/* A line is defined by its end points.                                 */
/* This class shows examples of calling its own and other class         */
/* predicates.                                                          */

:- object(line(_Point1, _Point2)).


	:- info([
		authors is 'Paulo Moura',
		version is 1.0,
		date is 2000/4/22,
		comment is 'Parametric object for representing geometric lines.',
		parnames is ['Point1', 'Point2'],
		source is 'Example adopted from the POEM system by Ben Staveley-Taylor.']).


	:- public(length/1).
	:- mode(length(-number), one).

	:- public(intersects/1).
	:- mode(intersects(+nonvar), one).

	:- public(signed_distance/2).
	:- mode(signed_distance(+nonvar, -number), one).

	:- public(distance/2).
	:- mode(distance(+nonvar, -number), one).


	length(Length) :-
		/* sets Len to the length of the owner line                 */
		parameter(1, P1),
		parameter(2, P2),
		P1::distance(P2, Length).


    intersects(Line2) :-
		/* succeeds if Line2 intersects the owner line.             */
		/* this isn't necessarily a good method, but shows how to   */
		/* call class procedures from within the class definition.  */
		parameter(1, P1),
		parameter(2, P2),
		Line1 = line(P1, P2),
		Line2 = line(P3, P4),
		Line2::signed_distance(P1, D1),
		Line2::signed_distance(P2, D2),
		opposite_signs(D1, D2),
		Line1::signed_distance(P3, D3),
		Line1::signed_distance(P4, D4),
		opposite_signs(D3, D4).


	signed_distance(Point, Dist) :-
		/* finds the perpendicular distance from point to line.     */
		/* the sign of the answer depends on which side of the      */
		/* line the point is on.                                    */
		parameter(1, P1),
		parameter(2, P2),
		P1 = point(X1, Y1),
		P2 = point(X2, Y2),
		Point = point(X3, Y3),
		A is X2-X1,
		B is Y1-Y2,
		C is X1*Y2-X2*Y1,
		Dist is (A*Y3+B*X3+C)/sqrt(A*A+B*B).


	distance(Point, Dist) :-
		/* as 'signed_distance', but Dist always >= 0               */
		parameter(1, P1),
		parameter(2, P2),
		line(P1, P2)::signed_distance(Point, Temp),
		Dist is abs(Temp).


	/* 'opposite_signs' succeeds if its arguments are of opposite signs.    */
	/* It has a feature in that 'opposite_signs(0,0)' succeeds: this is     */
	/* because 0 is treated as having optional sign.                        */

	opposite_signs(A, B) :-
		o_s_aux(A, B).

	opposite_signs(A, B) :-
		o_s_aux(B, A).

	o_s_aux(A, B) :-
		A >= 0,
		B =< 0.


:- end_object.