diff options
Diffstat (limited to 'src/util/golombrice.h')
| -rw-r--r-- | src/util/golombrice.h | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/src/util/golombrice.h b/src/util/golombrice.h index 67d262406f..f14cb594db 100644 --- a/src/util/golombrice.h +++ b/src/util/golombrice.h @@ -40,4 +40,35 @@ uint64_t GolombRiceDecode(BitStreamReader<IStream>& bitreader, uint8_t P) return (q << P) + r; } +// 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/ +static inline uint64_t MapIntoRange(uint64_t x, uint64_t n) +{ +#ifdef __SIZEOF_INT128__ + return (static_cast<unsigned __int128>(x) * static_cast<unsigned __int128>(n)) >> 64; +#else + // To perform the calculation on 64-bit numbers without losing the + // result to overflow, split the numbers into the most significant and + // least significant 32 bits and perform multiplication piece-wise. + // + // See: https://stackoverflow.com/a/26855440 + const uint64_t x_hi = x >> 32; + const uint64_t x_lo = x & 0xFFFFFFFF; + const uint64_t n_hi = n >> 32; + const uint64_t n_lo = n & 0xFFFFFFFF; + + const uint64_t ac = x_hi * n_hi; + const uint64_t ad = x_hi * n_lo; + const uint64_t bc = x_lo * n_hi; + const uint64_t bd = x_lo * n_lo; + + const uint64_t mid34 = (bd >> 32) + (bc & 0xFFFFFFFF) + (ad & 0xFFFFFFFF); + const uint64_t upper64 = ac + (bc >> 32) + (ad >> 32) + (mid34 >> 32); + return upper64; +#endif +} + #endif // BITCOIN_UTIL_GOLOMBRICE_H |