source: code/trunk/vendor/google.golang.org/protobuf/internal/detrand/rand.go@ 822

// 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()
}