- * 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;
- }
-}