/**
 * An implementation of Euclid's GCD algorithm.
 * @author Karel P. Bergmann
 */
 
public class GCD1
{
    /**
	 * A driver for the GCD method.  Requires two integers as parameters.
	 * Output's the GCD of the two integers to the screen.
	 *
	 * @param args Integers a, b
	 * @throws ArrayIndexOutOfBoundsException If fewer than two parameters are provided.
	 * @throws NumberFormatException If either of the first two parameters is not an integer.
	 */
    public static void main (String [] args)
    {
        int a = 0;
        int b = 0;
        
        /* Try to acquire a */
        try
        {
            a = Integer.parseInt(args[0]);
        }
        catch (ArrayIndexOutOfBoundsException e)
        {
            System.out.println ("Must supply two command line arguments.");
            System.exit (1);
        }
        catch (NumberFormatException e)
        {
            System.out.println ("First argument is not an integer.");
            System.exit (1);
        }
        
        /* Try to acquire b */
        try
        {
            b = Integer.parseInt(args[1]);
        }
        catch (ArrayIndexOutOfBoundsException e)
        {
            System.out.println ("Must supply two command line arguments.");
            System.exit (1);
        }
        catch (NumberFormatException e)
        {
            System.out.println ("Second argument is not an integer.");
            System.exit (1);
        }
        
        /* Find the GCD of a and b */
        System.out.println (gcd (a, b));
    }

    /**
	 * Calculates the greatest common divisor of a and b using the Euclidean algorithm.
	 * GCD (0, a) or GCD (a, 0) = a, if either a < 0 or b < 0, then GCD (a, b) = GCD (|a|, |b|).
	 *
	 * @param a An integer.
	 * @param b Another integer.
	 */
    public static int gcd (int a, int b)
    {
        /* Test if either argument is 0 */
        if (a == 0)
            return (a);
        else if (b == 0)
            return (a);
        else
        {
            /* Take absolute values of a and b */
            int p = Math.abs (a);
            int q = Math.abs (b);
            //assert (gcd (a, b) == gcd (p, q));
            
            /* Euclidean algorithm */
            while (q != 0)
            {
                int r = p % q;
                p = q;
                q = r;
            }
            
            /* Return GCD */
            return (p);
        }
    }
}