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189 lines
6.7 KiB
189 lines
6.7 KiB
/*
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* Copyright (C) 2018 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "utils/hash/cityhash.h"
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#include <cstdint>
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#include "absl/base/internal/endian.h"
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#include "absl/numeric/int128.h"
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namespace tc3farmhash {
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namespace {
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// Some primes between 2^63 and 2^64 for various uses.
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static const uint64_t k0 = 0xa5b85c5e198ed849ULL;
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static const uint64_t k1 = 0x8d58ac26afe12e47ULL;
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static const uint64_t k2 = 0xc47b6e9e3a970ed3ULL;
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static const uint64_t k3 = 0xc70f6907e782aa0bULL;
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// Hash 128 input bits down to 64 bits of output.
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// This is intended to be a reasonably good hash function.
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// It may change from time to time.
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inline uint64_t Hash128to64(const absl::uint128 x) {
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// Murmur-inspired hashing.
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const uint64_t kMul = 0xc6a4a7935bd1e995ULL;
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uint64_t a = (absl::Uint128Low64(x) ^ absl::Uint128High64(x)) * kMul;
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a ^= (a >> 47);
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uint64_t b = (absl::Uint128High64(x) ^ a) * kMul;
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b ^= (b >> 47);
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b *= kMul;
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return b;
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}
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uint64_t HashLen16(uint64_t u, uint64_t v) {
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return Hash128to64(absl::MakeUint128(u, v));
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}
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static uint64_t Rotate(uint64_t val, size_t shift) {
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assert(shift <= 63);
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return (val >> shift) | (val << (-shift & 63));
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}
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static uint64_t ShiftMix(uint64_t val) { return val ^ (val >> 47); }
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uint64_t HashLen0to16(const char *s, size_t len) {
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assert(len <= 16);
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if (len > 8) {
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uint64_t a = absl::little_endian::Load64(s);
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uint64_t b = absl::little_endian::Load64(s + len - 8);
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return HashLen16(a, Rotate(b + len, len)) ^ b;
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}
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if (len >= 4) {
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uint64_t a = absl::little_endian::Load32(s);
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return HashLen16(len + (a << 3), absl::little_endian::Load32(s + len - 4));
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}
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if (len > 0) {
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uint8_t a = s[0];
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uint8_t b = s[len >> 1];
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uint8_t c = s[len - 1];
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uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
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uint32_t z = len + (static_cast<uint32_t>(c) << 2);
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return ShiftMix(y * k2 ^ z * k3) * k2;
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}
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return k2;
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}
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// Return a 16-byte hash for 48 bytes. Quick and dirty.
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// Callers do best to use "random-looking" values for a and b.
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// (For more, see the code review discussion of CL 18799087.)
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std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(uint64_t w, uint64_t x,
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uint64_t y, uint64_t z,
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uint64_t a, uint64_t b) {
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a += w;
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b = Rotate(b + a + z, 51);
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uint64_t c = a;
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a += x;
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a += y;
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b += Rotate(a, 23);
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return std::make_pair(a + z, b + c);
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}
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// Return a 16-byte hash for s[0] ... s[31], a, and b. Quick and dirty.
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std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(const char *s, uint64_t a,
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uint64_t b) {
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return WeakHashLen32WithSeeds(absl::little_endian::Load64(s),
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absl::little_endian::Load64(s + 8),
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absl::little_endian::Load64(s + 16),
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absl::little_endian::Load64(s + 24), a, b);
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}
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} // namespace
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// This probably works well for 16-byte strings as well, but it may be overkill
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// in that case.
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static uint64_t HashLen17to32(const char *s, size_t len) {
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assert(len >= 17);
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assert(len <= 32);
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uint64_t a = absl::little_endian::Load64(s) * k1;
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uint64_t b = absl::little_endian::Load64(s + 8);
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uint64_t c = absl::little_endian::Load64(s + len - 8) * k2;
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uint64_t d = absl::little_endian::Load64(s + len - 16) * k0;
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return HashLen16(Rotate(a - b, 43) + Rotate(c, 30) + d,
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a + Rotate(b ^ k3, 20) - c + len);
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}
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// Return an 8-byte hash for 33 to 64 bytes.
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static uint64_t HashLen33to64(const char *s, size_t len) {
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uint64_t z = absl::little_endian::Load64(s + 24);
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uint64_t a = absl::little_endian::Load64(s) +
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(len + absl::little_endian::Load64(s + len - 16)) * k0;
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uint64_t b = Rotate(a + z, 52);
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uint64_t c = Rotate(a, 37);
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a += absl::little_endian::Load64(s + 8);
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c += Rotate(a, 7);
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a += absl::little_endian::Load64(s + 16);
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uint64_t vf = a + z;
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uint64_t vs = b + Rotate(a, 31) + c;
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a = absl::little_endian::Load64(s + 16) +
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absl::little_endian::Load64(s + len - 32);
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z += absl::little_endian::Load64(s + len - 8);
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b = Rotate(a + z, 52);
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c = Rotate(a, 37);
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a += absl::little_endian::Load64(s + len - 24);
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c += Rotate(a, 7);
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a += absl::little_endian::Load64(s + len - 16);
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uint64_t wf = a + z;
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uint64_t ws = b + Rotate(a, 31) + c;
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uint64_t r = ShiftMix((vf + ws) * k2 + (wf + vs) * k0);
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return ShiftMix(r * k0 + vs) * k2;
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}
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uint64_t CityHash64(const char *s, size_t len) {
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if (len <= 32) {
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if (len <= 16) {
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return HashLen0to16(s, len);
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} else {
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return HashLen17to32(s, len);
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}
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} else if (len <= 64) {
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return HashLen33to64(s, len);
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}
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// For strings over 64 bytes we hash the end first, and then as we
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// loop we keep 56 bytes of state: v, w, x, y, and z.
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uint64_t x = absl::little_endian::Load64(s + len - 40);
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uint64_t y = absl::little_endian::Load64(s + len - 16) +
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absl::little_endian::Load64(s + len - 56);
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uint64_t z = HashLen16(absl::little_endian::Load64(s + len - 48) + len,
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absl::little_endian::Load64(s + len - 24));
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std::pair<uint64_t, uint64_t> v =
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WeakHashLen32WithSeeds(s + len - 64, len, z);
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std::pair<uint64_t, uint64_t> w =
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WeakHashLen32WithSeeds(s + len - 32, y + k1, x);
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x = x * k1 + absl::little_endian::Load64(s);
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// Decrease len to the nearest multiple of 64, and operate on 64-byte chunks.
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len = (len - 1) & ~static_cast<size_t>(63);
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assert(len > 0);
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assert(len == len / 64 * 64);
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do {
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x = Rotate(x + y + v.first + absl::little_endian::Load64(s + 8), 37) * k1;
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y = Rotate(y + v.second + absl::little_endian::Load64(s + 48), 42) * k1;
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x ^= w.second;
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y += v.first + absl::little_endian::Load64(s + 40);
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z = Rotate(z + w.first, 33) * k1;
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v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
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w = WeakHashLen32WithSeeds(s + 32, z + w.second,
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y + absl::little_endian::Load64(s + 16));
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std::swap(z, x);
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s += 64;
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len -= 64;
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} while (len != 0);
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return HashLen16(HashLen16(v.first, w.first) + ShiftMix(y) * k1 + z,
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HashLen16(v.second, w.second) + x);
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}
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} // namespace tc3farmhash
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