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// 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}
62