code: private static long lowMask(String chars) {
          int n = chars.length();
          long m = 0;
          for (int i = 0; i < n; i++) {
              char c = chars.charAt(i);
              if (c < 64)
                  m |= (1L << c);
          }
          return m;
      }
docs: Compute the low-order mask for the characters in the given string