forked from toolshed/abra
70 lines
1.6 KiB
Go
70 lines
1.6 KiB
Go
// 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()
|
|
}
|