import struct

class RTLFWriter:
    HEADER_FIXED_FORMAT = "<4sIII"
    RECORD_FORMAT = "<dddddddddd"

    MAGIC_NUMBER = b"RTLF"
    VERSION = 1

    def __init__(self, filename, car_id, track_name):
        self.filename = filename
        self.car_id = car_id
        self.track_name = track_name.encode('utf-8')
        self.file = open(filename, "wb")
        self._write_header()

    def _write_header(self):
        track_len = len(self.track_name)
        header_bytes = struct.pack(
            self.HEADER_FIXED_FORMAT,
            self.MAGIC_NUMBER,
            self.VERSION,
            self.car_id,
            track_len
        )
        self.file.write(header_bytes)
        self.file.write(self.track_name)

    def write_record(self, timestamp, long, lat, fx, fy, fz, ax, ay, az, speed):
        record_bytes = struct.pack(
            self.RECORD_FORMAT,
            timestamp, long, lat, fx, fy, fz, ax, ay, az, speed
        )
        self.file.write(record_bytes)

    def close(self):
        if self.file:
            self.file.close()
            self.file = None

    def __enter__(self): return self
    def __exit__(self, exc_type, exc_val, exc_tb): self.close()