pcg.cpp

// *Really* minimal PCG32 code / (c) 2014 M.E. O'Neill / pcg-random.org
// Licensed under Apache License 2.0 (NO WARRANTY, etc. see website)

#include "pcg.h"

uint32_t pcg32_random_r(pcg32_random_t* rng)
{
    uint64_t oldstate = rng->state;
    // Advance internal state
    rng->state = oldstate * 6364136223846793005ULL + (rng->inc|1);
    // Calculate output function (XSH RR), uses old state for max ILP
    uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
    uint32_t rot = oldstate >> 59u;
    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
}

// Source from http://www.pcg-random.org/downloads/pcg-c-basic-0.9.zip
void pcg32_srandom_r(pcg32_random_t* rng, uint64_t initstate, uint64_t initseq)
{
    rng->state = 0U;
    rng->inc = (initseq << 1u) | 1u;
    pcg32_random_r(rng);
    rng->state += initstate;
    pcg32_random_r(rng);
}

// Source from https://github.com/imneme/pcg-c-basic/blob/master/pcg_basic.c
// pcg32_boundedrand_r(rng, bound):
//     Generate a uniformly distributed number, r, where 0 <= r < bound
uint32_t pcg32_boundedrand_r(pcg32_random_t *rng, uint32_t bound) {
	// 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.
	// A naive scheme to calculate the threshold would be to do
	//
	//     uint32_t threshold = 0x100000000ull % bound;
	//
	// but 64-bit div/mod is slower than 32-bit div/mod (especially on
	// 32-bit platforms).  In essence, we do
	//
	//     uint32_t threshold = (0x100000000ull-bound) % bound;
	//
	// because this version will calculate the same modulus, but the LHS
	// value is less than 2^32.
	uint32_t threshold = -bound % bound;

	// Uniformity guarantees that this loop 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 the worst case, someone passes a bound of 2^31 + 1
	// (i.e., 2147483649), which invalidates almost 50% of the range.  In
	// practice, bounds are typically small and only a tiny amount of the range
	// is eliminated.
	for (;;) {
		uint32_t r = pcg32_random_r(rng);
		if (r >= threshold)
			return r % bound;
	}
}