syncthing/lib/util/random.go

60 lines
1.8 KiB
Go

// Copyright (C) 2014 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at http://mozilla.org/MPL/2.0/.
package util
import (
"crypto/md5"
cryptoRand "crypto/rand"
"encoding/binary"
"io"
mathRand "math/rand"
)
// randomCharset contains the characters that can make up a randomString().
const randomCharset = "01234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-"
// PredictableRandom is an RNG that will always have the same sequence. It
// will be seeded with the device ID during startup, so that the sequence is
// predictable but varies between instances.
var PredictableRandom = mathRand.New(mathRand.NewSource(42))
func init() {
// The default RNG should be seeded with something good.
mathRand.Seed(RandomInt64())
}
// RandomString returns a string of random characters (taken from
// randomCharset) of the specified length.
func RandomString(l int) string {
bs := make([]byte, l)
for i := range bs {
bs[i] = randomCharset[mathRand.Intn(len(randomCharset))]
}
return string(bs)
}
// RandomInt64 returns a strongly random int64, slowly
func RandomInt64() int64 {
var bs [8]byte
_, err := io.ReadFull(cryptoRand.Reader, bs[:])
if err != nil {
panic("randomness failure: " + err.Error())
}
return SeedFromBytes(bs[:])
}
// SeedFromBytes calculates a weak 64 bit hash from the given byte slice,
// suitable for use a predictable random seed.
func SeedFromBytes(bs []byte) int64 {
h := md5.New()
h.Write(bs)
s := h.Sum(nil)
// The MD5 hash of the byte slice is 16 bytes long. We interpret it as two
// uint64s and XOR them together.
return int64(binary.BigEndian.Uint64(s[0:]) ^ binary.BigEndian.Uint64(s[8:]))
}