path: root/absl/hash/internal/city.cc

// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// This file provides CityHash64() and related functions.
//
// It's probably possible to create even faster hash functions by
// writing a program that systematically explores some of the space of
// possible hash functions, by using SIMD instructions, or by
// compromising on hash quality.

#include "absl/hash/internal/city.h"

#include <string.h>  // for memcpy and memset
#include <algorithm>

#include "absl/base/config.h"
#include "absl/base/internal/endian.h"
#include "absl/base/internal/unaligned_access.h"
#include "absl/base/optimization.h"

namespace absl {
namespace hash_internal {

#ifdef ABSL_IS_BIG_ENDIAN
#define uint32_in_expected_order(x) (absl::gbswap_32(x))
#define uint64_in_expected_order(x) (absl::gbswap_64(x))
#else
#define uint32_in_expected_order(x) (x)
#define uint64_in_expected_order(x) (x)
#endif

static uint64_t Fetch64(const char *p) {
  return uint64_in_expected_order(ABSL_INTERNAL_UNALIGNED_LOAD64(p));
}

static uint32_t Fetch32(const char *p) {
  return uint32_in_expected_order(ABSL_INTERNAL_UNALIGNED_LOAD32(p));
}

// Some primes between 2^63 and 2^64 for various uses.
static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
static const uint64_t k1 = 0xb492b66fbe98f273ULL;
static const uint64_t k2 = 0x9ae16a3b2f90404fULL;

// Magic numbers for 32-bit hashing.  Copied from Murmur3.
static const uint32_t c1 = 0xcc9e2d51;
static const uint32_t c2 = 0x1b873593;

// A 32-bit to 32-bit integer hash copied from Murmur3.
static uint32_t fmix(uint32_t h) {
  h ^= h >> 16;
  h *= 0x85ebca6b;
  h ^= h >> 13;
  h *= 0xc2b2ae35;
  h ^= h >> 16;
  return h;
}

static uint32_t Rotate32(uint32_t val, int shift) {
  // Avoid shifting by 32: doing so yields an undefined result.
  return shift == 0 ? val : ((val >> shift) | (val << (32 - shift)));
}

#undef PERMUTE3
#define PERMUTE3(a, b, c) \
  do {                    \
    std::swap(a, b);      \
    std::swap(a, c);      \
  } while (0)

static uint32_t Mur(uint32_t a, uint32_t h) {
  // Helper from Murmur3 for combining two 32-bit values.
  a *= c1;
  a = Rotate32(a, 17);
  a *= c2;
  h ^= a;
  h = Rotate32(h, 19);
  return h * 5 + 0xe6546b64;
}

static uint32_t Hash32Len13to24(const char *s, size_t len) {
  uint32_t a = Fetch32(s - 4 + (len >> 1));
  uint32_t b = Fetch32(s + 4);
  uint32_t c = Fetch32(s + len - 8);
  uint32_t d = Fetch32(s + (len >> 1));
  uint32_t e = Fetch32(s);
  uint32_t f = Fetch32(s + len - 4);
  uint32_t h = len;

  return fmix(Mur(f, Mur(e, Mur(d, Mur(c, Mur(b, Mur(a, h)))))));
}

static uint32_t Hash32Len0to4(const char *s, size_t len) {
  uint32_t b = 0;
  uint32_t c = 9;
  for (size_t i = 0; i < len; i++) {
    signed char v = s[i];
    b = b * c1 + v;
    c ^= b;
  }
  return fmix(Mur(b, Mur(len, c)));
}

static uint32_t Hash32Len5to12(const char *s, size_t len) {
  uint32_t a = len, b = len * 5, c = 9, d = b;
  a += Fetch32(s);
  b += Fetch32(s + len - 4);
  c += Fetch32(s + ((len >> 1) & 4));
  return fmix(Mur(c, Mur(b, Mur(a, d))));
}

uint32_t CityHash32(const char *s, size_t len) {
  if (len <= 24) {
    return len <= 12
               ? (len <= 4 ? Hash32Len0to4(s, len) : Hash32Len5to12(s, len))
               : Hash32Len13to24(s, len);
  }

  // len > 24
  uint32_t h = len, g = c1 * len, f = g;

  uint32_t a0 = Rotate32(Fetch32(s + len - 4) * c1, 17) * c2;
  uint32_t a1 = Rotate32(Fetch32(s + len - 8) * c1, 17) * c2;
  uint32_t a2 = Rotate32(Fetch32(s + len - 16) * c1, 17) * c2;
  uint32_t a3 = Rotate32(Fetch32(s + len - 12) * c1, 17) * c2;
  uint32_t a4 = Rotate32(Fetch32(s + len - 20) * c1, 17) * c2;
  h ^= a0;
  h = Rotate32(h, 19);
  h = h * 5 + 0xe6546b64;
  h ^= a2;
  h = Rotate32(h, 19);
  h = h * 5 + 0xe6546b64;
  g ^= a1;
  g = Rotate32(g, 19);
  g = g * 5 + 0xe6546b64;
  g ^= a3;
  g = Rotate32(g, 19);
  g = g * 5 + 0xe6546b64;
  f += a4;
  f = Rotate32(f, 19);
  f = f * 5 + 0xe6546b64;
  size_t iters = (len - 1) / 20;
  do {
    uint32_t b0 = Rotate32(Fetch32(s) * c1, 17) * c2;
    uint32_t b1 = Fetch32(s + 4);
    uint32_t b2 = Rotate32(Fetch32(s + 8) * c1, 17) * c2;
    uint32_t b3 = Rotate32(Fetch32(s + 12) * c1, 17) * c2;
    uint32_t b4 = Fetch32(s + 16);
    h ^= b0;
    h = Rotate32(h, 18);
    h = h * 5 + 0xe6546b64;
    f += b1;
    f = Rotate32(f, 19);
    f = f * c1;
    g += b2;
    g = Rotate32(g, 18);
    g = g * 5 + 0xe6546b64;
    h ^= b3 + b1;
    h = Rotate32(h, 19);
    h = h * 5 + 0xe6546b64;
    g ^= b4;
    g = absl::gbswap_32(g) * 5;
    h += b4 * 5;
    h = absl::gbswap_32(h);
    f += b0;
    PERMUTE3(f, h, g);
    s += 20;
  } while (--iters != 0);
  g = Rotate32(g, 11) * c1;
  g = Rotate32(g, 17) * c1;
  f = Rotate32(f, 11) * c1;
  f = Rotate32(f, 17) * c1;
  h = Rotate32(h + g, 19);
  h = h * 5 + 0xe6546b64;
  h = Rotate32(h, 17) * c1;
  h = Rotate32(h + f, 19);
  h = h * 5 + 0xe6546b64;
  h = Rotate32(h, 17) * c1;
  return h;
}

// Bitwise right rotate.  Normally this will compile to a single
// instruction, especially if the shift is a manifest constant.
static uint64_t Rotate(uint64_t val, int shift) {
  // Avoid shifting by 64: doing so yields an undefined result.
  return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
}

static uint64_t ShiftMix(uint64_t val) { return val ^ (val >> 47); }

static uint64_t HashLen16(uint64_t u, uint64_t v) {
  return Hash128to64(uint128(u, v));
}

static uint64_t HashLen16(uint64_t u, uint64_t v, uint64_t mul) {
  // Murmur-inspired hashing.
  uint64_t a = (u ^ v) * mul;
  a ^= (a >> 47);
  uint64_t b = (v ^ a) * mul;
  b ^= (b >> 47);
  b *= mul;
  return b;
}

static uint64_t HashLen0to16(const char *s, size_t len) {
  if (len >= 8) {
    uint64_t mul = k2 + len * 2;
    uint64_t a = Fetch64(s) + k2;
    uint64_t b = Fetch64(s + len - 8);
    uint64_t c = Rotate(b, 37) * mul + a;
    uint64_t d = (Rotate(a, 25) + b) * mul;
    return HashLen16(c, d, mul);
  }
  if (len >= 4) {
    uint64_t mul = k2 + len * 2;
    uint64_t a = Fetch32(s);
    return HashLen16(len + (a << 3), Fetch32(s + len - 4), mul);
  }
  if (len > 0) {
    uint8_t a = s[0];
    uint8_t b = s[len >> 1];
    uint8_t c = s[len - 1];
    uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
    uint32_t z = len + (static_cast<uint32_t>(c) << 2);
    return ShiftMix(y * k2 ^ z * k0) * k2;
  }
  return k2;
}

// This probably works well for 16-byte strings as well, but it may be overkill
// in that case.
static uint64_t HashLen17to32(const char *s, size_t len) {
  uint64_t mul = k2 + len * 2;
  uint64_t a = Fetch64(s) * k1;
  uint64_t b = Fetch64(s + 8);
  uint64_t c = Fetch64(s + len - 8) * mul;
  uint64_t d = Fetch64(s + len - 16) * k2;
  return HashLen16(Rotate(a + b, 43) + Rotate(c, 30) + d,
                   a + Rotate(b + k2, 18) + c, mul);
}

// Return a 16-byte hash for 48 bytes.  Quick and dirty.
// Callers do best to use "random-looking" values for a and b.
static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(uint64_t w, uint64_t x,
                                                        uint64_t y, uint64_t z,
                                                        uint64_t a, uint64_t b) {
  a += w;
  b = Rotate(b + a + z, 21);
  uint64_t c = a;
  a += x;
  a += y;
  b += Rotate(a, 44);
  return std::make_pair(a + z, b + c);
}

// Return a 16-byte hash for s[0] ... s[31], a, and b.  Quick and dirty.
static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(const char *s, uint64_t a,
                                                        uint64_t b) {
  return WeakHashLen32WithSeeds(Fetch64(s), Fetch64(s + 8), Fetch64(s + 16),
                                Fetch64(s + 24), a, b);
}

// Return an 8-byte hash for 33 to 64 bytes.
static uint64_t HashLen33to64(const char *s, size_t len) {
  uint64_t mul = k2 + len * 2;
  uint64_t a = Fetch64(s) * k2;
  uint64_t b = Fetch64(s + 8);
  uint64_t c = Fetch64(s + len - 24);
  uint64_t d = Fetch64(s + len - 32);
  uint64_t e = Fetch64(s + 16) * k2;
  uint64_t f = Fetch64(s + 24) * 9;
  uint64_t g = Fetch64(s + len - 8);
  uint64_t h = Fetch64(s + len - 16) * mul;
  uint64_t u = Rotate(a + g, 43) + (Rotate(b, 30) + c) * 9;
  uint64_t v = ((a + g) ^ d) + f + 1;
  uint64_t w = absl::gbswap_64((u + v) * mul) + h;
  uint64_t x = Rotate(e + f, 42) + c;
  uint64_t y = (absl::gbswap_64((v + w) * mul) + g) * mul;
  uint64_t z = e + f + c;
  a = absl::gbswap_64((x + z) * mul + y) + b;
  b = ShiftMix((z + a) * mul + d + h) * mul;
  return b + x;
}

uint64_t CityHash64(const char *s, size_t len) {
  if (len <= 32) {
    if (len <= 16) {
      return HashLen0to16(s, len);
    } else {
      return HashLen17to32(s, len);
    }
  } else if (len <= 64) {
    return HashLen33to64(s, len);
  }

  // For strings over 64 bytes we hash the end first, and then as we
  // loop we keep 56 bytes of state: v, w, x, y, and z.
  uint64_t x = Fetch64(s + len - 40);
  uint64_t y = Fetch64(s + len - 16) + Fetch64(s + len - 56);
  uint64_t z = HashLen16(Fetch64(s + len - 48) + len, Fetch64(s + len - 24));
  std::pair<uint64_t, uint64_t> v = WeakHashLen32WithSeeds(s + len - 64, len, z);
  std::pair<uint64_t, uint64_t> w = WeakHashLen32WithSeeds(s + len - 32, y + k1, x);
  x = x * k1 + Fetch64(s);

  // Decrease len to the nearest multiple of 64, and operate on 64-byte chunks.
  len = (len - 1) & ~static_cast<size_t>(63);
  do {
    x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
    y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
    x ^= w.second;
    y += v.first + Fetch64(s + 40);
    z = Rotate(z + w.first, 33) * k1;
    v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
    w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
    std::swap(z, x);
    s += 64;
    len -= 64;
  } while (len != 0);
  return HashLen16(HashLen16(v.first, w.first) + ShiftMix(y) * k1 + z,
                   HashLen16(v.second, w.second) + x);
}

uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed) {
  return CityHash64WithSeeds(s, len, k2, seed);
}

uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
                           uint64_t seed1) {
  return HashLen16(CityHash64(s, len) - seed0, seed1);
}

}  // namespace hash_internal
}  // namespace absl