ConstexprXXH3.h

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/** @noop License Comment
 *  @file
 *  @copyright
 *  BSD 2-Clause License
 *  @copyright
 *  constexpr-xxh3 - C++20 constexpr implementation of the XXH3 64-bit variant of xxHash
 *  Copyright (c) 2021-2023, chys <admin@chys.info> <chys87@github>
 *  All rights reserved.
 *  @copyright
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions are met:
 *  1. Redistributions of source code must retain the above copyright notice, this
 *     list of conditions and the following disclaimer.
 *  2. Redistributions in binary form must reproduce the above copyright notice,
 *     this list of conditions and the following disclaimer in the documentation
 *     and/or other materials provided with the distribution.
 *  @copyright
 *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 *  DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 *  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 *  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 *  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 *  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 *  OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 *  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *  @copyright
 *  This file uses code from Yann Collet's xxHash implementation.
 *  Original xxHash copyright notice:
 *  @copyright
 *  xxHash - Extremely Fast Hash algorithm
 *  Header File
 *  Copyright (C) 2012-2020 Yann Collet
 *  
 *  @author Chys <admin@chys.info> <chys87@github>; Yann Collet
 *  @date 2025
 */
 
#pragma once
 
#include <cstddef>
#include <cstdint>
#include <iterator>  // for std::data, std::size
#include <type_traits>
#include <utility>
 
namespace constexpr_xxh3 {
 
template <typename T>
concept ByteType = (std::is_integral_v<T> && sizeof(T) == 1)
#if defined __cpp_lib_byte && __cpp_lib_byte >= 201603
                   || std::is_same_v<T, std::byte>
#endif
    ;
 
template <typename T>
concept BytePtrType = requires (T ptr) {
  requires std::is_pointer_v<T>;
  requires ByteType<std::remove_cvref_t<decltype(*ptr)>>;
};
 
template <typename T>
concept BytesType = requires (const T& bytes) {
  { std::data(bytes) };
  requires BytePtrType<decltype(std::data(bytes))>;
  // -> std::convertible_to is not supported widely enough
  { static_cast<size_t>(std::size(bytes)) };
};
 
inline constexpr uint32_t swap32(uint32_t x) noexcept {
  return ((x << 24) & 0xff000000) | ((x << 8) & 0x00ff0000) |
         ((x >> 8) & 0x0000ff00) | ((x >> 24) & 0x000000ff);
}
 
template <typename T>
inline constexpr uint32_t readLE32(const T* ptr) noexcept {
  return uint8_t(ptr[0]) | uint32_t(uint8_t(ptr[1])) << 8 |
         uint32_t(uint8_t(ptr[2])) << 16 | uint32_t(uint8_t(ptr[3])) << 24;
}
 
inline constexpr uint64_t swap64(uint64_t x) noexcept {
  return ((x << 56) & 0xff00000000000000ULL) |
         ((x << 40) & 0x00ff000000000000ULL) |
         ((x << 24) & 0x0000ff0000000000ULL) |
         ((x << 8) & 0x000000ff00000000ULL) |
         ((x >> 8) & 0x00000000ff000000ULL) |
         ((x >> 24) & 0x0000000000ff0000ULL) |
         ((x >> 40) & 0x000000000000ff00ULL) |
         ((x >> 56) & 0x00000000000000ffULL);
}
 
template <typename T>
inline constexpr uint64_t readLE64(const T* ptr) noexcept {
  return readLE32(ptr) | uint64_t(readLE32(ptr + 4)) << 32;
}
 
inline constexpr void writeLE64(uint8_t* dst, uint64_t v) noexcept {
  for (int i = 0; i < 8; ++i) dst[i] = uint8_t(v >> (i * 8));
}
 
inline constexpr uint32_t PRIME32_1 = 0x9E3779B1U;
inline constexpr uint32_t PRIME32_2 = 0x85EBCA77U;
inline constexpr uint32_t PRIME32_3 = 0xC2B2AE3DU;
 
inline constexpr uint64_t PRIME64_1 = 0x9E3779B185EBCA87ULL;
inline constexpr uint64_t PRIME64_2 = 0xC2B2AE3D27D4EB4FULL;
inline constexpr uint64_t PRIME64_3 = 0x165667B19E3779F9ULL;
inline constexpr uint64_t PRIME64_4 = 0x85EBCA77C2B2AE63ULL;
inline constexpr uint64_t PRIME64_5 = 0x27D4EB2F165667C5ULL;
 
inline constexpr size_t SECRET_DEFAULT_SIZE = 192;
inline constexpr size_t SECRET_SIZE_MIN = 136;
 
inline constexpr uint8_t kSecret[SECRET_DEFAULT_SIZE]{
    0xb8, 0xfe, 0x6c, 0x39, 0x23, 0xa4, 0x4b, 0xbe, 0x7c, 0x01, 0x81, 0x2c,
    0xf7, 0x21, 0xad, 0x1c, 0xde, 0xd4, 0x6d, 0xe9, 0x83, 0x90, 0x97, 0xdb,
    0x72, 0x40, 0xa4, 0xa4, 0xb7, 0xb3, 0x67, 0x1f, 0xcb, 0x79, 0xe6, 0x4e,
    0xcc, 0xc0, 0xe5, 0x78, 0x82, 0x5a, 0xd0, 0x7d, 0xcc, 0xff, 0x72, 0x21,
    0xb8, 0x08, 0x46, 0x74, 0xf7, 0x43, 0x24, 0x8e, 0xe0, 0x35, 0x90, 0xe6,
    0x81, 0x3a, 0x26, 0x4c, 0x3c, 0x28, 0x52, 0xbb, 0x91, 0xc3, 0x00, 0xcb,
    0x88, 0xd0, 0x65, 0x8b, 0x1b, 0x53, 0x2e, 0xa3, 0x71, 0x64, 0x48, 0x97,
    0xa2, 0x0d, 0xf9, 0x4e, 0x38, 0x19, 0xef, 0x46, 0xa9, 0xde, 0xac, 0xd8,
    0xa8, 0xfa, 0x76, 0x3f, 0xe3, 0x9c, 0x34, 0x3f, 0xf9, 0xdc, 0xbb, 0xc7,
    0xc7, 0x0b, 0x4f, 0x1d, 0x8a, 0x51, 0xe0, 0x4b, 0xcd, 0xb4, 0x59, 0x31,
    0xc8, 0x9f, 0x7e, 0xc9, 0xd9, 0x78, 0x73, 0x64, 0xea, 0xc5, 0xac, 0x83,
    0x34, 0xd3, 0xeb, 0xc3, 0xc5, 0x81, 0xa0, 0xff, 0xfa, 0x13, 0x63, 0xeb,
    0x17, 0x0d, 0xdd, 0x51, 0xb7, 0xf0, 0xda, 0x49, 0xd3, 0x16, 0x55, 0x26,
    0x29, 0xd4, 0x68, 0x9e, 0x2b, 0x16, 0xbe, 0x58, 0x7d, 0x47, 0xa1, 0xfc,
    0x8f, 0xf8, 0xb8, 0xd1, 0x7a, 0xd0, 0x31, 0xce, 0x45, 0xcb, 0x3a, 0x8f,
    0x95, 0x16, 0x04, 0x28, 0xaf, 0xd7, 0xfb, 0xca, 0xbb, 0x4b, 0x40, 0x7e,
};
 
inline constexpr std::pair<uint64_t, uint64_t> mult64to128(
    uint64_t lhs, uint64_t rhs) noexcept {
  uint64_t lo_lo = uint64_t(uint32_t(lhs)) * uint32_t(rhs);
  uint64_t hi_lo = (lhs >> 32) * uint32_t(rhs);
  uint64_t lo_hi = uint32_t(lhs) * (rhs >> 32);
  uint64_t hi_hi = (lhs >> 32) * (rhs >> 32);
  uint64_t cross = (lo_lo >> 32) + uint32_t(hi_lo) + lo_hi;
  uint64_t upper = (hi_lo >> 32) + (cross >> 32) + hi_hi;
  uint64_t lower = (cross << 32) | uint32_t(lo_lo);
  return {lower, upper};
}
 
inline constexpr uint64_t mul128_fold64(uint64_t lhs, uint64_t rhs) noexcept {
#if defined __GNUC__ && __WORDSIZE >= 64
  // It appears both GCC and Clang support evaluating __int128 as constexpr
  auto product = static_cast<unsigned __int128>(lhs) * rhs;
  return uint64_t(product >> 64) ^ uint64_t(product);
#else
  auto product = mult64to128(lhs, rhs);
  return product.first ^ product.second;
#endif
}
 
inline constexpr uint64_t XXH64_avalanche(uint64_t h) noexcept {
  h = (h ^ (h >> 33)) * PRIME64_2;
  h = (h ^ (h >> 29)) * PRIME64_3;
  return h ^ (h >> 32);
}
 
inline constexpr uint64_t XXH3_avalanche(uint64_t h) noexcept {
  h = (h ^ (h >> 37)) * 0x165667919E3779F9ULL;
  return h ^ (h >> 32);
}
 
inline constexpr uint64_t rrmxmx(uint64_t h, uint64_t len) noexcept {
  h ^= ((h << 49) | (h >> 15)) ^ ((h << 24) | (h >> 40));
  h *= 0x9FB21C651E98DF25ULL;
  h ^= (h >> 35) + len;
  h *= 0x9FB21C651E98DF25ULL;
  return h ^ (h >> 28);
}
 
template <typename T, typename S>
constexpr uint64_t mix16B(const T* input, const S* secret,
                          uint64_t seed) noexcept {
  return mul128_fold64(readLE64(input) ^ (readLE64(secret) + seed),
                       readLE64(input + 8) ^ (readLE64(secret + 8) - seed));
}
 
inline constexpr size_t STRIPE_LEN = 64;
inline constexpr size_t SECRET_CONSUME_RATE = 8;
inline constexpr size_t ACC_NB = STRIPE_LEN / sizeof(uint64_t);
 
template <typename T, typename S>
constexpr void accumulate_512(uint64_t* acc, const T* input,
                              const S* secret) noexcept {
  for (size_t i = 0; i < ACC_NB; i++) {
    uint64_t data_val = readLE64(input + 8 * i);
    uint64_t data_key = data_val ^ readLE64(secret + i * 8);
    acc[i ^ 1] += data_val;
    acc[i] += uint32_t(data_key) * (data_key >> 32);
  }
}
 
template <typename T, typename S>
constexpr uint64_t hashLong_64b_internal(const T* input, size_t len,
                                         const S* secret,
                                         size_t secretSize) noexcept {
  uint64_t acc[ACC_NB]{PRIME32_3, PRIME64_1, PRIME64_2, PRIME64_3,
                       PRIME64_4, PRIME32_2, PRIME64_5, PRIME32_1};
  size_t nbStripesPerBlock = (secretSize - STRIPE_LEN) / SECRET_CONSUME_RATE;
  size_t block_len = STRIPE_LEN * nbStripesPerBlock;
  size_t nb_blocks = (len - 1) / block_len;
 
  for (size_t n = 0; n < nb_blocks; n++) {
    for (size_t i = 0; i < nbStripesPerBlock; i++)
      accumulate_512(acc, input + n * block_len + i * STRIPE_LEN,
                     secret + i * SECRET_CONSUME_RATE);
    for (size_t i = 0; i < ACC_NB; i++)
      acc[i] = (acc[i] ^ (acc[i] >> 47) ^
                readLE64(secret + secretSize - STRIPE_LEN + 8 * i)) *
               PRIME32_1;
  }
 
  size_t nbStripes = ((len - 1) - (block_len * nb_blocks)) / STRIPE_LEN;
  for (size_t i = 0; i < nbStripes; i++)
    accumulate_512(acc, input + nb_blocks * block_len + i * STRIPE_LEN,
                   secret + i * SECRET_CONSUME_RATE);
  accumulate_512(acc, input + len - STRIPE_LEN,
                 secret + secretSize - STRIPE_LEN - 7);
  uint64_t result = len * PRIME64_1;
  for (size_t i = 0; i < 4; i++)
    result +=
        mul128_fold64(acc[2 * i] ^ readLE64(secret + 11 + 16 * i),
                      acc[2 * i + 1] ^ readLE64(secret + 11 + 16 * i + 8));
  return XXH3_avalanche(result);
}
 
template <typename T, typename S, typename HashLong>
constexpr uint64_t XXH3_64bits_internal(const T* input, size_t len,
                                        uint64_t seed, const S* secret,
                                        size_t secretLen,
                                        HashLong f_hashLong) noexcept {
  if (len == 0) {
    return XXH64_avalanche(seed ^
                           (readLE64(secret + 56) ^ readLE64(secret + 64)));
  } else if (len < 4) {
    uint64_t keyed = ((uint32_t(uint8_t(input[0])) << 16) |
                      (uint32_t(uint8_t(input[len >> 1])) << 24) |
                      uint8_t(input[len - 1]) | (uint32_t(len) << 8)) ^
                     ((readLE32(secret) ^ readLE32(secret + 4)) + seed);
    return XXH64_avalanche(keyed);
  } else if (len <= 8) {
    uint64_t keyed =
        (readLE32(input + len - 4) + (uint64_t(readLE32(input)) << 32)) ^
        ((readLE64(secret + 8) ^ readLE64(secret + 16)) -
         (seed ^ (uint64_t(swap32(uint32_t(seed))) << 32)));
    return rrmxmx(keyed, len);
  } else if (len <= 16) {
    uint64_t input_lo =
        readLE64(input) ^
        ((readLE64(secret + 24) ^ readLE64(secret + 32)) + seed);
    uint64_t input_hi =
        readLE64(input + len - 8) ^
        ((readLE64(secret + 40) ^ readLE64(secret + 48)) - seed);
    uint64_t acc =
        len + swap64(input_lo) + input_hi + mul128_fold64(input_lo, input_hi);
    return XXH3_avalanche(acc);
  } else if (len <= 128) {
    uint64_t acc = len * PRIME64_1;
    size_t secret_off = 0;
    for (size_t i = 0, j = len; j > i; i += 16, j -= 16) {
      acc += mix16B(input + i, secret + secret_off, seed);
      acc += mix16B(input + j - 16, secret + secret_off + 16, seed);
      secret_off += 32;
    }
    return XXH3_avalanche(acc);
  } else if (len <= 240) {
    uint64_t acc = len * PRIME64_1;
    for (size_t i = 0; i < 128; i += 16)
      acc += mix16B(input + i, secret + i, seed);
    acc = XXH3_avalanche(acc);
    for (size_t i = 128; i < len / 16 * 16; i += 16)
      acc += mix16B(input + i, secret + (i - 128) + 3, seed);
    acc += mix16B(input + len - 16, secret + SECRET_SIZE_MIN - 17, seed);
    return XXH3_avalanche(acc);
  } else {
    return f_hashLong(input, len, seed, secret, secretLen);
  }
}
 
template <BytesType Bytes>
constexpr size_t bytes_size(const Bytes& bytes) noexcept {
  return std::size(bytes);
}
 
template <ByteType T, size_t N>
constexpr size_t bytes_size(T (&)[N]) noexcept {
  return (N ? N - 1 : 0);
}
 
/// Basic interfaces
 
template <ByteType T>
consteval uint64_t XXH3_64bits_const(const T* input, size_t len) noexcept {
  return XXH3_64bits_internal(
      input, len, 0, kSecret, sizeof(kSecret),
      [](const T* input, size_t len, uint64_t, const void*,
         size_t) constexpr noexcept {
        return hashLong_64b_internal(input, len, kSecret, sizeof(kSecret));
      });
}
 
template <ByteType T, ByteType S>
consteval uint64_t XXH3_64bits_withSecret_const(const T* input, size_t len,
                                                const S* secret,
                                                size_t secretSize) noexcept {
  return XXH3_64bits_internal(
      input, len, 0, secret, secretSize,
      [](const T* input, size_t len, uint64_t, const S* secret,
         size_t secretLen) constexpr noexcept {
        return hashLong_64b_internal(input, len, secret, secretLen);
      });
}
 
template <ByteType T>
consteval uint64_t XXH3_64bits_withSeed_const(const T* input, size_t len,
                                              uint64_t seed) noexcept {
  if (seed == 0) return XXH3_64bits_const(input, len);
  return XXH3_64bits_internal(
      input, len, seed, kSecret, sizeof(kSecret),
      [](const T* input, size_t len, uint64_t seed, const void*,
         size_t) constexpr noexcept {
        uint8_t secret[SECRET_DEFAULT_SIZE];
        for (size_t i = 0; i < SECRET_DEFAULT_SIZE; i += 16) {
          writeLE64(secret + i, readLE64(kSecret + i) + seed);
          writeLE64(secret + i + 8, readLE64(kSecret + i + 8) - seed);
        }
        return hashLong_64b_internal(input, len, secret, sizeof(secret));
      });
}
 
/// Convenient interfaces
 
template <BytesType Bytes>
consteval uint64_t XXH3_64bits_const(const Bytes& input) noexcept {
  return XXH3_64bits_const(std::data(input), bytes_size(input));
}
 
template <BytesType Bytes, BytesType Secret>
consteval uint64_t XXH3_64bits_withSecret_const(const Bytes& input,
                                                const Secret& secret) noexcept {
  return XXH3_64bits_withSecret_const(std::data(input), bytes_size(input),
                                      std::data(secret), bytes_size(secret));
}
 
template <BytesType Bytes>
consteval uint64_t XXH3_64bits_withSeed_const(const Bytes& input,
                                              uint64_t seed) noexcept {
  return XXH3_64bits_withSeed_const(std::data(input), bytes_size(input), seed);
}
 
}  // namespace constexpr_xxh3