crcutil.java

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package barontools.changethecolor;

/**
 * <p>Title: </p>
 * <p>Description: </p>
 * <p>Copyright: Copyright (c) 2007</p>
 * <p>Company: </p>
 * @author not attributable
 * @version 1.0
 */

final class CRCUtil {

  private static int[] crc_table;//CRC 表

  private static void make_crc_table() {
      int c;
      int n, k;
      crc_table = new int[256];
      for (n = 0; n < 256; n++) {
          c = n;
          for (k = 0; k < 8; k++) {
              if ((c & 1) == 1)
                  c = 0xedb88320 ^ (c >>> 1);
              else
                  c = c >>> 1;
          }
          crc_table[n] = c;
      }
  }

  private static int update_crc(byte[] buf, int off, int len) {
      int c = 0xffffffff;
      int n;

      if (crc_table == null) {
          make_crc_table();
      }
      for (n = off; n < len + off; n++) {
          c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >>> 8);
      }
      return c;
  }

  static int checksum(byte[] buf, int off, int len) {
      return update_crc(buf, off, len) ^ 0xffffffff;
  }

}