blob: a904dd1f91a2db0b463f61e3fa944ef2efbbccc9 [file] [log] [blame]
mpagenkoaf801632020-07-03 10:00:42 +00001// Copyright 2018 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.
4
5// Package detrand provides deterministically random functionality.
6//
7// The pseudo-randomness of these functions is seeded by the program binary
8// itself and guarantees that the output does not change within a program,
9// while ensuring that the output is unstable across different builds.
10package detrand
11
12import (
13 "encoding/binary"
14 "hash/fnv"
15 "os"
16)
17
18// Disable disables detrand such that all functions returns the zero value.
19// This function is not concurrent-safe and must be called during program init.
20func Disable() {
21 randSeed = 0
22}
23
24// Bool returns a deterministically random boolean.
25func Bool() bool {
26 return randSeed%2 == 1
27}
28
29// randSeed is a best-effort at an approximate hash of the Go binary.
30var randSeed = binaryHash()
31
32func binaryHash() uint64 {
33 // Open the Go binary.
34 s, err := os.Executable()
35 if err != nil {
36 return 0
37 }
38 f, err := os.Open(s)
39 if err != nil {
40 return 0
41 }
42 defer f.Close()
43
44 // Hash the size and several samples of the Go binary.
45 const numSamples = 8
46 var buf [64]byte
47 h := fnv.New64()
48 fi, err := f.Stat()
49 if err != nil {
50 return 0
51 }
52 binary.LittleEndian.PutUint64(buf[:8], uint64(fi.Size()))
53 h.Write(buf[:8])
54 for i := int64(0); i < numSamples; i++ {
55 if _, err := f.ReadAt(buf[:], i*fi.Size()/numSamples); err != nil {
56 return 0
57 }
58 h.Write(buf[:])
59 }
60 return h.Sum64()
61}