//

package edu.cmu.cs.vbc.prog.zipme;

/**
 * Computes Adler32 checksum for a stream of data. An Adler32
 * checksum is not as reliable as a CRC32 checksum, but a lot faster to
 * compute.
 * <p>
 * The specification for Adler32 may be found in RFC 1950. (ZLIB Compressed Data Format Specification version 3.3)
 * <p>
 * <p>
 * From that document:
 * <p>
 * "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.
 * <p>
 * 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."
 * <p>
 * "8.2. The Adler-32 algorithm
 * <p>
 * The Adler-32 algorithm is much faster than the CRC32 algorithm yet still provides an extremely low probability of undetected errors.
 * <p>
 * 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.)
 * <p>
 * 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
 * @see InflaterInputStream
 * @see DeflaterOutputStream
 * @since JDK 1.1
 */
public class Adler32 implements Checksum {

    /**
     * largest prime smaller than 65536
     */
    private static final int BASE = 65521;

    private int checksum;

    /**
     * Creates a new instance of the <code>Adler32</code> 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;
    }

    /**
     * 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) {
        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) {
        int s1 = checksum & 0xffff;
        int s2 = checksum >>> 16;
        while (len > 0) {
            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;
        }
        checksum = (s2 << 16) | s1;
    }

    /**
     * Returns the Adler32 data checksum computed so far.
     */
    public long getValue() {
        return (long) checksum & 0xffffffffL;
    }

}
