KegboardCrc.java

/*
 * Copyright 2014 Bevbot LLC <info@bevbot.com>
 *
 * This file is part of the Kegtab package from the Kegbot project. For
 * more information on Kegtab or Kegbot, see <http://kegbot.org/>.
 *
 * Kegtab 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, version 2.
 *
 * Kegtab 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 Kegtab. If not, see <http://www.gnu.org/licenses/>.
 */
package org.kegbot.kegboard;

import com.google.common.annotations.VisibleForTesting;

/**
 * @author mike wakerly (mike@wakerly.com)
 */
public class KegboardCrc {

  private static int[] sTable;

  public static int crc16CcittUpdate(int crc, byte b) {
    int bval = b & 0xff;
    bval ^= crc & 0x00ff;
    bval ^= (bval << 4) & 0x00ff;
    int result = ((bval << 8) & 0xffff) | ((crc >> 8) & 0x00ff);
    result ^= bval >> 4;
    result ^= (bval << 3) & 0xffff;
    result &= 0x0ffff;
    return result;
  }

  @VisibleForTesting
  protected static synchronized int[] getTable() {
    if (sTable == null) {
      sTable = new int[256];
      for (int i = 0; i < sTable.length; i++) {
        sTable[i] = crc16CcittUpdate(0, (byte) i);
      }
    }
    return sTable;
  }

  public static int crc16Ccitt(byte[] bytes, int length) {
    int crc = 0;
    final int[] table = getTable();
    for (int i = 0; i < length; i++) {
      final byte b = bytes[i];
      crc = (crc >> 8) ^ table[(crc ^ b) & 0x00ff];
      crc &= 0x0ffff;
    }
    return crc;
  }

}