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Add FastRange32 function and use it throughout the codebase

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This commit is contained in:
Pieter Wuille
2022-01-06 11:46:50 -05:00
parent 96ecd6fa3e
commit efab28b06b
3 changed files with 30 additions and 30 deletions
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20
src/util/fastrange.h
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@@ -7,11 +7,21 @@
#include <cstdint>
// Map a value x that is uniformly distributed in the range [0, 2^64) to a
// value uniformly distributed in [0, n) by returning the upper 64 bits of
// x * n.
//
// See: https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
/* This file offers implementations of the fast range reduction technique described
* in https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
*
* In short, they take an integer x and a range n, and return the upper bits of
* (x * n). If x is uniformly distributed over its domain, the result is as close to
* uniformly distributed over [0, n) as (x mod n) would be, but significantly faster.
*/
/** Fast range reduction with 32-bit input and 32-bit range. */
static inline uint32_t FastRange32(uint32_t x, uint32_t n)
{
return (uint64_t{x} * n) >> 32;
}
/** Fast range reduction with 64-bit input and 64-bit range. */
static inline uint64_t FastRange64(uint64_t x, uint64_t n)
{
#ifdef __SIZEOF_INT128__