code: function() {
      zip_ins_h = ((zip_ins_h << zip_H_SHIFT)
  		 ^ (zip_window[zip_strstart + zip_MIN_MATCH - 1] & 0xff))
  	& zip_HASH_MASK;
      zip_hash_head = zip_head1(zip_ins_h);
      zip_prev[zip_strstart & zip_WMASK] = zip_hash_head;
      zip_head2(zip_ins_h, zip_strstart);
  }
docs: /* ==========================================================================
  Insert string s in the dictionary and set match_head to the previous head
  of the hash chain (the most recent string with same hash key). Return
  the previous length of the hash chain.
  IN  assertion: all calls to to INSERT_STRING are made with consecutive
  input characters and the first MIN_MATCH bytes of s are valid
  (except for the last MIN_MATCH-1 bytes of the input file).