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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
// Stores state of MT19937 generator.
var mtGenSt []uint32 = make([]uint32, mtCoefN)
var mtIndex uint32 = mtCoefN + 1
var mtLowerMask uint32 = (1 << mtCoefR) - 1
var mtUpperMask uint32 = 0xFFFFFFFF & (^mtLowerMask)
func MTSeed(seed uint32) {
mtIndex = mtCoefN
mtGenSt[0] = seed
for i := uint32(1); i < mtCoefN; i++ {
mtGenSt[i] = (0xFFFFFFFF &
(mtF*(mtGenSt[i-1]^(mtGenSt[i-1]>>(mtCoefW-2))) + i))
}
}
func MTExtract() uint32 {
if mtIndex >= mtCoefN {
if mtIndex > mtCoefN {
MTSeed(5489)
}
mtTwist()
}
y := mtGenSt[mtIndex]
y = y ^ ((y >> mtCoefU) & mtCoefD)
y = y ^ ((y << mtCoefS) & mtCoefB)
y = y ^ ((y << mtCoefT) & mtCoefC)
y = y ^ (y >> mtCoefL)
mtIndex = mtIndex + 1
r := 0xFFFFFFFF & y
return r
}
func mtTwist() {
for i := uint32(0); i < mtCoefN-1; i++ {
x := (mtGenSt[i] & mtUpperMask) +
(mtGenSt[(i+1)%mtCoefN] & mtLowerMask)
xA := x >> 1
if x%2 != 0 { // lowest bit of x is 1
xA = xA ^ mtCoefA
}
mtGenSt[i] = mtGenSt[(i+mtCoefM)%mtCoefN] ^ xA
}
mtIndex = 0
}
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