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