* The specification for Adler32 may be found in RFC 1950. * (ZLIB Compressed Data Format Specification version 3.3) *
*
* From that document: *
* "ADLER32 (Adler-32 checksum) * This contains a checksum value of the uncompressed data * (excluding any dictionary data) computed according to Adler-32 * algorithm. This algorithm is a 32-bit extension and improvement * of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 * standard. *
* Adler-32 is composed of two sums accumulated per byte: s1 is * the sum of all bytes, s2 is the sum of all s1 values. Both sums * are done modulo 65521. s1 is initialized to 1, s2 to zero. The * Adler-32 checksum is stored as s2*65536 + s1 in most- * significant-byte first (network) order." *
* "8.2. The Adler-32 algorithm *
* The Adler-32 algorithm is much faster than the CRC32 algorithm yet * still provides an extremely low probability of undetected errors. *
* The modulo on unsigned long accumulators can be delayed for 5552 * bytes, so the modulo operation time is negligible. If the bytes * are a, b, c, the second sum is 3a + 2b + c + 3, and so is position * and order sensitive, unlike the first sum, which is just a * checksum. That 65521 is prime is important to avoid a possible * large class of two-byte errors that leave the check unchanged. * (The Fletcher checksum uses 255, which is not prime and which also * makes the Fletcher check insensitive to single byte changes 0 <-> * 255.) *
* The sum s1 is initialized to 1 instead of zero to make the length
* of the sequence part of s2, so that the length does not have to be
* checked separately. (Any sequence of zeroes has a Fletcher
* checksum of zero.)"
*
* @author John Leuner, Per Bothner
* @since JDK 1.1
*
* @see InflaterInputStream
* @see DeflaterOutputStream
*/
public class Adler32 implements Checksum
{
/** largest prime smaller than 65536 */
private static final int BASE = 65521;
private int checksum; //we do all in int.
//Note that java doesn't have unsigned integers,
//so we have to be careful with what arithmetic
//we do. We return the checksum as a long to
//avoid sign confusion.
/**
* Creates a new instance of the Adler32 class.
* The checksum starts off with a value of 1.
*/
public Adler32 ()
{
reset();
}
/**
* Resets the Adler32 checksum to the initial value.
*/
public void reset ()
{
checksum = 1; //Initialize to 1
}
/**
* Updates the checksum with the byte b.
*
* @param bval the data value to add. The high byte of the int is ignored.
*/
public void update (int bval)
{
//We could make a length 1 byte array and call update again, but I
//would rather not have that overhead
int s1 = checksum & 0xffff;
int s2 = checksum >>> 16;
s1 = (s1 + (bval & 0xFF)) % BASE;
s2 = (s1 + s2) % BASE;
checksum = (s2 << 16) + s1;
}
/**
* Updates the checksum with the bytes taken from the array.
*
* @param buffer an array of bytes
*/
public void update (byte[] buffer)
{
update(buffer, 0, buffer.length);
}
/**
* Updates the checksum with the bytes taken from the array.
*
* @param buf an array of bytes
* @param off the start of the data used for this update
* @param len the number of bytes to use for this update
*/
public void update (byte[] buf, int off, int len)
{
//(By Per Bothner)
int s1 = checksum & 0xffff;
int s2 = checksum >>> 16;
while (len > 0)
{
// We can defer the modulo operation:
// s1 maximally grows from 65521 to 65521 + 255 * 3800
// s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
int n = 3800;
if (n > len)
n = len;
len -= n;
while (--n >= 0)
{
s1 = s1 + (buf[off++] & 0xFF);
s2 = s2 + s1;
}
s1 %= BASE;
s2 %= BASE;
}
/*Old implementation, borrowed from somewhere:
int n;
while (len-- > 0) {
s1 = (s1 + (bs[offset++] & 0xff)) % BASE;
s2 = (s2 + s1) % BASE;
}*/
checksum = (s2 << 16) | s1;
}
/**
* Returns the Adler32 data checksum computed so far.
*/
public long getValue()
{
return (long) checksum & 0xffffffffL;
}
}