v/vlib/rand/pcg32.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

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)
}