// 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"

#include <algorithm>

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"

namespace util_hash {

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

static const uint64_t k0 = 0xa5b85c5e198ed849ULL;

static const uint64_t k1 = 0x8d58ac26afe12e47ULL;

static const uint64_t k2 = 0xc47b6e9e3a970ed3ULL;

static const uint64_t k3 = 0xc70f6907e782aa0bULL;

// 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) {

    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_t RotateByAtLeast1(uint64_t val, int shift) {

    DCHECK_GE(shift, 1);

    DCHECK_LE(shift, 63);

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

}

static uint64_t ShiftMix(uint64_t val) {

    return val ^ (val >> 47);

}

uint64_t HashLen16(uint64_t u, uint64_t v) {

    const uint64_t kMul = 0xc6a4a7935bd1e995ULL;

    uint64_t a = (u ^ v) * kMul;

    a ^= (a >> 47);

    uint64_t b = (u ^ a) * kMul;

    b ^= (b >> 47);

    b *= kMul;

    return b;

}

static uint64_t HashLen0to16(const char* s, size_t len) {

    DCHECK_GE(len, 0);

    DCHECK_LE(len, 16);

    if (len > 8) {

        uint64_t a = LittleEndian::Load64(s);

        uint64_t b = LittleEndian::Load64(s + len - 8);

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

    }

    if (len >= 4) {

        uint64_t a = LittleEndian::Load32(s);

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

    }

    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 * k3) * 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) {

    DCHECK_GE(len, 17);

    DCHECK_LE(len, 32);

    uint64_t a = LittleEndian::Load64(s) * k1;

    uint64_t b = LittleEndian::Load64(s + 8);

    uint64_t c = LittleEndian::Load64(s + len - 8) * k2;

    uint64_t 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_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, 51);

    uint64_t 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_t, uint64_t> WeakHashLen32WithSeeds(const char* s, uint64_t a, uint64_t 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_t HashLen33to64(const char* s, size_t len) {

    uint64_t z = LittleEndian::Load64(s + 24);

    uint64_t a = LittleEndian::Load64(s) + (len + LittleEndian::Load64(s + len - 16)) * k0;

    uint64_t b = Rotate(a + z, 52);

    uint64_t c = Rotate(a, 37);

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

    c += Rotate(a, 7);

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

    uint64_t vf = a + z;

    uint64_t 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_t wf = a + z;

    uint64_t ws = b + Rotate(a, 31) + c;

    uint64_t r = ShiftMix((vf + ws) * k2 + (wf + vs) * k0);

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

}

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

    uint64_t y = LittleEndian::Load64(s + len - 16) + LittleEndian::Load64(s + len - 56);

    uint64_t z =

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

    pair<uint64_t, uint64_t> v = WeakHashLen32WithSeeds(s + len - 64, len, z);

    pair<uint64_t, uint64_t> 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_t CityHash64WithSeeds(const char* s, size_t len, uint64_t seed0, uint64_t seed1) {

    return HashLen16(CityHash64(s, len) - seed0, seed1);

}

uint64_t CityHash64WithSeed(const char* s, size_t len, uint64_t seed) {

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

}

} // namespace util_hash