PacketBuilder.java

package com.nightscout.core.dexcom;

//import com.google.common.primitives.Bytes;

import net.tribe7.common.primitives.Bytes;

import java.util.ArrayList;
import java.util.List;

public class PacketBuilder {
    public static final int MAX_PAYLOAD = 1584;
    public static final int MIN_LEN = 6;
    public static final int MAX_LEN = MAX_PAYLOAD + MIN_LEN;
    public static final byte SOF = 0x01;
    public static final int OFFSET_SOF = 0;
    public static final int OFFSET_LENGTH = 1;
    public static final int OFFSET_NULL = 2;
    public static final byte NULL = 0x00;
    public static final int OFFSET_CMD = 3;
    public static final int OFFSET_PAYLOAD = 4;
    public static final int CRC_LEN = 2;
    public static final int HEADER_LEN = 4;
    private List<Byte> packet;
    private Command command;
    private List<Byte> payload;

    public PacketBuilder(Command command) {
        this.command = command;
    }

    public PacketBuilder(Command command, List<Byte> payload) {
        this.command = command;
        this.payload = payload;
    }

    public byte[] build() {
        packet = new ArrayList<>();
        packet.add(OFFSET_SOF, SOF);
        packet.add(OFFSET_LENGTH, getLength());
        packet.add(OFFSET_NULL, NULL);
        packet.add(OFFSET_CMD, command.getValue());
        if (this.payload != null) {
            this.packet.addAll(OFFSET_PAYLOAD, this.payload);
        }
        byte[] crc16 = CRC16.calculate(toBytes(), 0, this.packet.size());
        this.packet.add(crc16[0]);
        this.packet.add(crc16[1]);
        return this.toBytes();
    }

    private byte getLength() {
        int packetSize = payload == null ? MIN_LEN : payload.size() + CRC_LEN + HEADER_LEN;

        if (packetSize > MAX_LEN) {
            throw new IndexOutOfBoundsException(packetSize + " bytes, but packet must between "
                    + MIN_LEN + " and " + MAX_LEN + " bytes.");
        }

        return (byte) packetSize;
    }

    private byte[] toBytes() {
        return Bytes.toArray(this.packet);
    }
}