home/src/hash.cc

#include "hash.hh"

#include <cstdint>
#include <cstring>

#include "unit_tests.hh"
#include "assert.hh"

namespace Kakoune
{

[[gnu::always_inline]]
static inline uint32_t rotl(uint32_t x, int8_t r)
{
    return (x << r) | (x >> (32 - r));
}

[[gnu::always_inline]]
static inline uint32_t fmix(uint32_t h)
{
    h ^= h >> 16;
    h *= 0x85ebca6b;
    h ^= h >> 13;
    h *= 0xc2b2ae35;
    h ^= h >> 16;

    return h;
}

// murmur3 hash, based on https://github.com/PeterScott/murmur3
size_t hash_data(const char* input, size_t len)
{
    const uint8_t* data = reinterpret_cast<const uint8_t*>(input);
    uint32_t hash = 0x1235678;
    constexpr uint32_t c1 = 0xcc9e2d51;
    constexpr uint32_t c2 = 0x1b873593;

    const ptrdiff_t nblocks = len / 4;
    const uint8_t* blocks = data + nblocks*4;

    for (ptrdiff_t i = -nblocks; i; ++i)
    {
        uint32_t key;
        memcpy(&key, blocks + 4*i, 4);
        key *= c1;
        key = rotl(key, 15);
        key *= c2;

        hash ^= key;
        hash = rotl(hash, 13);
        hash = hash * 5 + 0xe6546b64;
    }

    const uint8_t* tail = data + nblocks * 4;
    uint32_t key = 0;
    switch (len & 0b11)
    {
        case 3: key ^= tail[2] << 16; [[fallthrough]];
        case 2: key ^= tail[1] << 8;  [[fallthrough]];
        case 1: key ^= tail[0];
                key *= c1;
                key = rotl(key,15);
                key *= c2;
                hash ^= key;
    }

    hash ^= len;
    hash = fmix(hash);

    return hash;
}

UnitTest test_murmur_hash{[] {
    {
        constexpr char data[] = "Hello, World!";
        kak_assert(hash_data(data, strlen(data)) == 0xf816f95b);
    }
    {
        constexpr char data[] = "xxxxxxxxxxxxxxxxxxxxxxxxxxxx";
        kak_assert(hash_data(data, strlen(data)) == 3551113186);
    }
    kak_assert(hash_data("", 0) == 2572747774);
}};

}