// Copyright (c) 2003-2004 Brian Wellington (bwelling@xbill.org)

package org.xbill.DNS;

/**
 * Helper functions for doing serial arithmetic.  These should be used when
 * setting/checking SOA serial numbers.  SOA serial number arithmetic is
 * defined in RFC 1982.
 *
 * @author Brian Wellington
 */

public final class Serial {

    private static final long MAX32 = 0xFFFFFFFFL;

    private Serial() {
    }

    /**
     * Compares two numbers using serial arithmetic.  The numbers are assumed
     * to be 32 bit unsigned integers stored in longs.
     *
     * @param serial1 The first integer
     * @param serial2 The second integer
     * @return 0 if the 2 numbers are equal, a positive number if serial1 is greater
     * than serial2, and a negative number if serial2 is greater than serial1.
     * @throws IllegalArgumentException serial1 or serial2 is out of range
     */
    public static int
    compare(long serial1, long serial2) {
        if (serial1 < 0 || serial1 > MAX32)
            throw new IllegalArgumentException(serial1 + " out of range");
        if (serial2 < 0 || serial2 > MAX32)
            throw new IllegalArgumentException(serial2 + " out of range");
        long diff = serial1 - serial2;
        if (diff >= MAX32)
            diff -= (MAX32 + 1);
        else if (diff < -MAX32)
            diff += (MAX32 + 1);
        return (int) diff;
    }

    /**
     * Increments a serial number.  The number is assumed to be a 32 bit unsigned
     * integer stored in a long.  This basically adds 1 and resets the value to
     * 0 if it is 2^32.
     *
     * @param serial The serial number
     * @return The incremented serial number
     * @throws IllegalArgumentException serial is out of range
     */
    public static long
    increment(long serial) {
        if (serial < 0 || serial > MAX32)
            throw new IllegalArgumentException(serial + " out of range");
        if (serial == MAX32)
            return 0;
        return serial + 1;
    }

}
