// Copyright 2010 Google Inc. All Rights Reserved.

// Authors: gpike@google.com (Geoff Pike), jyrki@google.com (Jyrki Alakuijala)

//

// This file provides CityHash64() and related functions.

//

// The externally visible functions follow the naming conventions of

// hash.h, where the size of the output is part of the name.  For

// example, CityHash64 returns a 64-bit hash.  The internal helpers do

// not have the return type in their name, but instead have names like

// HashLenXX or HashLenXXtoYY, where XX and YY are input string lengths.

//

// Most of the constants and tricks here were copied from murmur.cc or

// hash.h, or discovered by trial and error.  It's probably possible to further

// optimize the code here by writing a program that systematically explores

// more of the space of possible hash functions, or by using SIMD instructions.

#include "gutil/hash/city.h"

// IWYU pragma: no_include <pstl/glue_algorithm_defs.h>

#include <sys/types.h>

#include <algorithm>

#include <iterator>

using std::copy;

using std::max;

using std::min;

using std::reverse;

using std::sort;

using std::swap;

#include <utility>

using std::make_pair;

using std::pair;

#include "common/logging.h"

#include "gutil/endian.h"

#include "gutil/hash/hash128to64.h"

#include "gutil/int128.h"

#include "gutil/integral_types.h"

#include "gutil/port.h"

namespace util_hash {

// Some primes between 2^63 and 2^64 for various uses.

static const uint64 k0 = 0xa5b85c5e198ed849ULL;

static const uint64 k1 = 0x8d58ac26afe12e47ULL;

static const uint64 k2 = 0xc47b6e9e3a970ed3ULL;

static const uint64 k3 = 0xc70f6907e782aa0bULL;

// Bitwise right rotate.  Normally this will compile to a single

// instruction, especially if the shift is a manifest constant.

static uint64 Rotate(uint64 val, int shift) {

    DCHECK_GE(shift, 0);

    DCHECK_LE(shift, 63);

    // Avoid shifting by 64: doing so yields an undefined result.

    return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));

}

// Equivalent to Rotate(), but requires the second arg to be non-zero.

// On x86-64, and probably others, it's possible for this to compile

// to a single instruction if both args are already in registers.

static uint64 RotateByAtLeast1(uint64 val, int shift) {

    DCHECK_GE(shift, 1);

    DCHECK_LE(shift, 63);

    return (val >> shift) | (val << (64 - shift));

}

static uint64 ShiftMix(uint64 val) {

    return val ^ (val >> 47);

}

static uint64 HashLen16(uint64 u, uint64 v) {

    return Hash128to64(uint128(u, v));

}

static uint64 HashLen0to16(const char* s, size_t len) {

    DCHECK_GE(len, 0);

    DCHECK_LE(len, 16);

    if (len > 8) {

        uint64 a = LittleEndian::Load64(s);

        uint64 b = LittleEndian::Load64(s + len - 8);

        return HashLen16(a, RotateByAtLeast1(b + len, len)) ^ b;

    }

    if (len >= 4) {

        uint64 a = LittleEndian::Load32(s);

        return HashLen16(len + (a << 3), LittleEndian::Load32(s + len - 4));

    }

    if (len > 0) {

        uint8 a = s[0];

        uint8 b = s[len >> 1];

        uint8 c = s[len - 1];

        uint32 y = static_cast<uint32>(a) + (static_cast<uint32>(b) << 8);

        uint32 z = len + (static_cast<uint32>(c) << 2);

        return ShiftMix(y * k2 ^ z * k3) * k2;

    }

    return k2;

}

// This probably works well for 16-byte strings as well, but it may be overkill

// in that case.

static uint64 HashLen17to32(const char* s, size_t len) {

    DCHECK_GE(len, 17);

    DCHECK_LE(len, 32);

    uint64 a = LittleEndian::Load64(s) * k1;

    uint64 b = LittleEndian::Load64(s + 8);

    uint64 c = LittleEndian::Load64(s + len - 8) * k2;

    uint64 d = LittleEndian::Load64(s + len - 16) * k0;

    return HashLen16(Rotate(a - b, 43) + Rotate(c, 30) + d, a + Rotate(b ^ k3, 20) - c + len);

}

// Return a 16-byte hash for 48 bytes.  Quick and dirty.

// Callers do best to use "random-looking" values for a and b.

// (For more, see the code review discussion of CL 18799087.)

static pair<uint64, uint64> WeakHashLen32WithSeeds(uint64 w, uint64 x, uint64 y, uint64 z, uint64 a,

                                                   uint64 b) {

    a += w;

    b = Rotate(b + a + z, 51);

    uint64 c = a;

    a += x;

    a += y;

    b += Rotate(a, 23);

    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<uint64, uint64> WeakHashLen32WithSeeds(const char* s, uint64 a, uint64 b) {

    return WeakHashLen32WithSeeds(LittleEndian::Load64(s), LittleEndian::Load64(s + 8),

                                  LittleEndian::Load64(s + 16), LittleEndian::Load64(s + 24), a, b);

}

// Return an 8-byte hash for 33 to 64 bytes.

static uint64 HashLen33to64(const char* s, size_t len) {

    uint64 z = LittleEndian::Load64(s + 24);

    uint64 a = LittleEndian::Load64(s) + (len + LittleEndian::Load64(s + len - 16)) * k0;

    uint64 b = Rotate(a + z, 52);

    uint64 c = Rotate(a, 37);

    a += LittleEndian::Load64(s + 8);

    c += Rotate(a, 7);

    a += LittleEndian::Load64(s + 16);

    uint64 vf = a + z;

    uint64 vs = b + Rotate(a, 31) + c;

    a = LittleEndian::Load64(s + 16) + LittleEndian::Load64(s + len - 32);

    z += LittleEndian::Load64(s + len - 8);

    b = Rotate(a + z, 52);

    c = Rotate(a, 37);

    a += LittleEndian::Load64(s + len - 24);

    c += Rotate(a, 7);

    a += LittleEndian::Load64(s + len - 16);

    uint64 wf = a + z;

    uint64 ws = b + Rotate(a, 31) + c;

    uint64 r = ShiftMix((vf + ws) * k2 + (wf + vs) * k0);

    return ShiftMix(r * k0 + vs) * k2;

}

uint64 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 x = LittleEndian::Load64(s + len - 40);

    uint64 y = LittleEndian::Load64(s + len - 16) + LittleEndian::Load64(s + len - 56);

    uint64 z =

            HashLen16(LittleEndian::Load64(s + len - 48) + len, LittleEndian::Load64(s + len - 24));

    pair<uint64, uint64> v = WeakHashLen32WithSeeds(s + len - 64, len, z);

    pair<uint64, uint64> w = WeakHashLen32WithSeeds(s + len - 32, y + k1, x);

    x = x * k1 + LittleEndian::Load64(s);

    // Decrease len to the nearest multiple of 64, and operate on 64-byte chunks.

    len = (len - 1) & ~static_cast<size_t>(63);

    DCHECK_GT(len, 0);

    DCHECK_EQ(len, len / 64 * 64);

    do {

        x = Rotate(x + y + v.first + LittleEndian::Load64(s + 8), 37) * k1;

        y = Rotate(y + v.second + LittleEndian::Load64(s + 48), 42) * k1;

        x ^= w.second;

        y += v.first + LittleEndian::Load64(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 + LittleEndian::Load64(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 CityHash64WithSeed(const char* s, size_t len, uint64 seed) {

    return CityHash64WithSeeds(s, len, k2, seed);

}

uint64 CityHash64WithSeeds(const char* s, size_t len, uint64 seed0, uint64 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 ssize_t.  Based on City and Murmur128.

static uint128 CityMurmur(const char* s, size_t len, uint128 seed) {

    uint64 a = Uint128Low64(seed);

    uint64 b = Uint128High64(seed);

    uint64 c = 0;

    uint64 d = 0;

    ssize_t l = len - 16;

    if (l <= 0) { // len <= 16

        c = b * k1 + HashLen0to16(s, len);

        d = Rotate(a + (len >= 8 ? LittleEndian::Load64(s) : c), 32);

    } else { // len > 16

        c = HashLen16(LittleEndian::Load64(s + len - 8) + k1, a);

        d = HashLen16(b + len, c + LittleEndian::Load64(s + len - 16));

        a += d;

        do {

            a ^= ShiftMix(LittleEndian::Load64(s) * k1) * k1;

            a *= k1;

            b ^= a;

            c ^= ShiftMix(LittleEndian::Load64(s + 8) * k1) * k1;

            c *= k1;

            d ^= c;

            s += 16;

            l -= 16;

        } while (l > 0);

    }

    a = HashLen16(a, c);

    b = HashLen16(d, b);

    return uint128(a ^ b, HashLen16(b, a));

}

uint128 CityHash128WithSeed(const char* s, size_t len, uint128 seed) {

    // TODO(user): As of February 2011, there's a beta of Murmur3 that would

    // most likely be useful here.  E.g., if (len < 900) return Murmur3(...)

    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<uint64, uint64> v, w;

    uint64 x = Uint128Low64(seed);

    uint64 y = Uint128High64(seed);

    uint64 z = len * k1;

    v.first = Rotate(y ^ k1, 49) * k1 + LittleEndian::Load64(s);

    v.second = Rotate(v.first, 42) * k1 + LittleEndian::Load64(s + 8);

    w.first = Rotate(y + z, 35) * k1 + x;

    w.second = Rotate(x + LittleEndian::Load64(s + 88), 53) * k1;

    // This is similar to the inner loop of CityHash64(), manually unrolled.

    do {

        x = Rotate(x + y + v.first + LittleEndian::Load64(s + 16), 37) * k1;

        y = Rotate(y + v.second + LittleEndian::Load64(s + 48), 42) * k1;

        x ^= w.second;

        y ^= v.first;

        z = Rotate(z ^ w.first, 33);

        v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);

        w = WeakHashLen32WithSeeds(s + 32, z + w.second, y);

        std::swap(z, x);

        s += 64;

        x = Rotate(x + y + v.first + LittleEndian::Load64(s + 16), 37) * k1;

        y = Rotate(y + v.second + LittleEndian::Load64(s + 48), 42) * k1;

        x ^= w.second;

        y ^= v.first;

        z = Rotate(z ^ w.first, 33);

        v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);

        w = WeakHashLen32WithSeeds(s + 32, z + w.second, y);

        std::swap(z, x);

        s += 64;

        len -= 128;

    } while (PREDICT_TRUE(len >= 128));

    y += Rotate(w.first, 37) * k0 + z;

    x += Rotate(v.first + z, 49) * 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(y - x, 42) * k0 + v.second;

        w.first += LittleEndian::Load64(s + len - tail_done + 16);

        x = Rotate(x, 49) * k0 + w.first;

        w.first += v.first;

        v = WeakHashLen32WithSeeds(s + len - tail_done, v.first, v.second);

    }

    // At this point our 48 bytes of state should contain more than

    // enough information for a strong 128-bit hash.  We use two

    // different 48-byte-to-8-byte hashes to get a 16-byte final result.

    x = HashLen16(x, v.first);

    y = HashLen16(y, w.first);

    return uint128(HashLen16(x + v.second, w.second) + y, HashLen16(x + w.second, y + v.second));

}

uint128 CityHash128(const char* s, size_t len) {

    if (len >= 16) {

        return CityHash128WithSeed(

                s + 16, len - 16,

                uint128(LittleEndian::Load64(s) ^ k3, LittleEndian::Load64(s + 8)));

    } else if (len >= 8) {

        return CityHash128WithSeed(nullptr, 0,

                                   uint128(LittleEndian::Load64(s) ^ (len * k0),

                                           LittleEndian::Load64(s + len - 8) ^ k1));

    } else {

        return CityHash128WithSeed(s, len, uint128(k0, k1));

    }

}

} // namespace util_hash