// 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 FastRange64(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