1 // 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.
10 package detrand
11 12 import (
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.
20 func Disable() {
21 randSeed = 0
22 }
23 24 // Bool returns a deterministically random boolean.
25 func Bool() bool {
26 return randSeed%2 == 1
27 }
28 29 // Intn returns a deterministically random integer between 0 and n-1, inclusive.
30 func Intn(n int) int {
31 if n <= 0 {
32 panic("must be positive")
33 }
34 return int(randSeed % uint64(n))
35 }
36 37 // randSeed is a best-effort at an approximate hash of the Go binary.
38 var randSeed = binaryHash()
39 40 func binaryHash() uint64 {
41 // Open the Go binary.
42 s, err := os.Executable()
43 if err != nil {
44 return 0
45 }
46 f, err := os.Open(s)
47 if err != nil {
48 return 0
49 }
50 defer f.Close()
51 52 // Hash the size and several samples of the Go binary.
53 const numSamples = 8
54 var buf [64]byte
55 h := fnv.New64()
56 fi, err := f.Stat()
57 if err != nil {
58 return 0
59 }
60 binary.LittleEndian.PutUint64(buf[:8], uint64(fi.Size()))
61 h.Write(buf[:8])
62 for i := int64(0); i < numSamples; i++ {
63 if _, err := f.ReadAt(buf[:], i*fi.Size()/numSamples); err != nil {
64 return 0
65 }
66 h.Write(buf[:])
67 }
68 return h.Sum64()
69 }
70