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// Copyright 2018 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 detrand provides deterministically random functionality.
//
// The pseudo-randomness of these functions is seeded by the program binary
// itself and guarantees that the output does not change within a program,
// while ensuring that the output is unstable across different builds.
package detrand
import (
"encoding/binary"
"hash/fnv"
"os"
)
// Disable disables detrand such that all functions returns the zero value.
// This function is not concurrent-safe and must be called during program init.
func Disable() {
randSeed = 0
}
// Bool returns a deterministically random boolean.
func Bool() bool {
return randSeed%2 == 1
}
// Intn returns a deterministically random integer between 0 and n-1, inclusive.
func Intn(n int) int {
if n <= 0 {
panic("must be positive")
}
return int(randSeed % uint64(n))
}
// randSeed is a best-effort at an approximate hash of the Go binary.
var randSeed = binaryHash()
func binaryHash() uint64 {
// Open the Go binary.
s, err := os.Executable()
if err != nil {
return 0
}
f, err := os.Open(s)
if err != nil {
return 0
}
defer f.Close()
// Hash the size and several samples of the Go binary.
const numSamples = 8
var buf [64]byte
h := fnv.New64()
fi, err := f.Stat()
if err != nil {
return 0
}
binary.LittleEndian.PutUint64(buf[:8], uint64(fi.Size()))
h.Write(buf[:8])
for i := int64(0); i < numSamples; i++ {
if _, err := f.ReadAt(buf[:], i*fi.Size()/numSamples); err != nil {
return 0
}
h.Write(buf[:])
}
return h.Sum64()
}
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