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