CRC32.java

//

package net.sf.zipme;

/** 
 * Computes CRC32 data checksum of a data stream.
 * The actual CRC32 algorithm is described in RFC 1952
 * (GZIP file format specification version 4.3).
 * Can be used to get the CRC32 over a stream if used with checked input/output
 * streams.
 * @see InflaterInputStream
 * @see DeflaterOutputStream
 * @author Per Bothner
 * @date April 1, 1999.
 */
public class CRC32 implements Checksum {
  /** 
 * The crc data checksum so far. 
 */
  private int crc=0;
  /** 
 * The fast CRC table. Computed once when the CRC32 class is loaded. 
 */
  private static int[] crc_table=make_crc_table();
  /** 
 * Make the table for a fast CRC. 
 */
  private static int[] make_crc_table(){
    int[] crc_table=new int[256];
    for (int n=0; n < 256; n++) {
      int c=n;
      for (int k=8; --k >= 0; ) {
        if ((c & 1) != 0)         c=0xedb88320 ^ (c >>> 1);
 else         c=c >>> 1;
      }
      crc_table[n]=c;
    }
    return crc_table;
  }
  /** 
 * Returns the CRC32 data checksum computed so far.
 */
  public long getValue(){
    return (long)crc & 0xffffffffL;
  }
  /** 
 * Resets the CRC32 data checksum as if no update was ever called.
 */
  public void reset(){
    crc=0;
  }
  /** 
 * Updates the checksum with the int bval. 
 * @param bval (the byte is taken as the lower 8 bits of bval)
 */
  public void update(  int bval){
    int c=~crc;
    c=crc_table[(c ^ bval) & 0xff] ^ (c >>> 8);
    crc=~c;
  }
  /** 
 * Adds the byte array to the data checksum.
 * @param buf the buffer which contains the data
 * @param off the offset in the buffer where the data starts
 * @param len the length of the data
 */
  public void update(  byte[] buf,  int off,  int len){
    int c=~crc;
    while (--len >= 0)     c=crc_table[(c ^ buf[off++]) & 0xff] ^ (c >>> 8);
    crc=~c;
  }
  /** 
 * Adds the complete byte array to the data checksum.
 */
  public void update(  byte[] buf){
    update(buf,0,buf.length);
  }
}