:- module(topsort, [topsort/2]).

:- use_module(library(dgraphs),
	      [dgraph_new/1,
	       dgraph_add_edges/3,
	       dgraph_top_sort/2]).

/* simple implementation of a topological sorting algorithm */
/* graph is as Node-[Parents] */

topsort(Graph0, Sorted) :-
	mkedge_list(Graph0, EdgeList, []),
	dgraph_new(DGraph0),
	dgraph_add_edges(DGraph0, EdgeList,  DGraph1),
	dgraph_top_sort(DGraph1, Sorted).

mkedge_list([]) --> [].
mkedge_list([V-Parents|More]) -->
	add_edges(Parents, V),
	mkedge_list(More).

add_edges([], _V) --> [].
add_edges([P|Parents], V) --> [P-V],
	add_edges(Parents, V).