First implementation

vendor/google.golang.org/protobuf/internal/detrand/rand.go

@@ -0,0 +1,69 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package detrand provides deterministically random functionality.
+//
+// The pseudo-randomness of these functions is seeded by the program binary
+// itself and guarantees that the output does not change within a program,
+// while ensuring that the output is unstable across different builds.
+package detrand
+
+import (
+	"encoding/binary"
+	"hash/fnv"
+	"os"
+)
+
+// Disable disables detrand such that all functions returns the zero value.
+// This function is not concurrent-safe and must be called during program init.
+func Disable() {
+	randSeed = 0
+}
+
+// Bool returns a deterministically random boolean.
+func Bool() bool {
+	return randSeed%2 == 1
+}
+
+// Intn returns a deterministically random integer between 0 and n-1, inclusive.
+func Intn(n int) int {
+	if n <= 0 {
+		panic("must be positive")
+	}
+	return int(randSeed % uint64(n))
+}
+
+// randSeed is a best-effort at an approximate hash of the Go binary.
+var randSeed = binaryHash()
+
+func binaryHash() uint64 {
+	// Open the Go binary.
+	s, err := os.Executable()
+	if err != nil {
+		return 0
+	}
+	f, err := os.Open(s)
+	if err != nil {
+		return 0
+	}
+	defer f.Close()
+
+	// Hash the size and several samples of the Go binary.
+	const numSamples = 8
+	var buf [64]byte
+	h := fnv.New64()
+	fi, err := f.Stat()
+	if err != nil {
+		return 0
+	}
+	binary.LittleEndian.PutUint64(buf[:8], uint64(fi.Size()))
+	h.Write(buf[:8])
+	for i := int64(0); i < numSamples; i++ {
+		if _, err := f.ReadAt(buf[:], i*fi.Size()/numSamples); err != nil {
+			return 0
+		}
+		h.Write(buf[:])
+	}
+	return h.Sum64()
+}