/* $Id: graphpaths.pl,v 321.2 2002-03-06 17:51:47-08 - - $ */

/*
* Define the links in the graph.
*/

link( a, b ).
link( a, d ).
link( b, c ).
link( d, e ).
link( e, c ).
link( e, f ).
link( f, a ).
link( f, g ).
link( f, j ).
link( g, h ).
link( h, i ).
link( i, j ).

/*
* Is there a path from one node to another?
*/

/*
* This is the old version, which does not work on the new set
* of facts.  It causes the message [WARNING: Out of local stack],
* presumably due to the loop in the graph.
*
ispath( L, L ).
ispath( L, M ) :- link( L,X ),ispath( X,M ).
*/

ispath( L, M ) :- ispath2( L, M, [] ).

ispath2( L, L, _ ).
ispath2( L, M, Path ) :-
	link( L, X ),
	not( member( X, Path )),
	ispath2( X, M, [L|Path] ).

/*
* Find a path from one node to another.
*/

writeallpaths( Node, Node ) :-
	write( Node ), write( ' is ' ), write( Node ), nl.
writeallpaths( Node, Next ) :-
	listpath( Node, Next, [Node], List ),
	write( Node ), write( ' to ' ), write( Next ), write( ' is ' ),
	writepath( List ),
	fail.

writepath( [] ) :-
	nl.
writepath( [Head|Tail] ) :-
	write( ' ' ), write( Head ), writepath( Tail ).

listpath( Node, End, Outlist ) :-
	listpath( Node, End, [Node], Outlist ).

listpath( Node, Node, _, [Node] ).
listpath( Node, End, Tried, [Node|List] ) :-
	link( Node, Next ),
	not( member( Next, Tried )),
	listpath( Next, End, [Next|Tried], List ).

/*
* member( Head, [Head|_] ).
* member( Head, [_|Tail] ) :- member( Head, Tail ).
*/

/*
* not( X ) :- X, !, fail.
* not( _ ).
*/