a string hash.c

// based on xxhash32
unsigned int hash(char *data, size_t len)
{
  unsigned int hash;
  if (len < 4) {
    // load 3 bytes, overlapping if len < 3
    static unsigned char offset1[4] = { 0,0,1,1 };
    static unsigned char offset2[4] = { 0,0,0,2 };
    unsigned int h = data[0] + (data[offset1[len]]<<8) + (data[offset2[len]]<<16);
    h = xxprime1 + h*xxprime2;
    hash = rotl(h,13) * xxprime1;
  } else if (len <= 8) {
    // load two 32-bit ints, overlapping if len < 8
    unsigned int h0 = xxprime1 + load_unaligned_32(data+  0  )*xxprime2;
    unsigned int h1 = xxprime2 + load_unaligned_32(data+len-4)*xxprime2;
    h0 = rotl(h0,13) * xxprime1;
    h1 = rotl(h1,13) * xxprime1;
    hash = rotl(h0,1) + rotl(h1,7);
  } else {
    unsigned int h0 = xxprime1 ^ xxprime2;
    unsigned int h1 = xxprime2;
    unsigned int h2 = 0;
    unsigned int h3 = xxprime1;
    if (len < 16) {
      // load four 32-bit ints, partially overlapping
      h0 = h0 + load_unaligned_32(data+    0) * xxprime2;
      h1 = h1 + load_unaligned_32(data+    4) * xxprime2;
      h2 = h2 + load_unaligned_32(data+len-8) * xxprime2;
      h3 = h3 + load_unaligned_32(data+len-4) * xxprime2;
      h0 = rotl(h0,13)*xxprime1;
      h1 = rotl(h1,13)*xxprime1;
      h2 = rotl(h2,13)*xxprime1;
      h3 = rotl(h3,13)*xxprime1;
    } else {
      while (len > 16) {
        h0 = h0 + load_unaligned_32(data+ 0) * xxprime2;
        h1 = h1 + load_unaligned_32(data+ 4) * xxprime2;
        h2 = h2 + load_unaligned_32(data+ 8) * xxprime2;
        h3 = h3 + load_unaligned_32(data+12) * xxprime2;
        h0 = rotl(h0,13)*xxprime1;
        h1 = rotl(h1,13)*xxprime1;
        h2 = rotl(h2,13)*xxprime1;
        h3 = rotl(h3,13)*xxprime1;
        data += 16;
        len -= 16;
      }
      data = data + len - 16;
      h0 = h0 + load_unaligned_32(data+ 0) * xxprime2;
      h1 = h1 + load_unaligned_32(data+ 4) * xxprime2;
      h2 = h2 + load_unaligned_32(data+ 8) * xxprime2;
      h3 = h3 + load_unaligned_32(data+12) * xxprime2;
      h0 = rotl(h0,13)*xxprime1;
      h1 = rotl(h1,13)*xxprime1;
      h2 = rotl(h2,13)*xxprime1;
      h3 = rotl(h3,13)*xxprime1;
    }
    hash = rotl(h0,1)+rotl(h1,7)+rotl(h2,12)+rotl(h3,18);
  }
  hash += len;
  // XXH hash avalanche
  hash ^= hash >> 15;
  hash *= xxprime2;
  hash ^= hash >> 13;
  hash *= xxprime3;
  hash ^= hash >> 16;
  return hash;
}