rand.go (1683B)
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 }