code: function INSERT_STRING() {
  		ins_h = ((ins_h << H_SHIFT) ^ (window[strstart + MIN_MATCH - 1] & 0xff)) & HASH_MASK;
  		hash_head = head1(ins_h);
  		prev[strstart & WMASK] = hash_head;
  		head2(ins_h, 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).