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use much faster get_random_{type}_uniform approach

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This commit is contained in:
Daniel Micay 2018-08-30 01:16:58 -04:00
parent 1c181557c7
commit 99fcddc223
3 changed files with 231 additions and 41 deletions
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48
random.c
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@ -69,20 +69,20 @@ uint16_t get_random_u16(struct random_state *state) {
return value;
}
// based on OpenBSD arc4random_uniform
// See Fast Random Integer Generation in an Interval by Daniel Lemire
uint16_t get_random_u16_uniform(struct random_state *state, uint16_t bound) {
if (bound < 2) {
return 0;
uint32_t random = get_random_u16(state);
uint32_t multiresult = random * bound;
uint16_t leftover = multiresult;
if (leftover < bound) {
uint16_t threshold = -bound % bound;
while (leftover < threshold) {
random = get_random_u16(state);
multiresult = random * bound;
leftover = (uint16_t)multiresult;
}
}
uint16_t min = (uint16_t)-bound % bound;
uint16_t r;
do {
r = get_random_u16(state);
} while (unlikely(r < min));
return r % bound;
return multiresult >> 16;
}
uint64_t get_random_u64(struct random_state *state) {
@ -96,18 +96,18 @@ uint64_t get_random_u64(struct random_state *state) {
return value;
}
// based on OpenBSD arc4random_uniform
// See Fast Random Integer Generation in an Interval by Daniel Lemire
uint64_t get_random_u64_uniform(struct random_state *state, uint64_t bound) {
if (bound < 2) {
return 0;
unsigned __int128 random = get_random_u64(state);
unsigned __int128 multiresult = random * bound;
uint64_t leftover = multiresult;
if (leftover < bound) {
uint64_t threshold = -bound % bound;
while (leftover < threshold) {
random = get_random_u64(state);
multiresult = random * bound;
leftover = multiresult;
}
}
uint64_t min = -bound % bound;
uint64_t r;
do {
r = get_random_u64(state);
} while (unlikely(r < min));
return r % bound;
return multiresult >> 64;
}