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// Copyright © 2021 rsiddharth <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package lib
// MT19937 coefficients.
var mtCoefW uint32 = 32
var mtCoefN uint32 = 624
var mtCoefM uint32 = 397
var mtCoefR uint32 = 31
var mtCoefA uint32 = 0x9908B0DF
var mtCoefU uint32 = 11
var mtCoefD uint32 = 0xFFFFFFFF
var mtCoefS uint32 = 7
var mtCoefB uint32 = 0x9D2C5680
var mtCoefT uint32 = 15
var mtCoefC uint32 = 0xEFC60000
var mtCoefL uint32 = 18
var mtF uint32 = 1812433253
// MT19937 instance struct
type MTRand struct {
GenSt [624]uint32
Index uint32
initialized bool
}
var mtLowerMask uint32 = (1 << mtCoefR) - 1
var mtUpperMask uint32 = 0xFFFFFFFF & (^mtLowerMask)
func (r *MTRand) Seed(s uint32) {
r.Index = mtCoefN
r.GenSt[0] = s
for i := uint32(1); i < mtCoefN; i++ {
r.GenSt[i] = (0xFFFFFFFF &
(mtF*(r.GenSt[i-1]^(r.GenSt[i-1]>>(mtCoefW-2))) + i))
}
r.initialized = true
}
func (r *MTRand) Extract() uint32 {
if !r.initialized || r.Index >= mtCoefN {
if !r.initialized {
r.Seed(5489)
}
r.twist()
}
y := r.GenSt[r.Index]
y = y ^ ((y >> mtCoefU) & mtCoefD)
y = y ^ ((y << mtCoefS) & mtCoefB)
y = y ^ ((y << mtCoefT) & mtCoefC)
y = y ^ (y >> mtCoefL)
r.Index = r.Index + 1
y = 0xFFFFFFFF & y
return y
}
func (r *MTRand) twist() {
for i := uint32(0); i < mtCoefN-1; i++ {
x := (r.GenSt[i] & mtUpperMask) +
(r.GenSt[(i+1)%mtCoefN] & mtLowerMask)
xA := x >> 1
if x%2 != 0 { // lowest bit of x is 1
xA = xA ^ mtCoefA
}
r.GenSt[i] = r.GenSt[(i+mtCoefM)%mtCoefN] ^ xA
}
r.Index = 0
}
func (r *MTRand) UnTemper(y uint32) uint32 {
y = y ^ (y >> mtCoefL)
y0 := y
y = y0 ^ ((y0 << mtCoefT) & mtCoefC)
y = y0 ^ ((y << mtCoefT) & mtCoefC)
y0 = y
y = y0 ^ ((y0 << mtCoefS) & mtCoefB)
y = y0 ^ ((y << mtCoefS) & mtCoefB)
y = y0 ^ ((y << mtCoefS) & mtCoefB)
y = y0 ^ ((y << mtCoefS) & mtCoefB)
y0 = y
y = y0 ^ (y0 >> mtCoefU)
y = y0 ^ (y >> mtCoefU)
y = 0xFFFFFFFF & y
return y
}
// Returns a stream function. The stream function returns a random
// byte when invoked.
//
// The `seed` argument must be exactly bytes.
func keystream(seed []byte) func() byte {
if len(seed) != 2 {
return nil
}
// Pack seed into a uint32
s := (uint32(seed[0]) << 8) ^ uint32(seed[1])
// Init MT19937.
mtR := new(MTRand)
mtR.Seed(s)
// Stream func.
n := uint32(0)
return func() byte {
if n == uint32(0) {
n = mtR.Extract()
}
r := byte(n & 0xFF) // Extract last 8 bits.
n = n >> 8 // Get rid of the last 8 bits.
return r
}
}
// XORs `stream` with the MT19937 keystream seeded with `seed`.
func MTXORStream(stream, seed []byte) []byte {
if len(stream) == 0 {
return []byte{}
}
if len(seed) != 2 {
return nil
}
ks := keystream(seed)
if ks == nil {
return nil
}
s := make([]byte, 0)
for i := 0; i < len(stream); i++ {
s = append(s, stream[i]^ks())
}
return s
}
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