static int parseHexChar(char c) {
  if (c >= '0' && c <= '9') ret charDiff(c, '0');
  if (c >= 'a' && c <= 'f') ret charDiff(c, 'a')+10;
  if (c >= 'A' && c <= 'F') ret charDiff(c, 'A')+10;
  ret -1;
}