/*


  Jacksum 3.6.0 - a checksum utility in Java
  Copyright (c) 2001-2023 Dipl.-Inf. (FH) Johann N. Löfflmann,
  All Rights Reserved, <https://jacksum.net>.

  This program is free software: you can redistribute it and/or modify it under
  the terms of the GNU General Public License as published by the Free Software
  Foundation, either version 3 of the License, or (at your option) any later
  version.

  This program is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
  details.

  You should have received a copy of the GNU General Public License along with
  this program. If not, see <https://www.gnu.org/licenses/>.


 */

package net.jacksum.algorithms.crcs;

import net.jacksum.algorithms.AbstractChecksum;
import net.jacksum.formats.Encoding;

/**
 * A class that can be used to compute the CRC-16 of a data stream.
 * This is a 100% Java implementation.
 */

public class CRC16 extends AbstractChecksum implements CrcInfo {

    private int value;

    // CRC-16 look-up table
    protected final int[] crc16tab = {
        0x0000,0xC0C1,0xC181,0x0140,0xC301,0x03C0,0x0280,0xC241,
        0xC601,0x06C0,0x0780,0xC741,0x0500,0xC5C1,0xC481,0x0440,
        0xCC01,0x0CC0,0x0D80,0xCD41,0x0F00,0xCFC1,0xCE81,0x0E40,
        0x0A00,0xCAC1,0xCB81,0x0B40,0xC901,0x09C0,0x0880,0xC841,
        0xD801,0x18C0,0x1980,0xD941,0x1B00,0xDBC1,0xDA81,0x1A40,
        0x1E00,0xDEC1,0xDF81,0x1F40,0xDD01,0x1DC0,0x1C80,0xDC41,
        0x1400,0xD4C1,0xD581,0x1540,0xD701,0x17C0,0x1680,0xD641,
        0xD201,0x12C0,0x1380,0xD341,0x1100,0xD1C1,0xD081,0x1040,
        0xF001,0x30C0,0x3180,0xF141,0x3300,0xF3C1,0xF281,0x3240,
        0x3600,0xF6C1,0xF781,0x3740,0xF501,0x35C0,0x3480,0xF441,
        0x3C00,0xFCC1,0xFD81,0x3D40,0xFF01,0x3FC0,0x3E80,0xFE41,
        0xFA01,0x3AC0,0x3B80,0xFB41,0x3900,0xF9C1,0xF881,0x3840,
        0x2800,0xE8C1,0xE981,0x2940,0xEB01,0x2BC0,0x2A80,0xEA41,
        0xEE01,0x2EC0,0x2F80,0xEF41,0x2D00,0xEDC1,0xEC81,0x2C40,
        0xE401,0x24C0,0x2580,0xE541,0x2700,0xE7C1,0xE681,0x2640,
        0x2200,0xE2C1,0xE381,0x2340,0xE101,0x21C0,0x2080,0xE041,
        0xA001,0x60C0,0x6180,0xA141,0x6300,0xA3C1,0xA281,0x6240,
        0x6600,0xA6C1,0xA781,0x6740,0xA501,0x65C0,0x6480,0xA441,
        0x6C00,0xACC1,0xAD81,0x6D40,0xAF01,0x6FC0,0x6E80,0xAE41,
        0xAA01,0x6AC0,0x6B80,0xAB41,0x6900,0xA9C1,0xA881,0x6840,
        0x7800,0xB8C1,0xB981,0x7940,0xBB01,0x7BC0,0x7A80,0xBA41,
        0xBE01,0x7EC0,0x7F80,0xBF41,0x7D00,0xBDC1,0xBC81,0x7C40,
        0xB401,0x74C0,0x7580,0xB541,0x7700,0xB7C1,0xB681,0x7640,
        0x7200,0xB2C1,0xB381,0x7340,0xB101,0x71C0,0x7080,0xB041,
        0x5000,0x90C1,0x9181,0x5140,0x9301,0x53C0,0x5280,0x9241,
        0x9601,0x56C0,0x5780,0x9741,0x5500,0x95C1,0x9481,0x5440,
        0x9C01,0x5CC0,0x5D80,0x9D41,0x5F00,0x9FC1,0x9E81,0x5E40,
        0x5A00,0x9AC1,0x9B81,0x5B40,0x9901,0x59C0,0x5880,0x9841,
        0x8801,0x48C0,0x4980,0x8941,0x4B00,0x8BC1,0x8A81,0x4A40,
        0x4E00,0x8EC1,0x8F81,0x4F40,0x8D01,0x4DC0,0x4C80,0x8C41,
        0x4400,0x84C1,0x8581,0x4540,0x8701,0x47C0,0x4680,0x8641,
        0x8201,0x42C0,0x4380,0x8341,0x4100,0x81C1,0x8081,0x4040
    };

    public CRC16() {
        super();
        bitWidth = 16;        
        value = 0;
        formatPreferences.setHashEncoding(Encoding.DEC);
        formatPreferences.setSeparator(" ");
        formatPreferences.setFilesizeWanted(true);
    }

    @Override
    public void reset() {
        value = 0;
        length = 0;
    }

    @Override
    public void update(byte[] bytes, int offset, int length) {
        for (int i = offset; i < length + offset; i++) {
            value = (value >>> 8) ^ crc16tab[(value ^ bytes[i]) & 0xff];
        }
        this.length += length;
    }

    @Override
    public long getValue() {
        return (((long)value)&0xFFFFL);
    }

    @Override
    public byte[] getByteArray() {
        return new byte[]
        {(byte)((value>>8)&0xff),
         (byte)(value&0xff)};
    }

    @Override
    public byte[] getPolyAsBytes() {
        return new byte[] { (byte)0x80, (byte)0x05 };
    }

    @Override
    public int getWidth() {
        return bitWidth;
    }

    @Override
    public long getInitialValue() {
        return 0;
    }

    @Override
    public boolean isRefIn() {
        return true;
    }

    @Override
    public boolean isRefOut() {
        return true;
    }

    @Override
    public long getXorOut() {
        return 0;
    }
}
