export function HASH<T>(key: T): u32 {
  if (isString<T>()) {
    return hashStr(changetype<string>(key));
  } else if (isReference<T>()) {
    if (sizeof<T>() == 4) return hash32(changetype<u32>(key));
    if (sizeof<T>() == 8) return hash64(changetype<u64>(key));
  } else if (isFloat<T>()) {
    if (sizeof<T>() == 4) return hash32(reinterpret<u32>(f32(key)));
    if (sizeof<T>() == 8) return hash64(reinterpret<u64>(f64(key)));
  } else {
    if (sizeof<T>() <= 4) return hash32(u32(key), sizeof<T>());
    if (sizeof<T>() == 8) return hash64(u64(key));
  }
  return unreachable();
}

// XXHash 32-bit as a starting point, see: https://cyan4973.github.io/xxHash

// primes
// @ts-ignore: decorator
@inline const XXH32_P1: u32 = 2654435761;
// @ts-ignore: decorator
@inline const XXH32_P2: u32 = 2246822519;
// @ts-ignore: decorator
@inline const XXH32_P3: u32 = 3266489917;
// @ts-ignore: decorator
@inline const XXH32_P4: u32 = 668265263;
// @ts-ignore: decorator
@inline const XXH32_P5: u32 = 374761393;
// @ts-ignore: decorator
@inline const XXH32_SEED: u32 = 0;

// @ts-ignore: decorator
@inline
function hash32(key: u32, len: u32 = 4): u32 {
  var h: u32 = XXH32_SEED + XXH32_P5 + len;
  h += key * XXH32_P3;
  h  = rotl(h, 17) * XXH32_P4;
  h ^= h >> 15;
  h *= XXH32_P2;
  h ^= h >> 13;
  h *= XXH32_P3;
  h ^= h >> 16;
  return h;
}

// @ts-ignore: decorator
@inline
function hash64(key: u64): u32 {
  var h: u32 = XXH32_SEED + XXH32_P5 + 8;
  h += <u32>key * XXH32_P3;
  h  = rotl(h, 17) * XXH32_P4;
  h += <u32>(key >> 32) * XXH32_P3;
  h  = rotl(h, 17) * XXH32_P4;
  h ^= h >> 15;
  h *= XXH32_P2;
  h ^= h >> 13;
  h *= XXH32_P3;
  h ^= h >> 16;
  return h;
}

// @ts-ignore: decorator
@inline
function mix(h: u32, key: u32): u32 {
  return rotl(h + key * XXH32_P2, 13) * XXH32_P1;
}

// @ts-ignore: decorator
@inline
function hashStr(key: string): u32 {
  if (key === null) return XXH32_SEED;

  var h: u32 = key.length << 1;
  var len: usize = h;
  var pos = changetype<usize>(key);

  if (len >= 16) {
    let s1 = XXH32_SEED + XXH32_P1 + XXH32_P2;
    let s2 = XXH32_SEED + XXH32_P2;
    let s3 = XXH32_SEED;
    let s4 = XXH32_SEED - XXH32_P1;

    let end = len + pos - 16;
    while (pos <= end) {
      s1 = mix(s1, load<u32>(pos    ));
      s2 = mix(s2, load<u32>(pos,  4));
      s3 = mix(s3, load<u32>(pos,  8));
      s4 = mix(s4, load<u32>(pos, 12));
      pos += 16;
    }
    h += rotl(s1, 1) + rotl(s2, 7) + rotl(s3, 12) + rotl(s4, 18);
  } else {
    h += XXH32_SEED + XXH32_P5;
  }

  var end = changetype<usize>(key) + len - 4;
  while (pos <= end) {
    h += load<u32>(pos) * XXH32_P3;
    h = rotl(h, 17) * XXH32_P4;
    pos += 4;
  }

  end = changetype<usize>(key) + len;
  while (pos < end) {
    h += <u32>load<u8>(pos) * XXH32_P5;
    h = rotl(h, 11) * XXH32_P1;
    pos++;
  }

  h ^= h >> 15;
  h *= XXH32_P2;
  h ^= h >> 13;
  h *= XXH32_P3;
  h ^= h >> 16;
  return h;
}