1 // Copyright 2021 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Package slices defines various functions useful with slices of any type.
13 // Equal reports whether two slices are equal: the same length and all
14 // elements equal. If the lengths are different, Equal returns false.
15 // Otherwise, the elements are compared in increasing index order, and the
16 // comparison stops at the first unequal pair.
17 // Floating point NaNs are not considered equal.
18 func Equal[E comparable](s1, s2 []E) bool {
19 if len(s1) != len(s2) {
30 // EqualFunc reports whether two slices are equal using a comparison
31 // function on each pair of elements. If the lengths are different,
32 // EqualFunc returns false. Otherwise, the elements are compared in
33 // increasing index order, and the comparison stops at the first index
34 // for which eq returns false.
35 func EqualFunc[E1, E2 any](s1 []E1, s2 []E2, eq func(E1, E2) bool) bool {
36 if len(s1) != len(s2) {
39 for i, v1 := range s1 {
48 // Compare compares the elements of s1 and s2, using [cmp.Compare] on each pair
49 // of elements. The elements are compared sequentially, starting at index 0,
50 // until one element is not equal to the other.
51 // The result of comparing the first non-matching elements is returned.
52 // If both slices are equal until one of them ends, the shorter slice is
53 // considered less than the longer one.
54 // The result is 0 if s1 == s2, -1 if s1 < s2, and +1 if s1 > s2.
55 func Compare[E cmp.Ordered](s1, s2 []E) int {
56 for i, v1 := range s1 {
61 if c := cmp.Compare(v1, v2); c != 0 {
65 if len(s1) < len(s2) {
71 // CompareFunc is like Compare but uses a custom comparison function on each
73 // The result is the first non-zero result of cmp; if cmp always
74 // returns 0 the result is 0 if len(s1) == len(s2), -1 if len(s1) < len(s2),
75 // and +1 if len(s1) > len(s2).
76 func CompareFunc[E1, E2 any](s1 []E1, s2 []E2, cmp func(E1, E2) int) int {
77 for i, v1 := range s1 {
82 if c := cmp(v1, v2); c != 0 {
86 if len(s1) < len(s2) {
92 // Index returns the index of the first occurrence of v in s,
93 // or -1 if not present.
94 func Index[E comparable](s []E, v E) int {
103 // IndexFunc returns the first index i satisfying f(s[i]),
105 func IndexFunc[E any](s []E, f func(E) bool) int {
114 // Contains reports whether v is present in s.
115 func Contains[E comparable](s []E, v E) bool {
116 return Index(s, v) >= 0
119 // ContainsFunc reports whether at least one
120 // element e of s satisfies f(e).
121 func ContainsFunc[E any](s []E, f func(E) bool) bool {
122 return IndexFunc(s, f) >= 0
125 // Insert inserts the values v... into s at index i,
126 // returning the modified slice.
127 // The elements at s[i:] are shifted up to make room.
128 // In the returned slice r, r[i] == v[0],
129 // and r[i+len(v)] == value originally at r[i].
130 // Insert panics if i is out of range.
131 // This function is O(len(s) + len(v)).
132 func Insert[S ~[]E, E any](s S, i int, v ...E) S {
139 return append(s, v...)
142 // Use append rather than make so that we bump the size of
143 // the slice up to the next storage class.
144 // This is what Grow does but we don't call Grow because
145 // that might copy the values twice.
146 s2 := append(s[:i], make(S, n+m-i)...)
148 copy(s2[i+m:], s[i:])
154 // s: aaaaaaaabbbbccccccccdddd
158 // s: aaaaaaaavvvvbbbbcccccccc
162 // a are the values that don't move in s.
163 // v are the values copied in from v.
164 // b and c are the values from s that are shifted up in index.
165 // d are the values that get overwritten, never to be seen again.
167 if !overlaps(v, s[i+m:]) {
168 // Easy case - v does not overlap either the c or d regions.
169 // (It might be in some of a or b, or elsewhere entirely.)
170 // The data we copy up doesn't write to v at all, so just do it.
175 // s: aaaaaaaabbbbbbbbcccccccc
178 // Note the b values are duplicated.
183 // s: aaaaaaaavvvvbbbbcccccccc
186 // That's the result we want.
190 // The hard case - v overlaps c or d. We can't just shift up
191 // the data because we'd move or clobber the values we're trying
193 // So instead, write v on top of d, then rotate.
197 // s: aaaaaaaabbbbccccccccvvvv
201 rotateRight(s[i:], m)
204 // s: aaaaaaaavvvvbbbbcccccccc
207 // That's the result we want.
211 // Delete removes the elements s[i:j] from s, returning the modified slice.
212 // Delete panics if s[i:j] is not a valid slice of s.
213 // Delete modifies the contents of the slice s; it does not create a new slice.
214 // Delete is O(len(s)-j), so if many items must be deleted, it is better to
215 // make a single call deleting them all together than to delete one at a time.
216 // Delete might not modify the elements s[len(s)-(j-i):len(s)]. If those
217 // elements contain pointers you might consider zeroing those elements so that
218 // objects they reference can be garbage collected.
219 func Delete[S ~[]E, E any](s S, i, j int) S {
220 _ = s[i:j] // bounds check
222 return append(s[:i], s[j:]...)
225 // DeleteFunc removes any elements from s for which del returns true,
226 // returning the modified slice.
227 // DeleteFunc modifies the contents of the slice s;
228 // it does not create a new slice.
229 // When DeleteFunc removes m elements, it might not modify the elements
230 // s[len(s)-m:len(s)]. If those elements contain pointers you might consider
231 // zeroing those elements so that objects they reference can be garbage
233 func DeleteFunc[S ~[]E, E any](s S, del func(E) bool) S {
234 // Don't start copying elements until we find one to delete.
235 for i, v := range s {
238 for i++; i < len(s); i++ {
251 // Replace replaces the elements s[i:j] by the given v, and returns the
252 // modified slice. Replace panics if s[i:j] is not a valid slice of s.
253 func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
254 _ = s[i:j] // verify that i:j is a valid subslice
257 return Insert(s, i, v...)
260 return append(s[:i], v...)
263 tot := len(s[:i]) + len(v) + len(s[j:])
265 // Too big to fit, allocate and copy over.
266 s2 := append(s[:i], make(S, tot-i)...) // See Insert
268 copy(s2[i+len(v):], s[j:])
275 // Easy, as v fits in the deleted portion.
278 copy(r[i+len(v):], s[j:])
283 // We are expanding (v is bigger than j-i).
284 // The situation is something like this:
285 // (example has i=4,j=8,len(s)=16,len(v)=6)
286 // s: aaaaxxxxbbbbbbbbyy
292 // y: area to expand into
294 if !overlaps(r[i+len(v):], v) {
295 // Easy, as v is not clobbered by the first copy.
296 copy(r[i+len(v):], s[j:])
301 // This is a situation where we don't have a single place to which
302 // we can copy v. Parts of it need to go to two different places.
303 // We want to copy the prefix of v into y and the suffix into x, then
304 // rotate |y| spots to the right.
308 // s: aaaavvvvbbbbbbbbvv
312 // If either of those two destinations don't alias v, then we're good.
313 y := len(v) - (j - i) // length of y portion
315 if !overlaps(r[i:j], v) {
317 copy(r[len(s):], v[:y])
318 rotateRight(r[i:], y)
321 if !overlaps(r[len(s):], v) {
322 copy(r[len(s):], v[:y])
324 rotateRight(r[i:], y)
328 // Now we know that v overlaps both x and y.
329 // That means that the entirety of b is *inside* v.
330 // So we don't need to preserve b at all; instead we
331 // can copy v first, then copy the b part of v out of
332 // v to the right destination.
333 k := startIdx(v, s[j:])
335 copy(r[i+len(v):], r[i+k:])
339 // Clone returns a copy of the slice.
340 // The elements are copied using assignment, so this is a shallow clone.
341 func Clone[S ~[]E, E any](s S) S {
342 // Preserve nil in case it matters.
346 return append(S([]E{}), s...)
349 // Compact replaces consecutive runs of equal elements with a single copy.
350 // This is like the uniq command found on Unix.
351 // Compact modifies the contents of the slice s; it does not create a new slice.
352 // When Compact discards m elements in total, it might not modify the elements
353 // s[len(s)-m:len(s)]. If those elements contain pointers you might consider
354 // zeroing those elements so that objects they reference can be garbage collected.
355 func Compact[S ~[]E, E comparable](s S) S {
360 for k := 1; k < len(s); k++ {
371 // CompactFunc is like Compact but uses a comparison function.
372 func CompactFunc[S ~[]E, E any](s S, eq func(E, E) bool) S {
377 for k := 1; k < len(s); k++ {
378 if !eq(s[k], s[k-1]) {
388 // Grow increases the slice's capacity, if necessary, to guarantee space for
389 // another n elements. After Grow(n), at least n elements can be appended
390 // to the slice without another allocation. If n is negative or too large to
391 // allocate the memory, Grow panics.
392 func Grow[S ~[]E, E any](s S, n int) S {
394 panic("cannot be negative")
396 if n -= cap(s) - len(s); n > 0 {
397 s = append(s[:cap(s)], make([]E, n)...)[:len(s)]
402 // Clip removes unused capacity from the slice, returning s[:len(s):len(s)].
403 func Clip[S ~[]E, E any](s S) S {
404 return s[:len(s):len(s)]
407 // Rotation algorithm explanation:
412 // split up like this
414 // swap first 2 and last 2
418 // recursively rotate first left part by 2
426 // split up like this
428 // swap first 2 and last 2
432 // recursively rotate second part left by 6
437 // TODO: There are other rotate algorithms.
438 // This algorithm has the desirable property that it moves each element exactly twice.
439 // The triple-reverse algorithm is simpler and more cache friendly, but takes more writes.
440 // The follow-cycles algorithm can be 1-write but it is not very cache friendly.
442 // rotateLeft rotates b left by n spaces.
443 // s_final[i] = s_orig[i+r], wrapping around.
444 func rotateLeft[S ~[]E, E any](s S, r int) {
445 for r != 0 && r != len(s) {
447 swap(s[:r], s[len(s)-r:])
450 swap(s[:len(s)-r], s[r:])
451 s, r = s[len(s)-r:], r*2-len(s)
455 func rotateRight[S ~[]E, E any](s S, r int) {
456 rotateLeft(s, len(s)-r)
459 // swap swaps the contents of x and y. x and y must be equal length and disjoint.
460 func swap[S ~[]E, E any](x, y S) {
461 for i := 0; i < len(x); i++ {
462 x[i], y[i] = y[i], x[i]
466 // overlaps reports whether the memory ranges a[0:len(a)] and b[0:len(b)] overlap.
467 func overlaps[S ~[]E, E any](a, b S) bool {
468 if len(a) == 0 || len(b) == 0 {
471 elemSize := unsafe.Sizeof(a[0])
475 // TODO: use a runtime/unsafe facility once one becomes available. See issue 12445.
476 // Also see crypto/internal/alias/alias.go:AnyOverlap
477 return uintptr(unsafe.Pointer(&a[0])) <= uintptr(unsafe.Pointer(&b[len(b)-1]))+(elemSize-1) &&
478 uintptr(unsafe.Pointer(&b[0])) <= uintptr(unsafe.Pointer(&a[len(a)-1]))+(elemSize-1)
481 // startIdx returns the index in haystack where the needle starts.
482 // prerequisite: the needle must be aliased entirely inside the haystack.
483 func startIdx[S ~[]E, E any](haystack, needle S) int {
485 for i := range haystack {
486 if p == &haystack[i] {
490 // TODO: what if the overlap is by a non-integral number of Es?
491 panic("needle not found")
494 // Reverse reverses the elements of the slice in place.
495 func Reverse[E any](s []E) {
496 for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
497 s[i], s[j] = s[j], s[i]