1/*  Part of ClioPatria SeRQL and SPARQL server
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@cs.vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (C): 2010, VU University Amsterdam
    7
    8    This program is free software; you can redistribute it and/or
    9    modify it under the terms of the GNU General Public License
   10    as published by the Free Software Foundation; either version 2
   11    of the License, or (at your option) any later version.
   12
   13    This program is distributed in the hope that it will be useful,
   14    but WITHOUT ANY WARRANTY; without even the implied warranty of
   15    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   16    GNU General Public License for more details.
   17
   18    You should have received a copy of the GNU General Public
   19    License along with this library; if not, write to the Free Software
   20    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   21
   22    As a special exception, if you link this library with other files,
   23    compiled with a Free Software compiler, to produce an executable, this
   24    library does not by itself cause the resulting executable to be covered
   25    by the GNU General Public License. This exception does not however
   26    invalidate any other reasons why the executable file might be covered by
   27    the GNU General Public License.
   28*/
   29
   30:- module(rdf_bnode,
   31	  [ bnode_vars/3		% +Graph0, -VarGraph, -BNodeVars
   32	  ]).   33:- use_module(library(semweb/rdf_db)).   34:- use_module(library(assoc)).   35
   36/** <module> RDF graph operations on bnodes
   37
   38This module operates on  RDF  graphs   represented  as  rdf(S,P,O)  that
   39contain blank nodes  (bnodes).  Bnodes   can  be  considered existential
   40variables in (sub-)graph,  which  motivates   replacing  them  by Prolog
   41variables.
   42*/
   43
   44%%	bnode_vars(+RDF, -RDFWithVars, -Vars) is det.
   45%
   46%	Consistently replace bnodes in  RDF   with  Prolog  variable and
   47%	unify Vars with a list of the  variables found. Note that, if we
   48%	perform matches with such graphs,   multiple variables may unify
   49%	to  the  same  concrete  resource.  One  might  consider  adding
   50%	constraints such as dif/2.
   51%
   52%	@param  RDF is a list rdf(S,P,O)
   53%	@param  Resolved is a list rdf(S,P,O), where resources may be
   54%		a variable
   55%	@param	NodeIDs is a list of variables representing the bnodes.
   56
   57bnode_vars(Graph0, Graph, NodeIDs) :-
   58	empty_assoc(Map0),		% BNodeID --> Var
   59	bnode_vars(Graph0, Graph, Map0, Map),
   60	assoc_to_values(Map, NodeIDs).
   61
   62bnode_vars([], [], Map, Map).
   63bnode_vars([rdf(S0,P0,O0)|T0], Graph, Map0, Map) :-
   64	(   rdf_is_bnode(S0)
   65	;   rdf_is_bnode(P0)
   66	;   rdf_is_bnode(O0)
   67	), !,
   68	Graph = [rdf(S,P,O)|T],
   69	bnode_var(S0, S, Map0, Map1),
   70	bnode_var(P0, P, Map1, Map2),
   71	bnode_var(O0, O, Map2, Map3),
   72	bnode_vars(T0, T, Map3, Map).
   73bnode_vars([Triple|T0], [Triple|T], Map0, Map) :-
   74	bnode_vars(T0, T, Map0, Map).
   75
   76
   77bnode_var(R0, BNodeID, Map0, Map) :-
   78	rdf_is_bnode(R0), !,
   79	(   get_assoc(R0, Map0, BNodeID)
   80	->  Map = Map0
   81	;   put_assoc(R0, Map0, BNodeID, Map)
   82	).
   83bnode_var(R, R, Map, Map)