import serial

MAX_PAYLOAD = 31

TYPE_FIRST  = 0b00
TYPE_MIDDLE = 0b01
TYPE_LAST   = 0b10
CRC4_POLY   = 0x13  # x^4 + x + 1

def crc4_bytes(data: bytes) -> int:
    reg = 0
    for byte in data:
        for i in range(7, -1, -1):
            bit = (byte >> i) & 1
            reg = ((reg << 1) | bit) & 0x1F
            if reg & 0x10:
                reg ^= CRC4_POLY
    for _ in range(4):
        reg = (reg << 1) & 0x1F
        if reg & 0x10:
            reg ^= CRC4_POLY
    return reg & 0x0F

def build_header(pkt_type, session, nibble):
    return ((nibble & 0x0F) << 4) | ((session & 3) << 2) | (pkt_type & 3)

def create_packets(msg: bytes, session: int):
    # Разбиваем сообщение
    chunks = [msg[i:i+MAX_PAYLOAD] for i in range(0, len(msg), MAX_PAYLOAD)]
    data_packets = max(1, len(chunks))

    # CRC
    crc = crc4_bytes(msg)

    packets = []

    # FIRST
    header = build_header(TYPE_FIRST, session, data_packets)
    packets.append(bytes([header]) + chunks[0])

    # MIDDLE
    for i in range(1, data_packets):
        header = build_header(TYPE_MIDDLE, session, i)
        packets.append(bytes([header]) + chunks[i])

    # LAST (payload пустой)
    header = build_header(TYPE_LAST, session, crc)
    packets.append(bytes([header]))

    return packets


def main():
    #port = input("COM-порт (пример: COM5 или /dev/ttyUSB0): ")
    port = "/dev/ttyACM0"
    #text = input("Введите строку для отправки: ")
    text = "Some long string to send probably it shouldn't fit into one packet so several of them should arrive"

    msg = text.encode("utf-8")
    session = 1

    packets = create_packets(msg, session)

    print("\nГотовые пакеты:")
    for i, pkt in enumerate(packets):
        print(f"{i}: " + " ".join(f"{b:02X}" for b in pkt))

    # отправка
    with serial.Serial(port, 115200, timeout=1) as ser:
        for pkt in packets:
            ser.write(pkt)
            ser.flush()
            print(f"Отправлено: {len(pkt)} bytes")

    print("Готово.")


if __name__ == "__main__":
    main()