#!/usr/bin/env swipl -s

/*
 * Rick van der Zwet
 * 0433373
 * Prolog programming assigment 1
 * Licence: BSD
 * $Id: assignment1.pro 299 2007-11-08 00:10:31Z rick $
 */

/* final call, [X,Y] member of Graph */
trans_dag(X, Y, Graph) :-
    member([X,Y], Graph).

/* Find all nodes which are findable from X */
trans_dag(X, Y, Graph) :-
    member([X,Z], Graph),
    trans_dag(Z, Y, Graph).

/* final call, [X,Y] member of Graph, do note initial path is set []
 * over here */
trans(X, Y, Graph, _, []) :-
    member([X,Y], Graph).

/* Find all nodes which are findable from X, but where
 * passed node is not member of Avoid, to avoid cycles
 * Path is constructed just the other way around
 */
trans(X, Y, Graph, Avoid, Path) :-
    not(member(X, Avoid)),
    member([X,Z], Graph),
    Path=[Z|PathTail],
    AvoidAdded=[X|Avoid],
    trans(Z, Y, Graph, AvoidAdded, PathTail).

/* init trans to trans 'system' call, which just ignored the Path and
 * makes the Avoid list starting []
 */
trans(X, Y, Graph) :-
    trans(X, Y, Graph, [], _).


/* init trans to path 'system' call
 * makes the Avoid list starting []
 */
path(X, Y, Graph, Path) :-
    trans(X, Y, Graph, [], Path).