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. 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// Intn returns a deterministically random integer between 0 and n-1, inclusive. 30func 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. 38var randSeed = binaryHash() 39 40func 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