1", since = "1.0.0")]
634 impl<T> BorrowMut<[T]> for Vec<T> {
635 fn borrow_mut(&mut self) -> &mut [T] {
640 #[stable(feature = "rust1", since = "1.0.0")]
641 impl<T: Clone> ToOwned for [T] {
644 fn to_owned(&self) -> Vec<T> {
649 fn to_owned(&self) -> Vec<T> {
653 fn clone_into(&self, target: &mut Vec<T>) {
654 // drop anything in target that will not be overwritten
655 target.truncate(self.len());
656 let len = target.len();
658 // reuse the contained values' allocations/resources.
659 target.clone_from_slice(&self[..len]);
661 // target.len <= self.len due to the truncate above, so the
662 // slice here is always in-bounds.
663 target.extend_from_slice(&self[len..]);
667 ////////////////////////////////////////////////////////////////////////////////
669 ////////////////////////////////////////////////////////////////////////////////
671 /// Inserts `v[0]` into pre-sorted sequence `v[1..]` so that whole `v[..]` becomes sorted.
673 /// This is the integral subroutine of insertion sort.
674 fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
675 where F: FnMut(&T, &T) -> bool
677 if v.len() >= 2 && is_less(&v[1], &v[0]) {
679 // There are three ways to implement insertion here:
681 // 1. Swap adjacent elements until the first one gets to its final destination.
682 // However, this way we copy data around more than is necessary. If elements are big
683 // structures (costly to copy), this method will be slow.
685 // 2. Iterate until the right place for the first element is found. Then shift the
686 // elements succeeding it to make room for it and finally place it into the
687 // remaining hole. This is a good method.
689 // 3. Copy the first element into a temporary variable. Iterate until the right place
690 // for it is found. As we go along, copy every traversed element into the slot
691 // preceding it. Finally, copy data from the temporary variable into the remaining
692 // hole. This method is very good. Benchmarks demonstrated slightly better
693 // performance than with the 2nd method.
695 // All methods were benchmarked, and the 3rd showed best results. So we chose that one.
696 let mut tmp = mem::ManuallyDrop::new(ptr::read(&v[0]));
698 // Intermediate state of the insertion process is always tracked by `hole`, which
699 // serves two purposes:
700 // 1. Protects integrity of `v` from panics in `is_less`.
701 // 2. Fills the remaining hole in `v` in the end.
705 // If `is_less` panics at any point during the process, `hole` will get dropped and
706 // fill the hole in `v` with `tmp`, thus ensuring that `v` still holds every object it
707 // initially held exactly once.
708 let mut hole = InsertionHole {
712 ptr::copy_nonoverlapping(&v[1], &mut v[0], 1);
714 for i in 2..v.len() {
715 if !is_less(&v[i], &*tmp) {
718 ptr::copy_nonoverlapping(&v[i], &mut v[i - 1], 1);
719 hole.dest = &mut v[i];
721 // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
725 // When dropped, copies from `src` into `dest`.
726 struct InsertionHole<T> {
731 impl<T> Drop for InsertionHole<T> {
733 unsafe { ptr::copy_nonoverlapping(self.src, self.dest, 1); }
738 /// Merges non-decreasing runs `v[..mid]` and `v[mid..]` using `buf` as temporary storage, and
739 /// stores the result into `v[..]`.
743 /// The two slices must be non-empty and `mid` must be in bounds. Buffer `buf` must be long enough
744 /// to hold a copy of the shorter slice. Also, `T` must not be a zero-sized type.
745 unsafe fn merge<T, F>(v: &mut [T], mid: usize, buf: *mut T, is_less: &mut F)
746 where F: FnMut(&T, &T) -> bool
749 let v = v.as_mut_ptr();
750 let v_mid = v.add(mid);
751 let v_end = v.add(len);
753 // The merge process first copies the shorter run into `buf`. Then it traces the newly copied
754 // run and the longer run forwards (or backwards), comparing their next unconsumed elements and
755 // copying the lesser (or greater) one into `v`.
757 // As soon as the shorter run is fully consumed, the process is done. If the longer run gets
758 // consumed first, then we must copy whatever is left of the shorter run into the remaining
761 // Intermediate state of the process is always tracked by `hole`, which serves two purposes:
762 // 1. Protects integrity of `v` from panics in `is_less`.
763 // 2. Fills the remaining hole in `v` if the longer run gets consumed first.
767 // If `is_less` panics at any point during the process, `hole` will get dropped and fill the
768 // hole in `v` with the unconsumed range in `buf`, thus ensuring that `v` still holds every
769 // object it initially held exactly once.
772 if mid <= len - mid {
773 // The left run is shorter.
774 ptr::copy_nonoverlapping(v, buf, mid);
781 // Initially, these pointers point to the beginnings of their arrays.
782 let left = &mut hole.start;
783 let mut right = v_mid;
784 let out = &mut hole.dest;
786 while *left < hole.end && right < v_end {
787 // Consume the lesser side.
788 // If equal, prefer the left run to maintain stability.
789 let to_copy = if is_less(&*right, &**left) {
790 get_and_increment(&mut right)
792 get_and_increment(left)
794 ptr::copy_nonoverlapping(to_copy, get_and_increment(out), 1);
797 // The right run is shorter.
798 ptr::copy_nonoverlapping(v_mid, buf, len - mid);
801 end: buf.add(len - mid),
805 // Initially, these pointers point past the ends of their arrays.
806 let left = &mut hole.dest;
807 let right = &mut hole.end;
810 while v < *left && buf < *right {
811 // Consume the greater side.
812 // If equal, prefer the right run to maintain stability.
813 let to_copy = if is_less(&*right.offset(-1), &*left.offset(-1)) {
814 decrement_and_get(left)
816 decrement_and_get(right)
818 ptr::copy_nonoverlapping(to_copy, decrement_and_get(&mut out), 1);
821 // Finally, `hole` gets dropped. If the shorter run was not fully consumed, whatever remains of
822 // it will now be copied into the hole in `v`.
824 unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T {
826 *ptr = ptr.offset(1);
830 unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T {
831 *ptr = ptr.offset(-1);
835 // When dropped, copies the range `start..end` into `dest..`.
836 struct MergeHole<T> {
842 impl<T> Drop for MergeHole<T> {
844 // `T` is not a zero-sized type, so it's okay to divide by its size.
845 let len = (self.end as usize - self.start as usize) / mem::size_of::<T>();
846 unsafe { ptr::copy_nonoverlapping(self.start, self.dest, len); }
851 /// This merge sort borrows some (but not all) ideas from TimSort, which is described in detail
852 /// [here](http://svn.python.org/projects/python/trunk/Objects/listsort.txt).
854 /// The algorithm identifies strictly descending and non-descending subsequences, which are called
855 /// natural runs. There is a stack of pending runs yet to be merged. Each newly found run is pushed
856 /// onto the stack, and then some pairs of adjacent runs are merged until these two invariants are
859 /// 1. for every `i` in `1..runs.len()`: `runs[i - 1].len > runs[i].len`
860 /// 2. for every `i` in `2..runs.len()`: `runs[i - 2].len > runs[i - 1].len + runs[i].len`
862 /// The invariants ensure that the total running time is `O(n log n)` worst-case.
863 fn merge_sort<T, F>(v: &mut [T], mut is_less: F)
864 where F: FnMut(&T, &T) -> bool
866 // Slices of up to this length get sorted using insertion sort.
867 const MAX_INSERTION: usize = 20;
868 // Very short runs are extended using insertion sort to span at least this many elements.
869 const MIN_RUN: usize = 10;
871 // Sorting has no meaningful behavior on zero-sized types.
872 if size_of::<T>() == 0 {
878 // Short arrays get sorted in-place via insertion sort to avoid allocations.
879 if len <= MAX_INSERTION {
881 for i in (0..len-1).rev() {
882 insert_head(&mut v[i..], &mut is_less);
888 // Allocate a buffer to use as scratch memory. We keep the length 0 so we can keep in it
889 // shallow copies of the contents of `v` without risking the dtors running on copies if
890 // `is_less` panics. When merging two sorted runs, this buffer holds a copy of the shorter run,
891 // which will always have length at most `len / 2`.
892 let mut buf = Vec::with_capacity(len / 2);
894 // In order to identify natural runs in `v`, we traverse it backwards. That might seem like a
895 // strange decision, but consider the fact that merges more often go in the opposite direction
896 // (forwards). According to benchmarks, merging forwards is slightly faster than merging
897 // backwards. To conclude, identifying runs by traversing backwards improves performance.
898 let mut runs = vec![];
901 // Find the next natural run, and reverse it if it's strictly descending.
902 let mut start = end - 1;
906 if is_less(v.get_unchecked(start + 1), v.get_unchecked(start)) {
907 while start > 0 && is_less(v.get_unchecked(start),
908 v.get_unchecked(start - 1)) {
911 v[start..end].reverse();
913 while start > 0 && !is_less(v.get_unchecked(start),
914 v.get_unchecked(start - 1)) {
921 // Insert some more elements into the run if it's too short. Insertion sort is faster than
922 // merge sort on short sequences, so this significantly improves performance.
923 while start > 0 && end - start < MIN_RUN {
925 insert_head(&mut v[start..end], &mut is_less);
928 // Push this run onto the stack.
935 // Merge some pairs of adjacent runs to satisfy the invariants.
936 while let Some(r) = collapse(&runs) {
937 let left = runs[r + 1];
940 merge(&mut v[left.start .. right.start + right.len], left.len, buf.as_mut_ptr(),
945 len: left.len + right.len,
951 // Finally, exactly one run must remain in the stack.
952 debug_assert!(runs.len() == 1 && runs[0].start == 0 && runs[0].len == len);
954 // Examines the stack of runs and identifies the next pair of runs to merge. More specifically,
955 // if `Some(r)` is returned, that means `runs[r]` and `runs[r + 1]` must be merged next. If the
956 // algorithm should continue building a new run instead, `None` is returned.
958 // TimSort is infamous for its buggy implementations, as described here:
959 // http://envisage-project.eu/timsort-specification-and-verification/
961 // The gist of the story is: we must enforce the invariants on the top four runs on the stack.
962 // Enforcing them on just top three is not sufficient to ensure that the invariants will still
963 // hold for *all* runs in the stack.
965 // This function correctly checks invariants for the top four runs. Additionally, if the top
966 // run starts at index 0, it will always demand a merge operation until the stack is fully
967 // collapsed, in order to complete the sort.
969 fn collapse(runs: &[Run]) -> Option<usize> {
971 if n >= 2 && (runs[n - 1].start == 0 ||
972 runs[n - 2].len <= runs[n - 1].len ||
973 (n >= 3 && runs[n - 3].len <= runs[n - 2].len + runs[n - 1].len) ||
974 (n >= 4 && runs[n - 4].len <= runs[n - 3].len + runs[n - 2].len)) {
975 if n >= 3 && runs[n - 3].len < runs[n - 1].len {
985 #[derive(Clone, Copy)]