{

using clock = std::chrono::steady_clock;

using ms = std::chrono::milliseconds;

const auto deadline = (timeout_ms < 0)

? clock::time_point::max()

: clock::now() + ms(timeout_ms);

auto ms_left = [&]() -> int {

if (timeout_ms < 0) return -1;

auto rem = std::chrono::duration_cast<ms>(deadline - clock::now()).count();

return (rem > 0) ? static_cast<int>(rem) : 0;

};

if (!port_.wait_readable(ms_left()))

return std::nullopt;

uint8_t b1 = port_.read_byte();

// Sliding-window sync search: advance one byte at a time until a parser

// recognises the (b1, b2) pair. This handles leading garbage bytes

// (e.g. a stray '\n' before a '$' in an NMEA stream) without losing sync.

for (;;) {

int left = ms_left();

if (left == 0)

return std::nullopt;

if (!port_.wait_readable(left))

return std::nullopt;

uint8_t b2 = port_.read_byte();

for (auto& parser : parsers_) {

if (parser->matches(b1, b2))

return parser->parse(port_, b1, b2);

}

b1 = b2; // slide: discard b1, try again with b2 as the new b1

}