1 //! A dynamically-sized view into a contiguous sequence, `[T]`.
3 //! *[See also the slice primitive type](../../std/primitive.slice.html).*
5 //! Slices are a view into a block of memory represented as a pointer and a
10 //! let vec = vec![1, 2, 3];
11 //! let int_slice = &vec[..];
12 //! // coercing an array to a slice
13 //! let str_slice: &[&str] = &["one", "two", "three"];
16 //! Slices are either mutable or shared. The shared slice type is `&[T]`,
17 //! while the mutable slice type is `&mut [T]`, where `T` represents the element
18 //! type. For example, you can mutate the block of memory that a mutable slice
22 //! let x = &mut [1, 2, 3];
24 //! assert_eq!(x, &[1, 7, 3]);
27 //! Here are some of the things this module contains:
31 //! There are several structs that are useful for slices, such as [`Iter`], which
32 //! represents iteration over a slice.
34 //! ## Trait Implementations
36 //! There are several implementations of common traits for slices. Some examples
40 //! * [`Eq`], [`Ord`] - for slices whose element type are [`Eq`] or [`Ord`].
41 //! * [`Hash`] - for slices whose element type is [`Hash`].
45 //! The slices implement `IntoIterator`. The iterator yields references to the
49 //! let numbers = &[0, 1, 2];
50 //! for n in numbers {
51 //! println!("{} is a number!", n);
55 //! The mutable slice yields mutable references to the elements:
58 //! let mut scores = [7, 8, 9];
59 //! for score in &mut scores[..] {
64 //! This iterator yields mutable references to the slice's elements, so while
65 //! the element type of the slice is `i32`, the element type of the iterator is
68 //! * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default
70 //! * Further methods that return iterators are [`.split`], [`.splitn`],
71 //! [`.chunks`], [`.windows`] and more.
73 //! [`Clone`]: ../../std/clone/trait.Clone.html
74 //! [`Eq`]: ../../std/cmp/trait.Eq.html
75 //! [`Ord`]: ../../std/cmp/trait.Ord.html
76 //! [`Iter`]: struct.Iter.html
77 //! [`Hash`]: ../../std/hash/trait.Hash.html
78 //! [`.iter`]: ../../std/primitive.slice.html#method.iter
79 //! [`.iter_mut`]: ../../std/primitive.slice.html#method.iter_mut
80 //! [`.split`]: ../../std/primitive.slice.html#method.split
81 //! [`.splitn`]: ../../std/primitive.slice.html#method.splitn
82 //! [`.chunks`]: ../../std/primitive.slice.html#method.chunks
83 //! [`.windows`]: ../../std/primitive.slice.html#method.windows
84 #![stable(feature = "rust1", since = "1.0.0")]
85 // Many of the usings in this module are only used in the test configuration.
86 // It's cleaner to just turn off the unused_imports warning than to fix them.
87 #![cfg_attr(test, allow(unused_imports, dead_code))]
89 use core::borrow::{Borrow, BorrowMut};
90 use core::cmp::Ordering::{self, Less};
91 use core::mem::{self, size_of};
94 use crate::borrow::ToOwned;
95 use crate::boxed::Box;
98 #[stable(feature = "slice_get_slice", since = "1.28.0")]
99 pub use core::slice::SliceIndex;
100 #[stable(feature = "from_ref", since = "1.28.0")]
101 pub use core::slice::{from_mut, from_ref};
102 #[stable(feature = "rust1", since = "1.0.0")]
103 pub use core::slice::{from_raw_parts, from_raw_parts_mut};
104 #[stable(feature = "rust1", since = "1.0.0")]
105 pub use core::slice::{Chunks, Windows};
106 #[stable(feature = "chunks_exact", since = "1.31.0")]
107 pub use core::slice::{ChunksExact, ChunksExactMut};
108 #[stable(feature = "rust1", since = "1.0.0")]
109 pub use core::slice::{ChunksMut, Split, SplitMut};
110 #[stable(feature = "rust1", since = "1.0.0")]
111 pub use core::slice::{Iter, IterMut};
112 #[stable(feature = "rchunks", since = "1.31.0")]
113 pub use core::slice::{RChunks, RChunksExact, RChunksExactMut, RChunksMut};
114 #[stable(feature = "slice_rsplit", since = "1.27.0")]
115 pub use core::slice::{RSplit, RSplitMut};
116 #[stable(feature = "rust1", since = "1.0.0")]
117 pub use core::slice::{RSplitN, RSplitNMut, SplitN, SplitNMut};
119 ////////////////////////////////////////////////////////////////////////////////
120 // Basic slice extension methods
121 ////////////////////////////////////////////////////////////////////////////////
123 // HACK(japaric) needed for the implementation of `vec!` macro during testing
124 // N.B., see the `hack` module in this file for more details.
126 pub use hack::into_vec;
128 // HACK(japaric) needed for the implementation of `Vec::clone` during testing
129 // N.B., see the `hack` module in this file for more details.
131 pub use hack::to_vec;
133 // HACK(japaric): With cfg(test) `impl [T]` is not available, these three
134 // functions are actually methods that are in `impl [T]` but not in
135 // `core::slice::SliceExt` - we need to supply these functions for the
136 // `test_permutations` test
138 use crate::boxed::Box;
141 // We shouldn't add inline attribute to this since this is used in
142 // `vec!` macro mostly and causes perf regression. See #71204 for
143 // discussion and perf results.
144 pub fn into_vec<T>(b: Box<[T]>) -> Vec<T> {
147 let b = Box::into_raw(b);
148 Vec::from_raw_parts(b as *mut T, len, len)
153 pub fn to_vec<T>(s: &[T]) -> Vec<T>
157 let mut vec = Vec::with_capacity(s.len());
158 vec.extend_from_slice(s);
163 #[lang = "slice_alloc"]
168 /// This sort is stable (i.e., does not reorder equal elements) and `O(n * log(n))` worst-case.
170 /// When applicable, unstable sorting is preferred because it is generally faster than stable
171 /// sorting and it doesn't allocate auxiliary memory.
172 /// See [`sort_unstable`](#method.sort_unstable).
174 /// # Current implementation
176 /// The current algorithm is an adaptive, iterative merge sort inspired by
177 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
178 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
179 /// two or more sorted sequences concatenated one after another.
181 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
182 /// non-allocating insertion sort is used instead.
187 /// let mut v = [-5, 4, 1, -3, 2];
190 /// assert!(v == [-5, -3, 1, 2, 4]);
192 #[stable(feature = "rust1", since = "1.0.0")]
194 pub fn sort(&mut self)
198 merge_sort(self, |a, b| a.lt(b));
201 /// Sorts the slice with a comparator function.
203 /// This sort is stable (i.e., does not reorder equal elements) and `O(n * log(n))` worst-case.
205 /// The comparator function must define a total ordering for the elements in the slice. If
206 /// the ordering is not total, the order of the elements is unspecified. An order is a
207 /// total order if it is (for all `a`, `b` and `c`):
209 /// * total and antisymmetric: exactly one of `a < b`, `a == b` or `a > b` is true, and
210 /// * transitive, `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
212 /// For example, while [`f64`] doesn't implement [`Ord`] because `NaN != NaN`, we can use
213 /// `partial_cmp` as our sort function when we know the slice doesn't contain a `NaN`.
216 /// let mut floats = [5f64, 4.0, 1.0, 3.0, 2.0];
217 /// floats.sort_by(|a, b| a.partial_cmp(b).unwrap());
218 /// assert_eq!(floats, [1.0, 2.0, 3.0, 4.0, 5.0]);
221 /// When applicable, unstable sorting is preferred because it is generally faster than stable
222 /// sorting and it doesn't allocate auxiliary memory.
223 /// See [`sort_unstable_by`](#method.sort_unstable_by).
225 /// # Current implementation
227 /// The current algorithm is an adaptive, iterative merge sort inspired by
228 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
229 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
230 /// two or more sorted sequences concatenated one after another.
232 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
233 /// non-allocating insertion sort is used instead.
238 /// let mut v = [5, 4, 1, 3, 2];
239 /// v.sort_by(|a, b| a.cmp(b));
240 /// assert!(v == [1, 2, 3, 4, 5]);
242 /// // reverse sorting
243 /// v.sort_by(|a, b| b.cmp(a));
244 /// assert!(v == [5, 4, 3, 2, 1]);
246 #[stable(feature = "rust1", since = "1.0.0")]
248 pub fn sort_by<F>(&mut self, mut compare: F)
250 F: FnMut(&T, &T) -> Ordering,
252 merge_sort(self, |a, b| compare(a, b) == Less);
255 /// Sorts the slice with a key extraction function.
257 /// This sort is stable (i.e., does not reorder equal elements) and `O(m * n * log(n))`
258 /// worst-case, where the key function is `O(m)`.
260 /// For expensive key functions (e.g. functions that are not simple property accesses or
261 /// basic operations), [`sort_by_cached_key`](#method.sort_by_cached_key) is likely to be
262 /// significantly faster, as it does not recompute element keys.
264 /// When applicable, unstable sorting is preferred because it is generally faster than stable
265 /// sorting and it doesn't allocate auxiliary memory.
266 /// See [`sort_unstable_by_key`](#method.sort_unstable_by_key).
268 /// # Current implementation
270 /// The current algorithm is an adaptive, iterative merge sort inspired by
271 /// [timsort](https://en.wikipedia.org/wiki/Timsort).
272 /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
273 /// two or more sorted sequences concatenated one after another.
275 /// Also, it allocates temporary storage half the size of `self`, but for short slices a
276 /// non-allocating insertion sort is used instead.
281 /// let mut v = [-5i32, 4, 1, -3, 2];
283 /// v.sort_by_key(|k| k.abs());
284 /// assert!(v == [1, 2, -3, 4, -5]);
286 #[stable(feature = "slice_sort_by_key", since = "1.7.0")]
288 pub fn sort_by_key<K, F>(&mut self, mut f: F)
293 merge_sort(self, |a, b| f(a).lt(&f(b)));
296 /// Sorts the slice with a key extraction function.
298 /// During sorting, the key function is called only once per element.
300 /// This sort is stable (i.e., does not reorder equal elements) and `O(m * n + n * log(n))`
301 /// worst-case, where the key function is `O(m)`.
303 /// For simple key functions (e.g., functions that are property accesses or
304 /// basic operations), [`sort_by_key`](#method.sort_by_key) is likely to be
307 /// # Current implementation
309 /// The current algorithm is based on [pattern-defeating quicksort][pdqsort] by Orson Peters,
310 /// which combines the fast average case of randomized quicksort with the fast worst case of
311 /// heapsort, while achieving linear time on slices with certain patterns. It uses some
312 /// randomization to avoid degenerate cases, but with a fixed seed to always provide
313 /// deterministic behavior.
315 /// In the worst case, the algorithm allocates temporary storage in a `Vec<(K, usize)>` the
316 /// length of the slice.
321 /// let mut v = [-5i32, 4, 32, -3, 2];
323 /// v.sort_by_cached_key(|k| k.to_string());
324 /// assert!(v == [-3, -5, 2, 32, 4]);
327 /// [pdqsort]: https://github.com/orlp/pdqsort
328 #[stable(feature = "slice_sort_by_cached_key", since = "1.34.0")]
330 pub fn sort_by_cached_key<K, F>(&mut self, f: F)
335 // Helper macro for indexing our vector by the smallest possible type, to reduce allocation.
336 macro_rules! sort_by_key {
337 ($t:ty, $slice:ident, $f:ident) => {{
338 let mut indices: Vec<_> =
339 $slice.iter().map($f).enumerate().map(|(i, k)| (k, i as $t)).collect();
340 // The elements of `indices` are unique, as they are indexed, so any sort will be
341 // stable with respect to the original slice. We use `sort_unstable` here because
342 // it requires less memory allocation.
343 indices.sort_unstable();
344 for i in 0..$slice.len() {
345 let mut index = indices[i].1;
346 while (index as usize) < i {
347 index = indices[index as usize].1;
349 indices[i].1 = index;
350 $slice.swap(i, index as usize);
355 let sz_u8 = mem::size_of::<(K, u8)>();
356 let sz_u16 = mem::size_of::<(K, u16)>();
357 let sz_u32 = mem::size_of::<(K, u32)>();
358 let sz_usize = mem::size_of::<(K, usize)>();
360 let len = self.len();
364 if sz_u8 < sz_u16 && len <= (u8::MAX as usize) {
365 return sort_by_key!(u8, self, f);
367 if sz_u16 < sz_u32 && len <= (u16::MAX as usize) {
368 return sort_by_key!(u16, self, f);
370 if sz_u32 < sz_usize && len <= (u32::MAX as usize) {
371 return sort_by_key!(u32, self, f);
373 sort_by_key!(usize, self, f)
376 /// Copies `self` into a new `Vec`.
381 /// let s = [10, 40, 30];
382 /// let x = s.to_vec();
383 /// // Here, `s` and `x` can be modified independently.
385 #[rustc_conversion_suggestion]
386 #[stable(feature = "rust1", since = "1.0.0")]
388 pub fn to_vec(&self) -> Vec<T>
392 // N.B., see the `hack` module in this file for more details.
396 /// Converts `self` into a vector without clones or allocation.
398 /// The resulting vector can be converted back into a box via
399 /// `Vec<T>`'s `into_boxed_slice` method.
404 /// let s: Box<[i32]> = Box::new([10, 40, 30]);
405 /// let x = s.into_vec();
406 /// // `s` cannot be used anymore because it has been converted into `x`.
408 /// assert_eq!(x, vec![10, 40, 30]);
410 #[stable(feature = "rust1", since = "1.0.0")]
412 pub fn into_vec(self: Box<Self>) -> Vec<T> {
413 // N.B., see the `hack` module in this file for more details.
417 /// Creates a vector by repeating a slice `n` times.
421 /// This function will panic if the capacity would overflow.
428 /// assert_eq!([1, 2].repeat(3), vec![1, 2, 1, 2, 1, 2]);
431 /// A panic upon overflow:
434 /// // this will panic at runtime
435 /// b"0123456789abcdef".repeat(usize::MAX);
437 #[stable(feature = "repeat_generic_slice", since = "1.40.0")]
438 pub fn repeat(&self, n: usize) -> Vec<T>
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 capacity = self.len().checked_mul(n).expect("capacity overflow");
453 let mut buf = Vec::with_capacity(capacity);
455 // `2^expn` repetition is done by doubling `buf` `expn`-times.
459 // If `m > 0`, there are remaining bits up to the leftmost '1'.
461 // `buf.extend(buf)`:
463 ptr::copy_nonoverlapping(
465 (buf.as_mut_ptr() as *mut T).add(buf.len()),
468 // `buf` has capacity of `self.len() * n`.
469 let buf_len = buf.len();
470 buf.set_len(buf_len * 2);
477 // `rem` (`= n - 2^expn`) repetition is done by copying
478 // first `rem` repetitions from `buf` itself.
479 let rem_len = capacity - buf.len(); // `self.len() * rem`
481 // `buf.extend(buf[0 .. rem_len])`:
483 // This is non-overlapping since `2^expn > rem`.
484 ptr::copy_nonoverlapping(
486 (buf.as_mut_ptr() as *mut T).add(buf.len()),
489 // `buf.len() + rem_len` equals to `buf.capacity()` (`= self.len() * n`).
490 buf.set_len(capacity);
496 /// Flattens a slice of `T` into a single value `Self::Output`.
501 /// assert_eq!(["hello", "world"].concat(), "helloworld");
502 /// assert_eq!([[1, 2], [3, 4]].concat(), [1, 2, 3, 4]);
504 #[stable(feature = "rust1", since = "1.0.0")]
505 pub fn concat<Item: ?Sized>(&self) -> <Self as Concat<Item>>::Output
512 /// Flattens a slice of `T` into a single value `Self::Output`, placing a
513 /// given separator between each.
518 /// assert_eq!(["hello", "world"].join(" "), "hello world");
519 /// assert_eq!([[1, 2], [3, 4]].join(&0), [1, 2, 0, 3, 4]);
520 /// assert_eq!([[1, 2], [3, 4]].join(&[0, 0][..]), [1, 2, 0, 0, 3, 4]);
522 #[stable(feature = "rename_connect_to_join", since = "1.3.0")]
523 pub fn join<Separator>(&self, sep: Separator) -> <Self as Join<Separator>>::Output
525 Self: Join<Separator>,
527 Join::join(self, sep)
530 /// Flattens a slice of `T` into a single value `Self::Output`, placing a
531 /// given separator between each.
536 /// # #![allow(deprecated)]
537 /// assert_eq!(["hello", "world"].connect(" "), "hello world");
538 /// assert_eq!([[1, 2], [3, 4]].connect(&0), [1, 2, 0, 3, 4]);
540 #[stable(feature = "rust1", since = "1.0.0")]
541 #[rustc_deprecated(since = "1.3.0", reason = "renamed to join")]
542 pub fn connect<Separator>(&self, sep: Separator) -> <Self as Join<Separator>>::Output
544 Self: Join<Separator>,
546 Join::join(self, sep)
550 #[lang = "slice_u8_alloc"]
553 /// Returns a vector containing a copy of this slice where each byte
554 /// is mapped to its ASCII upper case equivalent.
556 /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
557 /// but non-ASCII letters are unchanged.
559 /// To uppercase the value in-place, use [`make_ascii_uppercase`].
561 /// [`make_ascii_uppercase`]: #method.make_ascii_uppercase
562 #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
564 pub fn to_ascii_uppercase(&self) -> Vec<u8> {
565 let mut me = self.to_vec();
566 me.make_ascii_uppercase();
570 /// Returns a vector containing a copy of this slice where each byte
571 /// is mapped to its ASCII lower case equivalent.
573 /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
574 /// but non-ASCII letters are unchanged.
576 /// To lowercase the value in-place, use [`make_ascii_lowercase`].
578 /// [`make_ascii_lowercase`]: #method.make_ascii_lowercase
579 #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
581 pub fn to_ascii_lowercase(&self) -> Vec<u8> {
582 let mut me = self.to_vec();
583 me.make_ascii_lowercase();
588 ////////////////////////////////////////////////////////////////////////////////
589 // Extension traits for slices over specific kinds of data
590 ////////////////////////////////////////////////////////////////////////////////
592 /// Helper trait for [`[T]::concat`](../../std/primitive.slice.html#method.concat).
594 /// Note: the `Item` type parameter is not used in this trait,
595 /// but it allows impls to be more generic.
596 /// Without it, we get this error:
599 /// error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predica
600 /// --> src/liballoc/slice.rs:608:6
602 /// 608 | impl<T: Clone, V: Borrow<[T]>> Concat for [V] {
603 /// | ^ unconstrained type parameter
606 /// This is because there could exist `V` types with multiple `Borrow<[_]>` impls,
607 /// such that multiple `T` types would apply:
610 /// # #[allow(dead_code)]
611 /// pub struct Foo(Vec<u32>, Vec<String>);
613 /// impl std::borrow::Borrow<[u32]> for Foo {
614 /// fn borrow(&self) -> &[u32] { &self.0 }
617 /// impl std::borrow::Borrow<[String]> for Foo {
618 /// fn borrow(&self) -> &[String] { &self.1 }
621 #[unstable(feature = "slice_concat_trait", issue = "27747")]
622 pub trait Concat<Item: ?Sized> {
623 #[unstable(feature = "slice_concat_trait", issue = "27747")]
624 /// The resulting type after concatenation
627 /// Implementation of [`[T]::concat`](../../std/primitive.slice.html#method.concat)
628 #[unstable(feature = "slice_concat_trait", issue = "27747")]
629 fn concat(slice: &Self) -> Self::Output;
632 /// Helper trait for [`[T]::join`](../../std/primitive.slice.html#method.join)
633 #[unstable(feature = "slice_concat_trait", issue = "27747")]
634 pub trait Join<Separator> {
635 #[unstable(feature = "slice_concat_trait", issue = "27747")]
636 /// The resulting type after concatenation
639 /// Implementation of [`[T]::join`](../../std/primitive.slice.html#method.join)
640 #[unstable(feature = "slice_concat_trait", issue = "27747")]
641 fn join(slice: &Self, sep: Separator) -> Self::Output;
644 #[unstable(feature = "slice_concat_ext", issue = "27747")]
645 impl<T: Clone, V: Borrow<[T]>> Concat<T> for [V] {
646 type Output = Vec<T>;
648 fn concat(slice: &Self) -> Vec<T> {
649 let size = slice.iter().map(|slice| slice.borrow().len()).sum();
650 let mut result = Vec::with_capacity(size);
652 result.extend_from_slice(v.borrow())
658 #[unstable(feature = "slice_concat_ext", issue = "27747")]
659 impl<T: Clone, V: Borrow<[T]>> Join<&T> for [V] {
660 type Output = Vec<T>;
662 fn join(slice: &Self, sep: &T) -> Vec<T> {
663 let mut iter = slice.iter();
664 let first = match iter.next() {
665 Some(first) => first,
666 None => return vec![],
668 let size = slice.iter().map(|v| v.borrow().len()).sum::<usize>() + slice.len() - 1;
669 let mut result = Vec::with_capacity(size);
670 result.extend_from_slice(first.borrow());
673 result.push(sep.clone());
674 result.extend_from_slice(v.borrow())
680 #[unstable(feature = "slice_concat_ext", issue = "27747")]
681 impl<T: Clone, V: Borrow<[T]>> Join<&[T]> for [V] {
682 type Output = Vec<T>;
684 fn join(slice: &Self, sep: &[T]) -> Vec<T> {
685 let mut iter = slice.iter();
686 let first = match iter.next() {
687 Some(first) => first,
688 None => return vec![],
691 slice.iter().map(|v| v.borrow().len()).sum::<usize>() + sep.len() * (slice.len() - 1);
692 let mut result = Vec::with_capacity(size);
693 result.extend_from_slice(first.borrow());
696 result.extend_from_slice(sep);
697 result.extend_from_slice(v.borrow())
703 ////////////////////////////////////////////////////////////////////////////////
704 // Standard trait implementations for slices
705 ////////////////////////////////////////////////////////////////////////////////
707 #[stable(feature = "rust1", since = "1.0.0")]
708 impl<T> Borrow<[T]> for Vec<T> {
709 fn borrow(&self) -> &[T] {
714 #[stable(feature = "rust1", since = "1.0.0")]
715 impl<T> BorrowMut<[T]> for Vec<T> {
716 fn borrow_mut(&mut self) -> &mut [T] {
721 #[stable(feature = "rust1", since = "1.0.0")]
722 impl<T: Clone> ToOwned for [T] {
725 fn to_owned(&self) -> Vec<T> {
730 fn to_owned(&self) -> Vec<T> {
734 fn clone_into(&self, target: &mut Vec<T>) {
735 // drop anything in target that will not be overwritten
736 target.truncate(self.len());
738 // target.len <= self.len due to the truncate above, so the
739 // slices here are always in-bounds.
740 let (init, tail) = self.split_at(target.len());
742 // reuse the contained values' allocations/resources.
743 target.clone_from_slice(init);
744 target.extend_from_slice(tail);
748 ////////////////////////////////////////////////////////////////////////////////
750 ////////////////////////////////////////////////////////////////////////////////
752 /// Inserts `v[0]` into pre-sorted sequence `v[1..]` so that whole `v[..]` becomes sorted.
754 /// This is the integral subroutine of insertion sort.
755 fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
757 F: FnMut(&T, &T) -> bool,
759 if v.len() >= 2 && is_less(&v[1], &v[0]) {
761 // There are three ways to implement insertion here:
763 // 1. Swap adjacent elements until the first one gets to its final destination.
764 // However, this way we copy data around more than is necessary. If elements are big
765 // structures (costly to copy), this method will be slow.
767 // 2. Iterate until the right place for the first element is found. Then shift the
768 // elements succeeding it to make room for it and finally place it into the
769 // remaining hole. This is a good method.
771 // 3. Copy the first element into a temporary variable. Iterate until the right place
772 // for it is found. As we go along, copy every traversed element into the slot
773 // preceding it. Finally, copy data from the temporary variable into the remaining
774 // hole. This method is very good. Benchmarks demonstrated slightly better
775 // performance than with the 2nd method.
777 // All methods were benchmarked, and the 3rd showed best results. So we chose that one.
778 let mut tmp = mem::ManuallyDrop::new(ptr::read(&v[0]));
780 // Intermediate state of the insertion process is always tracked by `hole`, which
781 // serves two purposes:
782 // 1. Protects integrity of `v` from panics in `is_less`.
783 // 2. Fills the remaining hole in `v` in the end.
787 // If `is_less` panics at any point during the process, `hole` will get dropped and
788 // fill the hole in `v` with `tmp`, thus ensuring that `v` still holds every object it
789 // initially held exactly once.
790 let mut hole = InsertionHole { src: &mut *tmp, dest: &mut v[1] };
791 ptr::copy_nonoverlapping(&v[1], &mut v[0], 1);
793 for i in 2..v.len() {
794 if !is_less(&v[i], &*tmp) {
797 ptr::copy_nonoverlapping(&v[i], &mut v[i - 1], 1);
798 hole.dest = &mut v[i];
800 // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
804 // When dropped, copies from `src` into `dest`.
805 struct InsertionHole<T> {
810 impl<T> Drop for InsertionHole<T> {
813 ptr::copy_nonoverlapping(self.src, self.dest, 1);
819 /// Merges non-decreasing runs `v[..mid]` and `v[mid..]` using `buf` as temporary storage, and
820 /// stores the result into `v[..]`.
824 /// The two slices must be non-empty and `mid` must be in bounds. Buffer `buf` must be long enough
825 /// to hold a copy of the shorter slice. Also, `T` must not be a zero-sized type.
826 unsafe fn merge<T, F>(v: &mut [T], mid: usize, buf: *mut T, is_less: &mut F)
828 F: FnMut(&T, &T) -> bool,
831 let v = v.as_mut_ptr();
832 let (v_mid, v_end) = unsafe { (v.add(mid), v.add(len)) };
834 // The merge process first copies the shorter run into `buf`. Then it traces the newly copied
835 // run and the longer run forwards (or backwards), comparing their next unconsumed elements and
836 // copying the lesser (or greater) one into `v`.
838 // As soon as the shorter run is fully consumed, the process is done. If the longer run gets
839 // consumed first, then we must copy whatever is left of the shorter run into the remaining
842 // Intermediate state of the process is always tracked by `hole`, which serves two purposes:
843 // 1. Protects integrity of `v` from panics in `is_less`.
844 // 2. Fills the remaining hole in `v` if the longer run gets consumed first.
848 // If `is_less` panics at any point during the process, `hole` will get dropped and fill the
849 // hole in `v` with the unconsumed range in `buf`, thus ensuring that `v` still holds every
850 // object it initially held exactly once.
853 if mid <= len - mid {
854 // The left run is shorter.
856 ptr::copy_nonoverlapping(v, buf, mid);
857 hole = MergeHole { start: buf, end: buf.add(mid), dest: v };
860 // Initially, these pointers point to the beginnings of their arrays.
861 let left = &mut hole.start;
862 let mut right = v_mid;
863 let out = &mut hole.dest;
865 while *left < hole.end && right < v_end {
866 // Consume the lesser side.
867 // If equal, prefer the left run to maintain stability.
869 let to_copy = if is_less(&*right, &**left) {
870 get_and_increment(&mut right)
872 get_and_increment(left)
874 ptr::copy_nonoverlapping(to_copy, get_and_increment(out), 1);
878 // The right run is shorter.
880 ptr::copy_nonoverlapping(v_mid, buf, len - mid);
881 hole = MergeHole { start: buf, end: buf.add(len - mid), dest: v_mid };
884 // Initially, these pointers point past the ends of their arrays.
885 let left = &mut hole.dest;
886 let right = &mut hole.end;
889 while v < *left && buf < *right {
890 // Consume the greater side.
891 // If equal, prefer the right run to maintain stability.
893 let to_copy = if is_less(&*right.offset(-1), &*left.offset(-1)) {
894 decrement_and_get(left)
896 decrement_and_get(right)
898 ptr::copy_nonoverlapping(to_copy, decrement_and_get(&mut out), 1);
902 // Finally, `hole` gets dropped. If the shorter run was not fully consumed, whatever remains of
903 // it will now be copied into the hole in `v`.
905 unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T {
907 *ptr = unsafe { ptr.offset(1) };
911 unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T {
912 *ptr = unsafe { ptr.offset(-1) };
916 // When dropped, copies the range `start..end` into `dest..`.
917 struct MergeHole<T> {
923 impl<T> Drop for MergeHole<T> {
925 // `T` is not a zero-sized type, so it's okay to divide by its size.
926 let len = (self.end as usize - self.start as usize) / mem::size_of::<T>();
928 ptr::copy_nonoverlapping(self.start, self.dest, len);
934 /// This merge sort borrows some (but not all) ideas from TimSort, which is described in detail
935 /// [here](http://svn.python.org/projects/python/trunk/Objects/listsort.txt).
937 /// The algorithm identifies strictly descending and non-descending subsequences, which are called
938 /// natural runs. There is a stack of pending runs yet to be merged. Each newly found run is pushed
939 /// onto the stack, and then some pairs of adjacent runs are merged until these two invariants are
942 /// 1. for every `i` in `1..runs.len()`: `runs[i - 1].len > runs[i].len`
943 /// 2. for every `i` in `2..runs.len()`: `runs[i - 2].len > runs[i - 1].len + runs[i].len`
945 /// The invariants ensure that the total running time is `O(n * log(n))` worst-case.
946 fn merge_sort<T, F>(v: &mut [T], mut is_less: F)
948 F: FnMut(&T, &T) -> bool,
950 // Slices of up to this length get sorted using insertion sort.
951 const MAX_INSERTION: usize = 20;
952 // Very short runs are extended using insertion sort to span at least this many elements.
953 const MIN_RUN: usize = 10;
955 // Sorting has no meaningful behavior on zero-sized types.
956 if size_of::<T>() == 0 {
962 // Short arrays get sorted in-place via insertion sort to avoid allocations.
963 if len <= MAX_INSERTION {
965 for i in (0..len - 1).rev() {
966 insert_head(&mut v[i..], &mut is_less);
972 // Allocate a buffer to use as scratch memory. We keep the length 0 so we can keep in it
973 // shallow copies of the contents of `v` without risking the dtors running on copies if
974 // `is_less` panics. When merging two sorted runs, this buffer holds a copy of the shorter run,
975 // which will always have length at most `len / 2`.
976 let mut buf = Vec::with_capacity(len / 2);
978 // In order to identify natural runs in `v`, we traverse it backwards. That might seem like a
979 // strange decision, but consider the fact that merges more often go in the opposite direction
980 // (forwards). According to benchmarks, merging forwards is slightly faster than merging
981 // backwards. To conclude, identifying runs by traversing backwards improves performance.
982 let mut runs = vec![];
985 // Find the next natural run, and reverse it if it's strictly descending.
986 let mut start = end - 1;
990 if is_less(v.get_unchecked(start + 1), v.get_unchecked(start)) {
991 while start > 0 && is_less(v.get_unchecked(start), v.get_unchecked(start - 1)) {
994 v[start..end].reverse();
996 while start > 0 && !is_less(v.get_unchecked(start), v.get_unchecked(start - 1))
1004 // Insert some more elements into the run if it's too short. Insertion sort is faster than
1005 // merge sort on short sequences, so this significantly improves performance.
1006 while start > 0 && end - start < MIN_RUN {
1008 insert_head(&mut v[start..end], &mut is_less);
1011 // Push this run onto the stack.
1012 runs.push(Run { start, len: end - start });
1015 // Merge some pairs of adjacent runs to satisfy the invariants.
1016 while let Some(r) = collapse(&runs) {
1017 let left = runs[r + 1];
1018 let right = runs[r];
1021 &mut v[left.start..right.start + right.len],
1027 runs[r] = Run { start: left.start, len: left.len + right.len };
1032 // Finally, exactly one run must remain in the stack.
1033 debug_assert!(runs.len() == 1 && runs[0].start == 0 && runs[0].len == len);
1035 // Examines the stack of runs and identifies the next pair of runs to merge. More specifically,
1036 // if `Some(r)` is returned, that means `runs[r]` and `runs[r + 1]` must be merged next. If the
1037 // algorithm should continue building a new run instead, `None` is returned.
1039 // TimSort is infamous for its buggy implementations, as described here:
1040 // http://envisage-project.eu/timsort-specification-and-verification/
1042 // The gist of the story is: we must enforce the invariants on the top four runs on the stack.
1043 // Enforcing them on just top three is not sufficient to ensure that the invariants will still
1044 // hold for *all* runs in the stack.
1046 // This function correctly checks invariants for the top four runs. Additionally, if the top
1047 // run starts at index 0, it will always demand a merge operation until the stack is fully
1048 // collapsed, in order to complete the sort.
1050 fn collapse(runs: &[Run]) -> Option<usize> {
1053 && (runs[n - 1].start == 0
1054 || runs[n - 2].len <= runs[n - 1].len
1055 || (n >= 3 && runs[n - 3].len <= runs[n - 2].len + runs[n - 1].len)
1056 || (n >= 4 && runs[n - 4].len <= runs[n - 3].len + runs[n - 2].len))
1058 if n >= 3 && runs[n - 3].len < runs[n - 1].len { Some(n - 3) } else { Some(n - 2) }
1064 #[derive(Clone, Copy)]