59 lines
2.2 KiB
C++
59 lines
2.2 KiB
C++
// *Really* minimal PCG32 code / (c) 2014 M.E. O'Neill / pcg-random.org
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// Licensed under Apache License 2.0 (NO WARRANTY, etc. see website)
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#include "pcg.h"
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uint32_t pcg32_random_r(pcg32_random_t* rng)
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{
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uint64_t oldstate = rng->state;
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// Advance internal state
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rng->state = oldstate * 6364136223846793005ULL + (rng->inc|1);
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// Calculate output function (XSH RR), uses old state for max ILP
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uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
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uint32_t rot = oldstate >> 59u;
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return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
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}
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// Source from http://www.pcg-random.org/downloads/pcg-c-basic-0.9.zip
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void pcg32_srandom_r(pcg32_random_t* rng, uint64_t initstate, uint64_t initseq)
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{
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rng->state = 0U;
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rng->inc = (initseq << 1u) | 1u;
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pcg32_random_r(rng);
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rng->state += initstate;
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pcg32_random_r(rng);
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}
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// Source from https://github.com/imneme/pcg-c-basic/blob/master/pcg_basic.c
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// pcg32_boundedrand_r(rng, bound):
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// Generate a uniformly distributed number, r, where 0 <= r < bound
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uint32_t pcg32_boundedrand_r(pcg32_random_t *rng, uint32_t bound) {
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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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// A naive scheme to calculate the threshold would be to do
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//
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// uint32_t threshold = 0x100000000ull % bound;
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//
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// but 64-bit div/mod is slower than 32-bit div/mod (especially on
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// 32-bit platforms). In essence, we do
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//
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// uint32_t threshold = (0x100000000ull-bound) % bound;
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//
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// because this version will calculate the same modulus, but the LHS
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// value is less than 2^32.
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uint32_t threshold = -bound % bound;
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// Uniformity guarantees that this loop 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 the worst case, someone passes a bound of 2^31 + 1
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// (i.e., 2147483649), which invalidates almost 50% of the range. In
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// practice, bounds are typically small and only a tiny amount of the range
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// is eliminated.
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for (;;) {
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uint32_t r = pcg32_random_r(rng);
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if (r >= threshold)
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return r % bound;
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}
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}
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