code: private static int[] toIdSet(boolean includeAlphaCharacters, String nonAlphaCharacters) {
          int[] bits = new int[4];  // 4*32=128 which covers all ASCII
          if (includeAlphaCharacters) {
              for (int ch = 'A'; ch <= 'Z'; ch++) {
                  bits[ch >>> 5] |= (1 << (ch & 31));
              }
              for (int ch = 'a'; ch <= 'z'; ch++) {
                  bits[ch >>> 5] |= (1 << (ch & 31));
              }
          }
          for (int i = 0; i < nonAlphaCharacters.length(); i++) {
              int ch = nonAlphaCharacters.charAt(i);
              if (ch >= 128) {
                  throw new AssertionError(); // out of range
              }
              bits[ch >>> 5] |= (1 << (ch & 31));
          }
          return bits;
      }
docs: Encodes a set of ASCII characters (< 127) as a bitset of 4 32-bit values.