File size: 10,480 Bytes
// SPDX-FileCopyrightText: 2011 Google, Inc.
// SPDX-FileContributor: Geoff Pike
// SPDX-FileContributor: Jyrki Alakuijala
// SPDX-License-Identifier: MIT

// CityHash, by Geoff Pike and Jyrki Alakuijala
//
// 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 <algorithm>
#include <cstring>
#include <utility>

#include "common/cityhash.h"
#include "common/swap.h"

// #include "config.h"
#ifdef __GNUC__
#define HAVE_BUILTIN_EXPECT 1
#endif
#ifdef COMMON_BIG_ENDIAN
#define WORDS_BIGENDIAN 1
#endif

using namespace std;

namespace Common {

static u64 unaligned_load64(const char* p) {
    u64 result;
    std::memcpy(&result, p, sizeof(result));
    return result;
}

static u32 unaligned_load32(const char* p) {
    u32 result;
    std::memcpy(&result, p, sizeof(result));
    return result;
}

#ifdef WORDS_BIGENDIAN
#define uint32_in_expected_order(x) (swap32(x))
#define uint64_in_expected_order(x) (swap64(x))
#else
#define uint32_in_expected_order(x) (x)
#define uint64_in_expected_order(x) (x)
#endif

#if !defined(LIKELY)
#if HAVE_BUILTIN_EXPECT
#define LIKELY(x) (__builtin_expect(!!(x), 1))
#else
#define LIKELY(x) (x)
#endif
#endif

static u64 Fetch64(const char* p) {
    return uint64_in_expected_order(unaligned_load64(p));
}

static u32 Fetch32(const char* p) {
    return uint32_in_expected_order(unaligned_load32(p));
}

// Some primes between 2^63 and 2^64 for various uses.
static constexpr u64 k0 = 0xc3a5c85c97cb3127ULL;
static constexpr u64 k1 = 0xb492b66fbe98f273ULL;
static constexpr u64 k2 = 0x9ae16a3b2f90404fULL;

// Bitwise right rotate.  Normally this will compile to a single
// instruction, especially if the shift is a manifest constant.
static u64 Rotate(u64 val, int shift) {
    // Avoid shifting by 64: doing so yields an undefined result.
    return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
}

static u64 ShiftMix(u64 val) {
    return val ^ (val >> 47);
}

static u64 HashLen16(u64 u, u64 v) {
    return Hash128to64(u128{u, v});
}

static u64 HashLen16(u64 u, u64 v, u64 mul) {
    // Murmur-inspired hashing.
    u64 a = (u ^ v) * mul;
    a ^= (a >> 47);
    u64 b = (v ^ a) * mul;
    b ^= (b >> 47);
    b *= mul;
    return b;
}

static u64 HashLen0to16(const char* s, size_t len) {
    if (len >= 8) {
        u64 mul = k2 + len * 2;
        u64 a = Fetch64(s) + k2;
        u64 b = Fetch64(s + len - 8);
        u64 c = Rotate(b, 37) * mul + a;
        u64 d = (Rotate(a, 25) + b) * mul;
        return HashLen16(c, d, mul);
    }
    if (len >= 4) {
        u64 mul = k2 + len * 2;
        u64 a = Fetch32(s);
        return HashLen16(len + (a << 3), Fetch32(s + len - 4), mul);
    }
    if (len > 0) {
        u8 a = s[0];
        u8 b = s[len >> 1];
        u8 c = s[len - 1];
        u32 y = static_cast<u32>(a) + (static_cast<u32>(b) << 8);
        u32 z = static_cast<u32>(len) + (static_cast<u32>(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 u64 HashLen17to32(const char* s, size_t len) {
    u64 mul = k2 + len * 2;
    u64 a = Fetch64(s) * k1;
    u64 b = Fetch64(s + 8);
    u64 c = Fetch64(s + len - 8) * mul;
    u64 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 pair<u64, u64> WeakHashLen32WithSeeds(u64 w, u64 x, u64 y, u64 z, u64 a, u64 b) {
    a += w;
    b = Rotate(b + a + z, 21);
    u64 c = a;
    a += x;
    a += y;
    b += Rotate(a, 44);
    return make_pair(a + z, b + c);
}

// Return a 16-byte hash for s[0] ... s[31], a, and b.  Quick and dirty.
static pair<u64, u64> WeakHashLen32WithSeeds(const char* s, u64 a, u64 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 u64 HashLen33to64(const char* s, size_t len) {
    u64 mul = k2 + len * 2;
    u64 a = Fetch64(s) * k2;
    u64 b = Fetch64(s + 8);
    u64 c = Fetch64(s + len - 24);
    u64 d = Fetch64(s + len - 32);
    u64 e = Fetch64(s + 16) * k2;
    u64 f = Fetch64(s + 24) * 9;
    u64 g = Fetch64(s + len - 8);
    u64 h = Fetch64(s + len - 16) * mul;
    u64 u = Rotate(a + g, 43) + (Rotate(b, 30) + c) * 9;
    u64 v = ((a + g) ^ d) + f + 1;
    u64 w = swap64((u + v) * mul) + h;
    u64 x = Rotate(e + f, 42) + c;
    u64 y = (swap64((v + w) * mul) + g) * mul;
    u64 z = e + f + c;
    a = swap64((x + z) * mul + y) + b;
    b = ShiftMix((z + a) * mul + d + h) * mul;
    return b + x;
}

u64 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.
    u64 x = Fetch64(s + len - 40);
    u64 y = Fetch64(s + len - 16) + Fetch64(s + len - 56);
    u64 z = HashLen16(Fetch64(s + len - 48) + len, Fetch64(s + len - 24));
    pair<u64, u64> v = WeakHashLen32WithSeeds(s + len - 64, len, z);
    pair<u64, u64> 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);
}

u64 CityHash64WithSeed(const char* s, size_t len, u64 seed) {
    return CityHash64WithSeeds(s, len, k2, seed);
}

u64 CityHash64WithSeeds(const char* s, size_t len, u64 seed0, u64 seed1) {
    return HashLen16(CityHash64(s, len) - seed0, seed1);
}

// A subroutine for CityHash128().  Returns a decent 128-bit hash for strings
// of any length representable in signed long.  Based on City and Murmur.
static u128 CityMurmur(const char* s, size_t len, u128 seed) {
    u64 a = seed[0];
    u64 b = seed[1];
    u64 c = 0;
    u64 d = 0;
    signed long l = static_cast<long>(len) - 16;
    if (l <= 0) { // len <= 16
        a = ShiftMix(a * k1) * k1;
        c = b * k1 + HashLen0to16(s, len);
        d = ShiftMix(a + (len >= 8 ? Fetch64(s) : c));
    } else { // len > 16
        c = HashLen16(Fetch64(s + len - 8) + k1, a);
        d = HashLen16(b + len, c + Fetch64(s + len - 16));
        a += d;
        do {
            a ^= ShiftMix(Fetch64(s) * k1) * k1;
            a *= k1;
            b ^= a;
            c ^= ShiftMix(Fetch64(s + 8) * k1) * k1;
            c *= k1;
            d ^= c;
            s += 16;
            l -= 16;
        } while (l > 0);
    }
    a = HashLen16(a, c);
    b = HashLen16(d, b);
    return u128{a ^ b, HashLen16(b, a)};
}

u128 CityHash128WithSeed(const char* s, size_t len, u128 seed) {
    if (len < 128) {
        return CityMurmur(s, len, seed);
    }

    // We expect len >= 128 to be the common case.  Keep 56 bytes of state:
    // v, w, x, y, and z.
    pair<u64, u64> v, w;
    u64 x = seed[0];
    u64 y = seed[1];
    u64 z = len * k1;
    v.first = Rotate(y ^ k1, 49) * k1 + Fetch64(s);
    v.second = Rotate(v.first, 42) * k1 + Fetch64(s + 8);
    w.first = Rotate(y + z, 35) * k1 + x;
    w.second = Rotate(x + Fetch64(s + 88), 53) * k1;

    // This is the same inner loop as CityHash64(), manually unrolled.
    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;
        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 -= 128;
    } while (LIKELY(len >= 128));
    x += Rotate(v.first + z, 49) * k0;
    y = y * k0 + Rotate(w.second, 37);
    z = z * k0 + Rotate(w.first, 27);
    w.first *= 9;
    v.first *= k0;
    // If 0 < len < 128, hash up to 4 chunks of 32 bytes each from the end of s.
    for (size_t tail_done = 0; tail_done < len;) {
        tail_done += 32;
        y = Rotate(x + y, 42) * k0 + v.second;
        w.first += Fetch64(s + len - tail_done + 16);
        x = x * k0 + w.first;
        z += w.second + Fetch64(s + len - tail_done);
        w.second += v.first;
        v = WeakHashLen32WithSeeds(s + len - tail_done, v.first + z, v.second);
        v.first *= k0;
    }
    // At this point our 56 bytes of state should contain more than
    // enough information for a strong 128-bit hash.  We use two
    // different 56-byte-to-8-byte hashes to get a 16-byte final result.
    x = HashLen16(x, v.first);
    y = HashLen16(y + z, w.first);
    return u128{HashLen16(x + v.second, w.second) + y, HashLen16(x + w.second, y + v.second)};
}

u128 CityHash128(const char* s, size_t len) {
    return len >= 16 ? CityHash128WithSeed(s + 16, len - 16, u128{Fetch64(s), Fetch64(s + 8) + k0})
                     : CityHash128WithSeed(s, len, u128{k0, k1});
}

} // namespace Common