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[moveonly] Move MapIntoRange() to separate util/fastrange.h

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This commit is contained in:
Pieter Wuille 2022-01-06 11:27:06 -05:00
parent e941507500
commit c6d15c45d9
3 changed files with 44 additions and 31 deletions
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1
src/Makefile.am
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@ -240,6 +240,7 @@ BITCOIN_CORE_H = \
util/check.h \
util/epochguard.h \
util/error.h \
util/fastrange.h \
util/fees.h \
util/getuniquepath.h \
util/golombrice.h \

41
src/util/fastrange.h Normal file
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@ -0,0 +1,41 @@
// Copyright (c) 2018-2020 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
#ifndef BITCOIN_UTIL_FASTRANGE_H
#define BITCOIN_UTIL_FASTRANGE_H
#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/
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_FASTRANGE_H

33
src/util/golombrice.h
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@ -5,6 +5,8 @@
#ifndef BITCOIN_UTIL_GOLOMBRICE_H
#define BITCOIN_UTIL_GOLOMBRICE_H
#include <util/fastrange.h>
#include <streams.h>
#include <cstdint>
@ -40,35 +42,4 @@ 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