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crrand: use Go prng to seed Perm64 #23

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Sep 14, 2025
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crrand: use Go prng to seed Perm64 #23

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29 changes: 13 additions & 16 deletions crrand/perm.go
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Original file line number Diff line number Diff line change
Expand Up @@ -16,27 +16,24 @@
// generation.
package crrand

import "math/bits"
import (
"math/bits"
"math/rand/v2"
)

// MakePerm64 constructs a new Perm64 from a 64-bit seed, providing a
// deterministic, pseudorandom, bijective mapping of 64-bit values X to 64-bit
// values Y.
func MakePerm64(seed uint64) Perm64 {
// derive 4 x 32-bit round keys from the 64-bit seed using only ARX ops.
const c0 = 0x9E3779B97F4A7C15 // golden ratio (used here as XOR salt)
const c1 = 0xC2B2AE3D27D4EB4F // a constant

var m Perm64
s0 := seed
s1 := bits.RotateLeft64(seed^c0, 13)
s2 := bits.RotateLeft64(seed^c1, 37)
s3 := bits.RotateLeft64(seed^c0^c1, 53)

m.seed[0] = uint32(s0)
m.seed[1] = uint32(s1 >> 32)
m.seed[2] = uint32(s2)
m.seed[3] = uint32(s3 >> 32)
return m
prng := rand.New(rand.NewPCG(seed, seed))
return Perm64{
seed: [4]uint32{
prng.Uint32(),
prng.Uint32(),
prng.Uint32(),
prng.Uint32(),
},
}
}

// A Perm64 provides a deterministic, pseudorandom permutation of 64-bit values.
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