A custom fast, non-cryptographic hash function

zxhash.c

#include <stdint.h>

static inline uint64_t multiply_mix(uint64_t x, uint64_t y) {
  __uint128_t m = (__uint128_t)x * (__uint128_t)y;

  uint64_t hi = m >> 64;
  uint64_t lo = m;

  return lo ^ hi;
}

static inline uint64_t zxhash(const void *key, int len, uint64_t seed) {
  const uint64_t secret = 0x9e3779b97f4a7c15;
  uint64_t h = multiply_mix(seed ^ 0x243f6a8885a308d3, len ^ 0x13198a2e03707344);

  if (len < 8) {
    const uint8_t *bytes = key;

    uint64_t block = 0;

    switch (len) {
      case 7: block |= (uint64_t)bytes[6] << 48;
      case 6: block |= (uint64_t)bytes[5] << 40;
      case 5: block |= (uint64_t)bytes[4] << 32;
      case 4: block |= (uint64_t)bytes[3] << 24;
      case 3: block |= (uint64_t)bytes[2] << 16;
      case 2: block |= (uint64_t)bytes[1] << 8;
      case 1: block |= (uint64_t)bytes[0];
    }

    h = multiply_mix(h, block ^ secret);
  } else if (len < 128) {
    const uint64_t *view = key;
    while (len > 16) {
      h = multiply_mix(h ^ view[0], view[1] ^ secret);

      view += 2;
      len -= 16;
    }

    h ^= *(const uint64_t *)((const uint8_t *)view + len - 8);
    const uint64_t block = (len > 8) ? view[0] : 0;
    h = multiply_mix(h, block ^ secret);
  } else {
    uint64_t s0 = 0xa4093822299f31d0;
    uint64_t s1 = 0x082efa98ec4e6c89;
    uint64_t s2 = 0x452821e638d01377;
    uint64_t s3 = 0xbe5466cf34e90c6c;

    const uint64_t *view = key;
    while (len > 64) {
      s0 = multiply_mix(s0 ^ view[0], view[1] ^ secret);
      s1 = multiply_mix(s1 ^ view[2], view[3] ^ secret);
      s2 = multiply_mix(s2 ^ view[4], view[5] ^ secret);
      s3 = multiply_mix(s3 ^ view[6], view[7] ^ secret);
      view += 8;
      len -= 64;
    }

    const uint64_t *final = (const uint64_t *)((const uint8_t *)view + len - 64);
    s0 = multiply_mix(s0 ^ final[0], s2 ^ final[1]);
    s1 = multiply_mix(s1 ^ final[2], s3 ^ final[3]);
    s0 = multiply_mix(s0 ^ final[4], s1 ^ final[5]);
    h = multiply_mix(h ^ final[6], s0 ^ final[7]);
  }

  return multiply_mix(h ^ len, 0xffebb71d94fcdaf9);
}