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// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package slices defines various functions useful with slices of any type.
package slices
import (
"unsafe"
"golang.org/x/exp/constraints"
)
// Equal reports whether two slices are equal: the same length and all
// elements equal. If the lengths are different, Equal returns false.
// Otherwise, the elements are compared in increasing index order, and the
// comparison stops at the first unequal pair.
// Floating point NaNs are not considered equal.
func Equal[S ~[]E, E comparable](s1, s2 S) bool {
if len(s1) != len(s2) {
return false
}
for i := range s1 {
if s1[i] != s2[i] {
return false
}
}
return true
}
// EqualFunc reports whether two slices are equal using an equality
// function on each pair of elements. If the lengths are different,
// EqualFunc returns false. Otherwise, the elements are compared in
// increasing index order, and the comparison stops at the first index
// for which eq returns false.
func EqualFunc[S1 ~[]E1, S2 ~[]E2, E1, E2 any](s1 S1, s2 S2, eq func(E1, E2) bool) bool {
if len(s1) != len(s2) {
return false
}
for i, v1 := range s1 {
v2 := s2[i]
if !eq(v1, v2) {
return false
}
}
return true
}
// Compare compares the elements of s1 and s2, using [cmp.Compare] on each pair
// of elements. The elements are compared sequentially, starting at index 0,
// until one element is not equal to the other.
// The result of comparing the first non-matching elements is returned.
// If both slices are equal until one of them ends, the shorter slice is
// considered less than the longer one.
// The result is 0 if s1 == s2, -1 if s1 < s2, and +1 if s1 > s2.
func Compare[S ~[]E, E constraints.Ordered](s1, s2 S) int {
for i, v1 := range s1 {
if i >= len(s2) {
return +1
}
v2 := s2[i]
if c := cmpCompare(v1, v2); c != 0 {
return c
}
}
if len(s1) < len(s2) {
return -1
}
return 0
}
// CompareFunc is like [Compare] but uses a custom comparison function on each
// pair of elements.
// The result is the first non-zero result of cmp; if cmp always
// returns 0 the result is 0 if len(s1) == len(s2), -1 if len(s1) < len(s2),
// and +1 if len(s1) > len(s2).
func CompareFunc[S1 ~[]E1, S2 ~[]E2, E1, E2 any](s1 S1, s2 S2, cmp func(E1, E2) int) int {
for i, v1 := range s1 {
if i >= len(s2) {
return +1
}
v2 := s2[i]
if c := cmp(v1, v2); c != 0 {
return c
}
}
if len(s1) < len(s2) {
return -1
}
return 0
}
// Index returns the index of the first occurrence of v in s,
// or -1 if not present.
func Index[S ~[]E, E comparable](s S, v E) int {
for i := range s {
if v == s[i] {
return i
}
}
return -1
}
// IndexFunc returns the first index i satisfying f(s[i]),
// or -1 if none do.
func IndexFunc[S ~[]E, E any](s S, f func(E) bool) int {
for i := range s {
if f(s[i]) {
return i
}
}
return -1
}
// Contains reports whether v is present in s.
func Contains[S ~[]E, E comparable](s S, v E) bool {
return Index(s, v) >= 0
}
// ContainsFunc reports whether at least one
// element e of s satisfies f(e).
func ContainsFunc[S ~[]E, E any](s S, f func(E) bool) bool {
return IndexFunc(s, f) >= 0
}
// Insert inserts the values v... into s at index i,
// returning the modified slice.
// The elements at s[i:] are shifted up to make room.
// In the returned slice r, r[i] == v[0],
// and r[i+len(v)] == value originally at r[i].
// Insert panics if i is out of range.
// This function is O(len(s) + len(v)).
func Insert[S ~[]E, E any](s S, i int, v ...E) S {
m := len(v)
if m == 0 {
return s
}
n := len(s)
if i == n {
return append(s, v...)
}
if n+m > cap(s) {
// Use append rather than make so that we bump the size of
// the slice up to the next storage class.
// This is what Grow does but we don't call Grow because
// that might copy the values twice.
s2 := append(s[:i], make(S, n+m-i)...)
copy(s2[i:], v)
copy(s2[i+m:], s[i:])
return s2
}
s = s[:n+m]
// before:
// s: aaaaaaaabbbbccccccccdddd
// ^ ^ ^ ^
// i i+m n n+m
// after:
// s: aaaaaaaavvvvbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
//
// a are the values that don't move in s.
// v are the values copied in from v.
// b and c are the values from s that are shifted up in index.
// d are the values that get overwritten, never to be seen again.
if !overlaps(v, s[i+m:]) {
// Easy case - v does not overlap either the c or d regions.
// (It might be in some of a or b, or elsewhere entirely.)
// The data we copy up doesn't write to v at all, so just do it.
copy(s[i+m:], s[i:])
// Now we have
// s: aaaaaaaabbbbbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
// Note the b values are duplicated.
copy(s[i:], v)
// Now we have
// s: aaaaaaaavvvvbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
// That's the result we want.
return s
}
// The hard case - v overlaps c or d. We can't just shift up
// the data because we'd move or clobber the values we're trying
// to insert.
// So instead, write v on top of d, then rotate.
copy(s[n:], v)
// Now we have
// s: aaaaaaaabbbbccccccccvvvv
// ^ ^ ^ ^
// i i+m n n+m
rotateRight(s[i:], m)
// Now we have
// s: aaaaaaaavvvvbbbbcccccccc
// ^ ^ ^ ^
// i i+m n n+m
// That's the result we want.
return s
}
// clearSlice sets all elements up to the length of s to the zero value of E.
// We may use the builtin clear func instead, and remove clearSlice, when upgrading
// to Go 1.21+.
func clearSlice[S ~[]E, E any](s S) {
var zero E
for i := range s {
s[i] = zero
}
}
// Delete removes the elements s[i:j] from s, returning the modified slice.
// Delete panics if j > len(s) or s[i:j] is not a valid slice of s.
// Delete is O(len(s)-i), so if many items must be deleted, it is better to
// make a single call deleting them all together than to delete one at a time.
// Delete zeroes the elements s[len(s)-(j-i):len(s)].
func Delete[S ~[]E, E any](s S, i, j int) S {
_ = s[i:j:len(s)] // bounds check
if i == j {
return s
}
oldlen := len(s)
s = append(s[:i], s[j:]...)
clearSlice(s[len(s):oldlen]) // zero/nil out the obsolete elements, for GC
return s
}
// DeleteFunc removes any elements from s for which del returns true,
// returning the modified slice.
// DeleteFunc zeroes the elements between the new length and the original length.
func DeleteFunc[S ~[]E, E any](s S, del func(E) bool) S {
i := IndexFunc(s, del)
if i == -1 {
return s
}
// Don't start copying elements until we find one to delete.
for j := i + 1; j < len(s); j++ {
if v := s[j]; !del(v) {
s[i] = v
i++
}
}
clearSlice(s[i:]) // zero/nil out the obsolete elements, for GC
return s[:i]
}
// Replace replaces the elements s[i:j] by the given v, and returns the
// modified slice. Replace panics if s[i:j] is not a valid slice of s.
// When len(v) < (j-i), Replace zeroes the elements between the new length and the original length.
func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
_ = s[i:j] // verify that i:j is a valid subslice
if i == j {
return Insert(s, i, v...)
}
if j == len(s) {
return append(s[:i], v...)
}
tot := len(s[:i]) + len(v) + len(s[j:])
if tot > cap(s) {
// Too big to fit, allocate and copy over.
s2 := append(s[:i], make(S, tot-i)...) // See Insert
copy(s2[i:], v)
copy(s2[i+len(v):], s[j:])
return s2
}
r := s[:tot]
if i+len(v) <= j {
// Easy, as v fits in the deleted portion.
copy(r[i:], v)
if i+len(v) != j {
copy(r[i+len(v):], s[j:])
}
clearSlice(s[tot:]) // zero/nil out the obsolete elements, for GC
return r
}
// We are expanding (v is bigger than j-i).
// The situation is something like this:
// (example has i=4,j=8,len(s)=16,len(v)=6)
// s: aaaaxxxxbbbbbbbbyy
// ^ ^ ^ ^
// i j len(s) tot
// a: prefix of s
// x: deleted range
// b: more of s
// y: area to expand into
if !overlaps(r[i+len(v):], v) {
// Easy, as v is not clobbered by the first copy.
copy(r[i+len(v):], s[j:])
copy(r[i:], v)
return r
}
// This is a situation where we don't have a single place to which
// we can copy v. Parts of it need to go to two different places.
// We want to copy the prefix of v into y and the suffix into x, then
// rotate |y| spots to the right.
//
// v[2:] v[:2]
// | |
// s: aaaavvvvbbbbbbbbvv
// ^ ^ ^ ^
// i j len(s) tot
//
// If either of those two destinations don't alias v, then we're good.
y := len(v) - (j - i) // length of y portion
if !overlaps(r[i:j], v) {
copy(r[i:j], v[y:])
copy(r[len(s):], v[:y])
rotateRight(r[i:], y)
return r
}
if !overlaps(r[len(s):], v) {
copy(r[len(s):], v[:y])
copy(r[i:j], v[y:])
rotateRight(r[i:], y)
return r
}
// Now we know that v overlaps both x and y.
// That means that the entirety of b is *inside* v.
// So we don't need to preserve b at all; instead we
// can copy v first, then copy the b part of v out of
// v to the right destination.
k := startIdx(v, s[j:])
copy(r[i:], v)
copy(r[i+len(v):], r[i+k:])
return r
}
// Clone returns a copy of the slice.
// The elements are copied using assignment, so this is a shallow clone.
func Clone[S ~[]E, E any](s S) S {
// Preserve nil in case it matters.
if s == nil {
return nil
}
return append(S([]E{}), s...)
}
// Compact replaces consecutive runs of equal elements with a single copy.
// This is like the uniq command found on Unix.
// Compact modifies the contents of the slice s and returns the modified slice,
// which may have a smaller length.
// Compact zeroes the elements between the new length and the original length.
func Compact[S ~[]E, E comparable](s S) S {
if len(s) < 2 {
return s
}
i := 1
for k := 1; k < len(s); k++ {
if s[k] != s[k-1] {
if i != k {
s[i] = s[k]
}
i++
}
}
clearSlice(s[i:]) // zero/nil out the obsolete elements, for GC
return s[:i]
}
// CompactFunc is like [Compact] but uses an equality function to compare elements.
// For runs of elements that compare equal, CompactFunc keeps the first one.
// CompactFunc zeroes the elements between the new length and the original length.
func CompactFunc[S ~[]E, E any](s S, eq func(E, E) bool) S {
if len(s) < 2 {
return s
}
i := 1
for k := 1; k < len(s); k++ {
if !eq(s[k], s[k-1]) {
if i != k {
s[i] = s[k]
}
i++
}
}
clearSlice(s[i:]) // zero/nil out the obsolete elements, for GC
return s[:i]
}
// Grow increases the slice's capacity, if necessary, to guarantee space for
// another n elements. After Grow(n), at least n elements can be appended
// to the slice without another allocation. If n is negative or too large to
// allocate the memory, Grow panics.
func Grow[S ~[]E, E any](s S, n int) S {
if n < 0 {
panic("cannot be negative")
}
if n -= cap(s) - len(s); n > 0 {
// TODO(https://go.dev/issue/53888): Make using []E instead of S
// to workaround a compiler bug where the runtime.growslice optimization
// does not take effect. Revert when the compiler is fixed.
s = append([]E(s)[:cap(s)], make([]E, n)...)[:len(s)]
}
return s
}
// Clip removes unused capacity from the slice, returning s[:len(s):len(s)].
func Clip[S ~[]E, E any](s S) S {
return s[:len(s):len(s)]
}
// Rotation algorithm explanation:
//
// rotate left by 2
// start with
// 0123456789
// split up like this
// 01 234567 89
// swap first 2 and last 2
// 89 234567 01
// join first parts
// 89234567 01
// recursively rotate first left part by 2
// 23456789 01
// join at the end
// 2345678901
//
// rotate left by 8
// start with
// 0123456789
// split up like this
// 01 234567 89
// swap first 2 and last 2
// 89 234567 01
// join last parts
// 89 23456701
// recursively rotate second part left by 6
// 89 01234567
// join at the end
// 8901234567
// TODO: There are other rotate algorithms.
// This algorithm has the desirable property that it moves each element exactly twice.
// The triple-reverse algorithm is simpler and more cache friendly, but takes more writes.
// The follow-cycles algorithm can be 1-write but it is not very cache friendly.
// rotateLeft rotates b left by n spaces.
// s_final[i] = s_orig[i+r], wrapping around.
func rotateLeft[E any](s []E, r int) {
for r != 0 && r != len(s) {
if r*2 <= len(s) {
swap(s[:r], s[len(s)-r:])
s = s[:len(s)-r]
} else {
swap(s[:len(s)-r], s[r:])
s, r = s[len(s)-r:], r*2-len(s)
}
}
}
func rotateRight[E any](s []E, r int) {
rotateLeft(s, len(s)-r)
}
// swap swaps the contents of x and y. x and y must be equal length and disjoint.
func swap[E any](x, y []E) {
for i := 0; i < len(x); i++ {
x[i], y[i] = y[i], x[i]
}
}
// overlaps reports whether the memory ranges a[0:len(a)] and b[0:len(b)] overlap.
func overlaps[E any](a, b []E) bool {
if len(a) == 0 || len(b) == 0 {
return false
}
elemSize := unsafe.Sizeof(a[0])
if elemSize == 0 {
return false
}
// TODO: use a runtime/unsafe facility once one becomes available. See issue 12445.
// Also see crypto/internal/alias/alias.go:AnyOverlap
return uintptr(unsafe.Pointer(&a[0])) <= uintptr(unsafe.Pointer(&b[len(b)-1]))+(elemSize-1) &&
uintptr(unsafe.Pointer(&b[0])) <= uintptr(unsafe.Pointer(&a[len(a)-1]))+(elemSize-1)
}
// startIdx returns the index in haystack where the needle starts.
// prerequisite: the needle must be aliased entirely inside the haystack.
func startIdx[E any](haystack, needle []E) int {
p := &needle[0]
for i := range haystack {
if p == &haystack[i] {
return i
}
}
// TODO: what if the overlap is by a non-integral number of Es?
panic("needle not found")
}
// Reverse reverses the elements of the slice in place.
func Reverse[S ~[]E, E any](s S) {
for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
s[i], s[j] = s[j], s[i]
}
}