/*  $Id: chrdif.chr,v 1.2 2004/09/21 14:30:24 jan Exp $

    Part of CHR (Constraint Handling Rules)

    Author:        Tom Schrijvers
    E-mail:        Tom.Schrijvers@cs.kuleuven.ac.be
    WWW:           http://www.swi-prolog.org
    Copyright (C): 2003-2004, K.U. Leuven

*/

:- module(chrdif,[chrdif/2]).
:- use_module(library(chr)).

:- constraints dif/2, dif2/3, or/2, or_seq/2, del_or/1.

chrdif(X,Y) :- dif(X,Y).

dif(X,Y) <=> compound(X), compound(Y) | dif1(X,Y).
dif(X,X) <=> fail.
dif(X,Y) <=> nonvar(X), nonvar(Y) /* X \== Y holds */ | true.

dif1(X,Y) :-
	( functor(X,F,A),
	  functor(Y,F,A) ->
		X =.. [_|XL],
		Y =.. [_|YL],
		dif1l(XL,YL,A)
	;
		true
	).

dif1l(Xs,Ys,N) :-
	or(Or,N),
	dif1l_2(Xs,Ys,Or).

dif1l_2([],[],_).
dif1l_2([X|Xs],[Y|Ys],Or) :-
	dif2(X,Y,Or),
	dif1l_2(Xs,Ys,Or).

or_seq(OrP,Or) \ or(Or,0), or(OrP,N) <=> M is N - 1, or_seq(OrP,M).
or(_,0) <=> fail.

dif2(X,Y,Or) <=> compound(X), compound(Y) | dif3(X,Y,Or).
dif2(X,X,Or), or(Or,N) <=> M is N - 1, or(Or,M).
dif2(X,Y,Or) <=> nonvar(X), nonvar(Y) /* X \== Y holds */ | del_or(Or).

del_or(Or) \ or_seq(OrP,Or) <=> del_or(OrP).
del_or(Or) \ or_seq(Or,OrC) <=> del_or(OrC).
del_or(Or) \ or(Or,_) <=> true.
del_or(Or) \ dif2(_,_,Or) <=> true.
del_or(Or) <=> true.

dif3(X,Y,Or) :-
	( functor(X,F,A),
	  functor(Y,F,A) ->
		X =.. [_|XL],
		Y =.. [_|YL],
		or_seq(Or,Or2),
		dif1l(XL,YL,A)
	;
		del_or(Or)
	).
