libcruft-util/hash/xxhash.cpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2016-2018 Danny Robson <danny@nerdcruft.net>
 */

#include "xxhash.hpp"

#include "../bitwise.hpp"
#include "../debug.hpp"
#include "../endian.hpp"

#include <cstring>

using cruft::hash::xxhash;


///////////////////////////////////////////////////////////////////////////////
template <typename T>
static
T
read_le (const void *ptr)
{
    return *static_cast<const T*> (ptr);
}


///////////////////////////////////////////////////////////////////////////////
template <typename T>
struct constants {
    static const T prime[5];
    static const T final_rotate[3];
    static const T round_rotate;
};


//-----------------------------------------------------------------------------
template <>
const uint32_t
constants<uint32_t>::prime[5] = {
        2654435761u,
        2246822519u,
        3266489917u,
         668265263u,
         374761393u,
};


template <>
const uint32_t
constants<uint32_t>::final_rotate[3] = {
    15, 13, 16
};


//-----------------------------------------------------------------------------
template <>
const uint32_t
constants<uint32_t>::round_rotate = 13;


//-----------------------------------------------------------------------------
template <>
const uint64_t constants<uint64_t>::prime[5] = {
    11400714785074694791ull,
    14029467366897019727ull,
     1609587929392839161ull,
     9650029242287828579ull,
     2870177450012600261ull,
};


template <>
const uint64_t
constants<uint64_t>::final_rotate[3] = {
    33, 29, 32
};


//-----------------------------------------------------------------------------
template <>
const uint64_t
constants<uint64_t>::round_rotate = 31;


///////////////////////////////////////////////////////////////////////////////
template <typename T>
static
T
round (T state, T input)
{
    state += input * constants<T>::prime[1];
    state  = cruft::rotatel (state, constants<T>::round_rotate);
    state *= constants<T>::prime[0];

    return state;
}


///////////////////////////////////////////////////////////////////////////////
template <typename WordT>
xxhash<WordT>::xxhash (WordT _seed):
    m_seed  (_seed)
{ ; }


///////////////////////////////////////////////////////////////////////////////
template <typename WordT>
typename xxhash<WordT>::digest_t
xxhash<WordT>::operator() (const cruft::view<const uint8_t*> data)
{
    word_t state[4] {
        m_seed + constants<WordT>::prime[0] + constants<WordT>::prime[1],
        m_seed +                              constants<WordT>::prime[1],
        m_seed,
        m_seed - constants<WordT>::prime[0],
    };

    // consume block sized chunks while they're available.
    // process each state word independently per block.
    auto cursor = std::cbegin (data);
    const auto last = std::cend (data);
    while (last - cursor > block_bytes) {
        for (int i = 0; i < 4; ++i) {
            state[i] = round<word_t> (state[i], read_le<word_t> (cursor));
            cursor += sizeof (word_t);
        }
    }

    // leave the remainder. it's used midway through finalisation. note that we
    // don't update the cursor as it's used to detect the remaining bytes
    // during finalisation.
    ;

    // compress the state and mix in the data size
    word_t h;
    if (data.size () < block_bytes) {
        h = state[2] + constants<WordT>::prime[4];
    } else {
        h = rotatel (state[0],  1) +
            rotatel (state[1],  7) +
            rotatel (state[2], 12) +
            rotatel (state[3], 18);

        if constexpr (std::is_same_v<WordT,uint64_t>) {
            h = (h ^ round<WordT> (0, state[0])) * constants<WordT>::prime[0] + constants<WordT>::prime[3];
            h = (h ^ round<WordT> (0, state[1])) * constants<WordT>::prime[0] + constants<WordT>::prime[3];
            h = (h ^ round<WordT> (0, state[2])) * constants<WordT>::prime[0] + constants<WordT>::prime[3];
            h = (h ^ round<WordT> (0, state[3])) * constants<WordT>::prime[0] + constants<WordT>::prime[3];
        }
    }

    h += static_cast<WordT> (data.size ());

    // drain the remainder of the data, first by words...
    while (cursor + sizeof (WordT) <= last) {
        if constexpr (std::is_same_v<WordT,uint32_t>) {
            h += read_le<WordT> (cursor) * constants<WordT>::prime[2];
            h  = rotatel (h, 17)         * constants<WordT>::prime[3];
        } else {
            h = rotatel (
                h ^ round<WordT> (0, read_le<WordT> (cursor)), 27
            ) * constants<WordT>::prime[0] + constants<WordT>::prime[3];
        }

        cursor += sizeof (WordT);
    }

    // ...then maybe by half words...
    if constexpr (std::is_same_v<WordT,uint64_t>) {
        while (cursor + sizeof (uint32_t) <= last) {
            h = rotatel (
                h ^ read_le<uint32_t> (cursor) * constants<WordT>::prime[0], 23
            ) * constants<WordT>::prime[1] + constants<WordT>::prime[2];

            cursor += sizeof (uint32_t);
        }
    }

    // ...then by bytes
    while (cursor != last) {
        if constexpr (std::is_same_v<WordT,uint32_t>) {
            h += *cursor         * constants<WordT>::prime[4];
            h  = rotatel (h, 11) * constants<WordT>::prime[0];
        } else {
            h = rotatel (h ^ *cursor * constants<WordT>::prime[4], 11) * constants<WordT>::prime[0];
        }

        ++cursor;
    }

    // everything should have been consumed by now
    CHECK_EQ (cursor, std::cend (data));

    // mix the result one last time before returning
    h ^= h >> constants<WordT>::final_rotate[0]; h *= constants<WordT>::prime[1];
    h ^= h >> constants<WordT>::final_rotate[1]; h *= constants<WordT>::prime[2];
    h ^= h >> constants<WordT>::final_rotate[2];

    return h;
};


///////////////////////////////////////////////////////////////////////////////
template class cruft::hash::xxhash<uint32_t>;
template class cruft::hash::xxhash<uint64_t>;