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// Copyright 2020 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 fuzz
import (
"math/bits"
"os"
"strconv"
"strings"
"sync/atomic"
"time"
)
type mutatorRand interface {
uint32() uint32
intn(int) int
uint32n(uint32) uint32
exp2() int
bool() bool
save(randState, randInc *uint64)
restore(randState, randInc uint64)
}
// The functions in pcg implement a 32 bit PRNG with a 64 bit period: pcg xsh rr
// 64 32. See https://www.pcg-random.org/ for more information. This
// implementation is geared specifically towards the needs of fuzzing: Simple
// creation and use, no reproducibility, no concurrency safety, just the
// necessary methods, optimized for speed.
var globalInc atomic.Uint64 // PCG stream
const multiplier uint64 = 6364136223846793005
// pcgRand is a PRNG. It should not be copied or shared. No Rand methods are
// concurrency safe.
type pcgRand struct {
noCopy noCopy // help avoid mistakes: ask vet to ensure that we don't make a copy
state uint64
inc uint64
}
func godebugSeed() *int {
debug := strings.Split(os.Getenv("GODEBUG"), ",")
for _, f := range debug {
if strings.HasPrefix(f, "fuzzseed=") {
seed, err := strconv.Atoi(strings.TrimPrefix(f, "fuzzseed="))
if err != nil {
panic("malformed fuzzseed")
}
return &seed
}
}
return nil
}
// newPcgRand generates a new, seeded Rand, ready for use.
func newPcgRand() *pcgRand {
r := new(pcgRand)
now := uint64(time.Now().UnixNano())
if seed := godebugSeed(); seed != nil {
now = uint64(*seed)
}
inc := globalInc.Add(1)
r.state = now
r.inc = (inc << 1) | 1
r.step()
r.state += now
r.step()
return r
}
func (r *pcgRand) step() {
r.state *= multiplier
r.state += r.inc
}
func (r *pcgRand) save(randState, randInc *uint64) {
*randState = r.state
*randInc = r.inc
}
func (r *pcgRand) restore(randState, randInc uint64) {
r.state = randState
r.inc = randInc
}
// uint32 returns a pseudo-random uint32.
func (r *pcgRand) uint32() uint32 {
x := r.state
r.step()
return bits.RotateLeft32(uint32(((x>>18)^x)>>27), -int(x>>59))
}
// intn returns a pseudo-random number in [0, n).
// n must fit in a uint32.
func (r *pcgRand) intn(n int) int {
if int(uint32(n)) != n {
panic("large Intn")
}
return int(r.uint32n(uint32(n)))
}
// uint32n returns a pseudo-random number in [0, n).
//
// For implementation details, see:
// https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction
// https://lemire.me/blog/2016/06/30/fast-random-shuffling
func (r *pcgRand) uint32n(n uint32) uint32 {
v := r.uint32()
prod := uint64(v) * uint64(n)
low := uint32(prod)
if low < n {
thresh := uint32(-int32(n)) % n
for low < thresh {
v = r.uint32()
prod = uint64(v) * uint64(n)
low = uint32(prod)
}
}
return uint32(prod >> 32)
}
// exp2 generates n with probability 1/2^(n+1).
func (r *pcgRand) exp2() int {
return bits.TrailingZeros32(r.uint32())
}
// bool generates a random bool.
func (r *pcgRand) bool() bool {
return r.uint32()&1 == 0
}
// noCopy may be embedded into structs which must not be copied
// after the first use.
//
// See https://golang.org/issues/8005#issuecomment-190753527
// for details.
type noCopy struct{}
// lock is a no-op used by -copylocks checker from `go vet`.
func (*noCopy) lock() {}
func (*noCopy) unlock() {}
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