1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2009-2022, University of Amsterdam
    7                              CWI, Amsterdam
    8                              SWI-Prolog Solutions b.v.
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   36
   37:- module(rdf_compare,
   38          [ rdf_equal_graphs/3          % +Graph1, +Graph2, -Substitutions
   39          ]).   40:- if(exists_source(library(semweb/rdf_db))).   41:- use_module(library(semweb/rdf_db),[lang_equal/2,rdf_is_bnode/1]).   42:- endif.   43:- autoload(library(apply),[partition/4,maplist/3]).   44:- use_module(library(debug),[debug/3]).   45:- autoload(library(lists),[select/3]).

 rdf_equal_graphs(+GraphA, +GraphB, -Substition) is semidet

True if GraphA and GraphB are the same under Substition. Substition is a list of BNodeA = BNodeB, where BNodeA is a blank node that appears in GraphA and BNodeB is a blank node that appears in GraphB.

Arguments:

GraphA	- is a list of rdf(S,P,O) terms
GraphB	- is a list of rdf(S,P,O) terms
Substition	- is a list if NodeA = NodeB terms.

To be done

- The current implementation is rather naive. After dealing with the subgraphs that contain no bnodes, it performs a fully non-deterministic substitution.

   72rdf_equal_graphs(A, B, Substitutions) :-
   73    sort(A, SA),
   74    sort(B, SB),
   75    partition(contains_bnodes, SA, VA, GA),
   76    partition(contains_bnodes, SB, VB, GB),
   77    (   GA == GB
   78    ->  true
   79    ;   maplist(compare_triple, GA, GB)
   80    ),
   81    compare_list(VA, VB, [], Substitutions),
   82    !.
   83
   84contains_bnodes(rdf(S,P,O)) :-
   85    (   node_id(S)
   86    ;   node_id(P)
   87    ;   node_id(O)
   88    ),
   89    !.
   90
   91compare_list([], [], S, S).
   92compare_list([H1|T1], In2, S0, S) :-
   93    select(H2, In2, T2),
   94    compare_triple(H1, H2, S0, S1),
   95    compare_list(T1, T2, S1, S).
   96
   97compare_triple(T1, T2) :-
   98    compare_triple(T1,T2,[],[]).
   99
  100compare_triple(rdf(Subj1,P1,O1), rdf(Subj2, P2, O2), S0, S) :-
  101    compare_field(Subj1, Subj2, S0, S1),
  102    compare_field(P1, P2, S1, S2),
  103    compare_field(O1, O2, S2, S).
  104
  105compare_field(X, X, S, S) :- !.
  106compare_field(literal(X), xml(X), S, S) :- !. % TBD
  107compare_field(literal(lang(L1,X)), literal(lang(L2,X)), S, S) :-
  108    !,
  109    lang_equal(L1, L2).
  110compare_field(X, Id, S, S) :-
  111    memberchk(X=Id, S),
  112    !.
  113compare_field(X, Y, S, [X=Y|S]) :-
  114    \+ memberchk(X=_, S),
  115    node_id(X),
  116    node_id(Y),
  117    debug(rdf_compare, 'Assume ~w = ~w~n', [X, Y]).
  118
  119node_id(node(_)) :- !.
  120node_id(X) :-
  121    rdf_is_bnode(X).
  122
  123:- if(\+current_predicate(rdf_is_bnode/1)).  124rdf_is_bnode(Node) :-
  125    atom(Node),
  126    sub_atom(Node, 0, _, _, '_:').
  127:- endif.  128:- if(\+current_predicate(lang_equal/2)).  129lang_equal(X, Y) :-
  130    downcase_atom(X, L),
  131    downcase_atom(Y, L).
  132:- endif.