com.microrisc.hdlcframing.v1.HDLC_DataTransformer.java Source code

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Here is the source code for com.microrisc.hdlcframing.v1.HDLC_DataTransformer.java

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/* 
 * Copyright 2014 MICRORISC s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.microrisc.hdlcframing.v1;

import java.util.LinkedList;
import java.util.List;
import org.apache.commons.lang.ArrayUtils;

/**
 * Transforms data to and reads data from HDLC stuffing protocol.
 * 
 * @author Michal Konopa
 */
public final class HDLC_DataTransformer {
    private static final short FLAG_SEQUENCE = 0x7e;
    private static final short CONTROL_ESCAPE = 0x7d;
    private static final short ESCAPE_BIT = 0x20;

    /** Checks specified request to be sent to HDLC. */
    private static void checkRequestData(short[] dataToTransform) {
        if (dataToTransform == null) {
            throw new IllegalArgumentException("Data to transform  cannot be NULL");
        }
    }

    /** Checks specified request to be sent to HDLC. */
    private static void checkUartDataFrame(short[] uartDataFrame) {
        if (uartDataFrame == null) {
            throw new IllegalArgumentException("Incomming UART data frame cannot be null");
        }
    }

    /**
     * Returns new value of CRC.
     * @param crc current value of CRC
     * @param value input data byte
     * @return updated value of CRC
     */
    private static short updateCRC(short crc, short value) {
        for (int bitLoop = 8; bitLoop != 0; --bitLoop, value >>= 1) {
            if (((crc ^ value) & 0x01) != 0) {
                crc = (short) ((crc >> 1) ^ 0x8C);
            } else {
                crc >>= 1;
            }
        }
        return crc;
    }

    /**
     * Converts and adds the converted value of specified byte into specified list. 
     * @param dataItem data item to convert to HDLC protocol
     * @param resultList target list to add the converted value into
     */
    private static void convertAndAddByte(short dataItem, List<Short> resultList) {
        if ((dataItem == FLAG_SEQUENCE) || (dataItem == CONTROL_ESCAPE)) {
            resultList.add(CONTROL_ESCAPE);
            Short secByte = (short) (dataItem ^ ESCAPE_BIT);
            resultList.add(secByte);
        } else {
            resultList.add(dataItem);
        }
    }

    public static short[] transformToHLDCFormat(short[] dataToTransform) {
        checkRequestData(dataToTransform);
        List<Short> convertedDataList = new LinkedList<>();

        // begining byte
        convertedDataList.add(FLAG_SEQUENCE);

        // data and computing checksum
        short checksum = dataToTransform[0];

        for (int dataId = 0; dataId < dataToTransform.length; dataId++) {
            if (dataId != 0) {
                checksum ^= dataToTransform[dataId];
            }
            convertAndAddByte(dataToTransform[dataId], convertedDataList);
        }

        // adding checksum
        checksum ^= 0x5F;
        convertAndAddByte(checksum, convertedDataList);

        // ending byte
        convertedDataList.add(FLAG_SEQUENCE);

        Short[] transformedData = convertedDataList.toArray(new Short[] {});
        return ArrayUtils.toPrimitive(transformedData);
    }

    /**
     * Retrieves and returns data from specified HDLC data frame.
     * @param uartDataFrame HDLC data frame, which the data to retrive from
     * @return data from specified HDLC data frame
     * @throws HDLC_FormatException if specified data frame has incorrect format
     */
    public static short[] getDataFromFrame(short[] uartDataFrame) throws HDLC_FormatException {
        checkUartDataFrame(uartDataFrame);
        List<Short> dataList = new LinkedList<>();

        if (uartDataFrame[0] != FLAG_SEQUENCE) {
            throw new HDLC_FormatException("First byte must be 0x7e");
        }

        if (uartDataFrame[uartDataFrame.length - 1] != FLAG_SEQUENCE) {
            throw new HDLC_FormatException("Last byte must be 0x7e");
        }

        boolean escapedByte = false;
        short countedChecksum = -1;

        // getting starting position of a checksum
        int checksumStartPos = uartDataFrame.length - 2;
        if (uartDataFrame[uartDataFrame.length - 3] == CONTROL_ESCAPE) {
            checksumStartPos = uartDataFrame.length - 3;
        }

        for (int dataId = 1; dataId < checksumStartPos; dataId++) {
            short dataItem = uartDataFrame[dataId];

            if (dataItem == CONTROL_ESCAPE) {
                escapedByte = true;
                continue;
            }

            // previous byte was CONTROL ESCAPE
            if (escapedByte) {
                if ((dataItem & ESCAPE_BIT) == 0) {
                    dataItem |= ESCAPE_BIT;
                } else {
                    dataItem ^= ESCAPE_BIT;
                }
                escapedByte = false;
            }

            if (countedChecksum == -1) {
                countedChecksum = (short) (0x5F ^ dataItem);
            } else {
                countedChecksum ^= dataItem;
            }

            dataList.add(dataItem);
        }

        short packetChecksum = uartDataFrame[uartDataFrame.length - 2];
        if (uartDataFrame[uartDataFrame.length - 3] == CONTROL_ESCAPE) {
            packetChecksum = uartDataFrame[uartDataFrame.length - 2];
            if ((packetChecksum & ESCAPE_BIT) == 0) {
                packetChecksum |= ESCAPE_BIT;
            } else {
                packetChecksum ^= ESCAPE_BIT;
            }
        }

        if (countedChecksum != packetChecksum) {
            throw new HDLC_FormatException(
                    "Checksum mismatch. " + "Counted: " + countedChecksum + " Get: " + packetChecksum);
        }

        Short[] data = dataList.toArray(new Short[] {});
        return ArrayUtils.toPrimitive(data);
    }
}