RS-232 & RS-485 Serial UART Communication

using System.IO.Ports;

public class SerialRfidParser
{
    // Standard Hardware Frame: [0xAA, Len, ReaderAddr, CmdCode, Data..., CRC16_Hi, CRC16_Lo]
    public const byte FRAME_HEADER = 0xAA;

    public static bool TryParseFrame(ReadOnlySpan<byte> buffer, out byte[] epcBytes)
    {
        epcBytes = Array.Empty<byte>();
        if (buffer.Length < 7 || buffer[0] != FRAME_HEADER) return false;

        byte len = buffer[1];
        if (buffer.Length < len + 2) return false;

        // Verify CRC-16 CCITT
        ushort calculatedCrc = ComputeCrc16(buffer.Slice(1, len - 1));
        ushort receivedCrc = (ushort)((buffer[len] << 8) | buffer[len + 1]);

        if (calculatedCrc != receivedCrc) return false;

        byte cmd = buffer[3];
        if (cmd == 0x01) // Inventory Read Command
        {
            byte epcLength = buffer[4];
            epcBytes = buffer.Slice(5, epcLength).ToArray();
            return true;
        }
        return false;
    }

    private static ushort ComputeCrc16(ReadOnlySpan<byte> data)
    {
        ushort crc = 0xFFFF;
        foreach (byte b in data)
        {
            crc ^= (ushort)(b << 8);
            for (int i = 0; i < 8; i++)
                crc = (crc & 0x8000) != 0 ? (ushort)((crc << 1) ^ 0x1021) : (ushort)(crc << 1);
        }
        return crc;
    }
}