mirrors/serenity
1
0
Fork 0
mirror of https://github.com/SerenityOS/serenity.git synced 2025-01-22 09:21:57 -05:00
Code Issues Projects Releases Packages Wiki Activity

AK: Improve performance of u64 randoms

Browse source
This commit is contained in:
Pavlo Ivashkov 2025-01-06 23:08:00 +02:00 committed by Nico Weber
parent 1e935911f0
commit 31daabcf0a
1 changed files with 9 additions and 11 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

20
AK/Random.cpp
View file

@ -5,21 +5,19 @@
*/
#include <AK/Random.h>
#include <AK/UFixedBigInt.h>
#include <AK/UFixedBigIntDivision.h>
namespace AK {
u32 get_random_uniform(u32 max_bounds)
{
// If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up
// with a group around 2**32-1 that is a bit too small. For this reason, the implementation
// `arc4random() % max_bounds` would be insufficient. Here we compute the last number of the
// last "full group". Note that if max_bounds is a divisor of UINT32_MAX,
// then we end up with UINT32_MAX:
u32 const max_usable = UINT32_MAX - (static_cast<u64>(UINT32_MAX) + 1) % max_bounds;
// with one group that is a bit too small. For this reason, the implementation
// `arc4random() % max_bounds` would be insufficient. Here we compute the first number of
// the first "full group" as 2**32 % max_bounds, and rely on the equivalence
// `2**32 % x == (2**32 - x) % x` to keep calculations in u32 domain:
u32 const threshold = static_cast<u32>(-max_bounds) % max_bounds;
auto random_value = get_random<u32>();
for (int i = 0; i < 20 && random_value > max_usable; ++i) {
for (int i = 0; i < 20 && random_value < threshold; ++i) {
// By chance we picked a value from the incomplete group. Note that this group has size at
// most 2**31-1, so picking this group has a chance of less than 50%.
// In practice, this means that for the worst possible input, there is still only a
@ -33,10 +31,10 @@ u32 get_random_uniform(u32 max_bounds)
u64 get_random_uniform_64(u64 max_bounds)
{
// Uses the same algorithm as `get_random_uniform`,
// by replacing u64 with u128 and u32 with u64.
u64 const max_usable = UINT64_MAX - static_cast<u64>((static_cast<u128>(UINT64_MAX) + 1) % max_bounds);
// by replacing u32 with u64.
u64 const threshold = static_cast<u64>(-max_bounds) % max_bounds;
auto random_value = get_random<u64>();
for (int i = 0; i < 20 && random_value > max_usable; ++i) {
for (int i = 0; i < 20 && random_value < threshold; ++i) {
random_value = get_random<u64>();
}
return random_value % max_bounds;