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v/vlib/rand/pcg32.v
2019-11-19 14:26:18 +03:00

59 lines
1.8 KiB
V

module rand
// Ported from http://www.pcg-random.org/download.html
// and https://github.com/imneme/pcg-c-basic/blob/master/pcg_basic.c
pub struct Pcg32 {
mut:
state u64
inc u64
}
/**
* new_pcg32 - a Pcg32 PRNG generator
* @param initstate - the initial state of the PRNG.
* @param initseq - the stream/step of the PRNG.
* @return a new Pcg32 PRNG instance
*/
pub fn new_pcg32(initstate u64, initseq u64) Pcg32 {
mut rng := Pcg32{}
rng.state = u64(0)
rng.inc = u64( u64(initseq << u64(1)) | u64(1) )
rng.next()
rng.state += initstate
rng.next()
return rng
}
/**
* Pcg32.next - update the PRNG state and get back the next random number
* @return the generated pseudo random number
*/
[inline] pub fn (rng mut Pcg32) next() u32 {
oldstate := rng.state
rng.state = oldstate * u64(6364136223846793005) + rng.inc
xorshifted := u32( u64( u64(oldstate >> u64(18)) ^ oldstate) >> u64(27) )
rot := u32( oldstate >> u64(59) )
return u32( (xorshifted >> rot) | (xorshifted << ((-rot) & u32(31))) )
}
/**
* Pcg32.bounded_next - update the PRNG state. Get the next number < bound
* @param bound - the returned random number will be < bound
* @return the generated pseudo random number
*/
[inline] pub fn (rng mut Pcg32) bounded_next(bound u32) u32 {
// To avoid bias, we need to make the range of the RNG a multiple of
// bound, which we do by dropping output less than a threshold.
threshold := u32( -bound % bound )
// Uniformity guarantees that loop below will terminate. In practice, it
// should usually terminate quickly; on average (assuming all bounds are
// equally likely), 82.25% of the time, we can expect it to require just
// one iteration. In practice, bounds are typically small and only a
// tiny amount of the range is eliminated.
for {
r := rng.next()
if r >= threshold {
return u32( r % bound )
}
}
return u32(0)
}