93 lines
2.4 KiB
Plaintext
93 lines
2.4 KiB
Plaintext
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/* A line is defined by its end points. */
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/* This class shows examples of calling its own and other class */
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/* predicates. */
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:- object(line(_Point1, _Point2)).
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:- info([
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authors is 'Paulo Moura',
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version is 1.0,
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date is 2000/4/22,
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comment is 'Parametric object for representing geometric lines.',
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parnames is ['Point1', 'Point2'],
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source is 'Example adopted from the POEM system by Ben Staveley-Taylor.']).
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:- public(length/1).
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:- mode(length(-number), one).
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:- public(intersects/1).
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:- mode(intersects(+nonvar), one).
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:- public(signed_distance/2).
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:- mode(signed_distance(+nonvar, -number), one).
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:- public(distance/2).
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:- mode(distance(+nonvar, -number), one).
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length(Length) :-
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/* sets Len to the length of the owner line */
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parameter(1, P1),
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parameter(2, P2),
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P1::distance(P2, Length).
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intersects(Line2) :-
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/* succeeds if Line2 intersects the owner line. */
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/* this isn't necessarily a good method, but shows how to */
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/* call class procedures from within the class definition. */
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parameter(1, P1),
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parameter(2, P2),
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Line1 = line(P1, P2),
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Line2 = line(P3, P4),
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Line2::signed_distance(P1, D1),
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Line2::signed_distance(P2, D2),
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opposite_signs(D1, D2),
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Line1::signed_distance(P3, D3),
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Line1::signed_distance(P4, D4),
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opposite_signs(D3, D4).
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signed_distance(Point, Dist) :-
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/* finds the perpendicular distance from point to line. */
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/* the sign of the answer depends on which side of the */
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/* line the point is on. */
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parameter(1, P1),
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parameter(2, P2),
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P1 = point(X1, Y1),
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P2 = point(X2, Y2),
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Point = point(X3, Y3),
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A is X2-X1,
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B is Y1-Y2,
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C is X1*Y2-X2*Y1,
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Dist is (A*Y3+B*X3+C)/sqrt(A*A+B*B).
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distance(Point, Dist) :-
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/* as 'signed_distance', but Dist always >= 0 */
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parameter(1, P1),
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parameter(2, P2),
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line(P1, P2)::signed_distance(Point, Temp),
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Dist is abs(Temp).
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/* 'opposite_signs' succeeds if its arguments are of opposite signs. */
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/* It has a feature in that 'opposite_signs(0,0)' succeeds: this is */
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/* because 0 is treated as having optional sign. */
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opposite_signs(A, B) :-
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o_s_aux(A, B).
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opposite_signs(A, B) :-
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o_s_aux(B, A).
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o_s_aux(A, B) :-
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A >= 0,
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B =< 0.
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:- end_object.
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