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