inline size_t codepoint_length(const char *s8, size_t l) {
  if (l) {
    auto b = static_cast<uint8_t>(s8[0]);
    if ((b & 0x80) == 0) {
      return 1;
    } else if ((b & 0xE0) == 0xC0) {
      return 2;
    } else if ((b & 0xF0) == 0xE0) {
      return 3;
    } else if ((b & 0xF8) == 0xF0) {
      return 4;
    }
  }
  return 0;
}
The software reads data past the end, or before the beginning, of the intended buffer.