````// Copyright 2009 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 subtle implements functions that are often useful in cryptographic`
`// code but require careful thought to use correctly.``package subtle`

`// ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents`
`// and 0 otherwise. The time taken is a function of the length of the slices and`
`// is independent of the contents.`
`func ConstantTimeCompare(x, y []byte) int {`
`	if len(x) != len(y) {`
`		return 0`
`	}`

`	var v byte`

`	for i := 0; i < len(x); i++ {`
`		v |= x[i] ^ y[i]`
`	}`

`	return ConstantTimeByteEq(v, 0)`
`}`

`// ConstantTimeSelect returns x if v == 1 and y if v == 0.`
`// Its behavior is undefined if v takes any other value.`
`func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }`

`// ConstantTimeByteEq returns 1 if x == y and 0 otherwise.`
`func ConstantTimeByteEq(x, y uint8) int {`
`	return int((uint32(x^y) - 1) >> 31)`
`}`

`// ConstantTimeEq returns 1 if x == y and 0 otherwise.`
`func ConstantTimeEq(x, y int32) int {`
`	return int((uint64(uint32(x^y)) - 1) >> 63)`
`}`

`// ConstantTimeCopy copies the contents of y into x (a slice of equal length)`
`// if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v`
`// takes any other value.`
`func ConstantTimeCopy(v int, x, y []byte) {`
`	if len(x) != len(y) {`
`		panic("subtle: slices have different lengths")`
`	}`

`	xmask := byte(v - 1)`
`	ymask := byte(^(v - 1))`
`	for i := 0; i < len(x); i++ {`
`		x[i] = x[i]&xmask | y[i]&ymask`
`	}`
`}`

`// ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.`
`// Its behavior is undefined if x or y are negative or > 2**31 - 1.`
`func ConstantTimeLessOrEq(x, y int) int {`
`	x32 := int32(x)`
`	y32 := int32(y)`
`	return int(((x32 - y32 - 1) >> 31) & 1)`
`}`
```