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