lib/connections: Refactor address listing into connection service

lib/util/random.go

@@ -0,0 +1,59 @@
+// 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:]))
+}