code: private int rollHash(int hashvalue, String str, int i) {
      // 'roll' hash
      char outchar = str.charAt(str.length() - 1 - i);
      char inchar = str.charAt(str.length() - _block - 1 - i);
      hashvalue = A * hashvalue + CHAR_HASHES[inchar] - _Apowblock * CHAR_HASHES[outchar];
      return hashvalue;
    }
docs: Update rolling hash values.
  @param hashvalue
  @param str
  @param i
  @return