Constexpr port of php's string hashing function

djbx33a.h

static constexpr zend_ulong DJBX33A(const char *str, size_t len){
    zend_ulong hash = 5381;
#if defined(_WIN32) || defined(__i386__) || defined(__x86_64__) || defined(__aarch64__)
    // Version with multiplication works better on modern CPUs
    for (; len >= 8; len -= 8, str += 8) {
        uint64_t chunk;
        std::memcpy(&chunk, str, sizeof(chunk));
        hash = hash * 33 * 33 * 33 * 33 + ((chunk >> (8 * 0)) & 0xff) * 33 * 33 * 33 + ((chunk >> (8 * 1)) & 0xff) * 33 * 33 +
               ((chunk >> (8 * 2)) & 0xff) * 33 + ((chunk >> (8 * 3)) & 0xff);
        hash = hash * 33 * 33 * 33 * 33 + ((chunk >> (8 * 4)) & 0xff) * 33 * 33 * 33 + ((chunk >> (8 * 5)) & 0xff) * 33 * 33 +
               ((chunk >> (8 * 6)) & 0xff) * 33 + ((chunk >> (8 * 7)) & 0xff);
    }
    // Remaining bytes
    for (; len >= 4; len -= 4, str += 4) {
        hash = hash * 33 * 33 * 33 * 33 + str[0] * 33 * 33 * 33 + str[1] * 33 * 33 + str[2] * 33 + str[3];
    }
    for (; len >= 2; len -= 2, str += 2) { hash = hash * 33 * 33 * 33 + str[0] * 33 * 33 + str[1] * 33; }
    if (len == 1) { hash = hash * 33 + *str; }
#else
    // Variant with the hash unrolled eight times
    for (; len >= 8; len -= 8) {
        hash = ((hash << 5) + hash) + *str++;
        hash = ((hash << 5) + hash) + *str++;
        hash = ((hash << 5) + hash) + *str++;
        hash = ((hash << 5) + hash) + *str++;
        hash = ((hash << 5) + hash) + *str++;
        hash = ((hash << 5) + hash) + *str++;
        hash = ((hash << 5) + hash) + *str++;
        hash = ((hash << 5) + hash) + *str++;
    }
    // Remaining bytes
    switch (len) {
        case 7: hash = ((hash << 5) + hash) + *str++; /* fallthrough... */
        case 6: hash = ((hash << 5) + hash) + *str++; /* fallthrough... */
        case 5: hash = ((hash << 5) + hash) + *str++; /* fallthrough... */
        case 4: hash = ((hash << 5) + hash) + *str++; /* fallthrough... */
        case 3: hash = ((hash << 5) + hash) + *str++; /* fallthrough... */
        case 2: hash = ((hash << 5) + hash) + *str++; /* fallthrough... */
        case 1: hash = ((hash << 5) + hash) + *str++; break;
        case 0:
            break;
            // EMPTY_SWITCH_DEFAULT_CASE()
    }
#endif
    if constexpr (sizeof(zend_ulong) == 8) return hash | 0x8000000000000000ULL;
    if constexpr (sizeof(zend_ulong) == 4) return hash | 0x80000000UL;
    static_assert(sizeof(zend_ulong) == 8 || sizeof(zend_ulong) == 4, "Unsupported hash size");
}