private static void checkSurrogatePair(CharSequence value, int offset, int len) {
char hi = value.charAt(offset);
if (len <= offset)
throw new RuntimeException(
String.format("Character sequence ends with single surrogate character: 0x%X", hi));
char lo = value.charAt(offset + 1);
if (!Character.isSurrogatePair(hi, lo))
throw new RuntimeException(String.format("Malformed surrogate pair: 0x%X - 0x%X", hi, lo));
}