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// Copyright 2017 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 rand
import (
"encoding/binary"
"io"
"math/bits"
)
// PCGSource is an implementation of a 64-bit permuted congruential
// generator as defined in
//
// PCG: A Family of Simple Fast Space-Efficient Statistically Good
// Algorithms for Random Number Generation
// Melissa E. O’Neill, Harvey Mudd College
// http://www.pcg-random.org/pdf/toms-oneill-pcg-family-v1.02.pdf
//
// The generator here is the congruential generator PCG XSL RR 128/64 (LCG)
// as found in the software available at http://www.pcg-random.org/.
// It has period 2^128 with 128 bits of state, producing 64-bit values.
// Is state is represented by two uint64 words.
type PCGSource struct {
low uint64
high uint64
}
const (
maxUint32 = (1 << 32) - 1
multiplier = 47026247687942121848144207491837523525
mulHigh = multiplier >> 64
mulLow = multiplier & maxUint64
increment = 117397592171526113268558934119004209487
incHigh = increment >> 64
incLow = increment & maxUint64
// TODO: Use these?
initializer = 245720598905631564143578724636268694099
initHigh = initializer >> 64
initLow = initializer & maxUint64
)
// Seed uses the provided seed value to initialize the generator to a deterministic state.
func (pcg *PCGSource) Seed(seed uint64) {
pcg.low = seed
pcg.high = seed // TODO: What is right?
}
// Uint64 returns a pseudo-random 64-bit unsigned integer as a uint64.
func (pcg *PCGSource) Uint64() uint64 {
pcg.multiply()
pcg.add()
// XOR high and low 64 bits together and rotate right by high 6 bits of state.
return bits.RotateLeft64(pcg.high^pcg.low, -int(pcg.high>>58))
}
func (pcg *PCGSource) add() {
var carry uint64
pcg.low, carry = bits.Add64(pcg.low, incLow, 0)
pcg.high, _ = bits.Add64(pcg.high, incHigh, carry)
}
func (pcg *PCGSource) multiply() {
hi, lo := bits.Mul64(pcg.low, mulLow)
hi += pcg.high * mulLow
hi += pcg.low * mulHigh
pcg.low = lo
pcg.high = hi
}
// MarshalBinary returns the binary representation of the current state of the generator.
func (pcg *PCGSource) MarshalBinary() ([]byte, error) {
var buf [16]byte
binary.BigEndian.PutUint64(buf[:8], pcg.high)
binary.BigEndian.PutUint64(buf[8:], pcg.low)
return buf[:], nil
}
// UnmarshalBinary sets the state of the generator to the state represented in data.
func (pcg *PCGSource) UnmarshalBinary(data []byte) error {
if len(data) < 16 {
return io.ErrUnexpectedEOF
}
pcg.low = binary.BigEndian.Uint64(data[8:])
pcg.high = binary.BigEndian.Uint64(data[:8])
return nil
}
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