/* AOP solution to invariance aspect */

aspect invariance
{

    // advice code to throw an error upon bad input

    before(int n) : (call(int base.fib(..)) || call(int base.fact(..)))
	&& args(n,..)
    {
	if (n<0) { System.out.println("bad input "+n); System.exit(1); }
    }
    
    boolean around(int b) : call(boolean base.divisible(..)) && args(int,b)
	{
	    return (b!=0) && proceed(b);
	}

    int around(int ax) : call(int base.fact(int,int)) && args(int,ax)
	                        && !withincode(int base.fact(int,int))
	{
	    return proceed(1); //  note 1 arg to proceed, not 2.
	}

} //invariance

/*
    A "join point" is everywhere in the program where something "interesting"
    with respect to a certain aspect is happening.  In other words, they
    represent points in the program where different concerns "crosscut".
    Here, the join points
    are right before either fib or fact is called, and when divisible
    is called with a boolean value expected.  These joinpoints are identified
    using AspectJ's language of "pointcuts".

    The first before advice can be said to "crosscut" multiple sections
    of the base program, as it applies both to the calling of fib and fact

    The advice code is "weaved" into the execution of the program at the
    join points identified by the pointcuts predicating each advice

    Finally, the "concern" of invariance has been modularly seperated from
    the rest of the program (though the oop code may also be able to 
    claim this for this small example).  However, this invariance aspect
    is more loosely coupled from the rest of the program.  Its use is 
    entirely optional. Use it by compile it together with any program that
    uses the base class:

    ajc mathfuns.java invariance.java
*/