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Optimize pcg32_boundedrand_r() #9

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Optimize pcg32_boundedrand_r() #9

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27 changes: 9 additions & 18 deletions pcg_basic.c
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Original file line number Diff line number Diff line change
Expand Up @@ -78,22 +78,6 @@ uint32_t pcg32_random()

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
Expand All @@ -103,8 +87,15 @@ uint32_t pcg32_boundedrand_r(pcg32_random_t* rng, uint32_t bound)
// is eliminated.
for (;;) {
uint32_t r = pcg32_random_r(rng);
if (r >= threshold)
return r % bound;

// Start of bound-multiple segment which holds r
uint32_t rint = r / bound * bound;

// Check if the end of that segment do not overflow,
// i.e. rint + bound <= 2^32 or
// rint <= 2^32 - bound = (uint32_t)(-bound)
if (rint <= (uint32_t)(-bound))
return r - rint;
}
}

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