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)  2009-2020, University of Amsterdam
    7                              CWI, Amsterdam
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   35
   36:- module(rdf_compare,
   37          [ rdf_equal_graphs/3          % +Graph1, +Graph2, -Substitutions
   38          ]).   39:- use_module(library(semweb/rdf_db),[lang_equal/2,rdf_is_bnode/1]).   40:- autoload(library(apply),[partition/4,maplist/3]).   41:- autoload(library(debug),[debug/3]).   42:- 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.

   69rdf_equal_graphs(A, B, Substitutions) :-
   70    sort(A, SA),
   71    sort(B, SB),
   72    partition(contains_bnodes, SA, VA, GA),
   73    partition(contains_bnodes, SB, VB, GB),
   74    (   GA == GB
   75    ->  true
   76    ;   maplist(compare_triple, GA, GB)
   77    ),
   78    compare_list(VA, VB, [], Substitutions),
   79    !.
   80
   81contains_bnodes(rdf(S,P,O)) :-
   82    (   node_id(S)
   83    ;   node_id(P)
   84    ;   node_id(O)
   85    ),
   86    !.
   87
   88compare_list([], [], S, S).
   89compare_list([H1|T1], In2, S0, S) :-
   90    select(H2, In2, T2),
   91    compare_triple(H1, H2, S0, S1),
   92    compare_list(T1, T2, S1, S).
   93
   94compare_triple(T1, T2) :-
   95    compare_triple(T1,T2,[],[]).
   96
   97compare_triple(rdf(Subj1,P1,O1), rdf(Subj2, P2, O2), S0, S) :-
   98    compare_field(Subj1, Subj2, S0, S1),
   99    compare_field(P1, P2, S1, S2),
  100    compare_field(O1, O2, S2, S).
  101
  102compare_field(X, X, S, S) :- !.
  103compare_field(literal(X), xml(X), S, S) :- !. % TBD
  104compare_field(literal(lang(L1,X)), literal(lang(L2,X)), S, S) :-
  105    !,
  106    lang_equal(L1, L2).
  107compare_field(X, Id, S, S) :-
  108    memberchk(X=Id, S),
  109    !.
  110compare_field(X, Y, S, [X=Y|S]) :-
  111    \+ memberchk(X=_, S),
  112    node_id(X),
  113    node_id(Y),
  114    debug(rdf_compare, 'Assume ~w = ~w~n', [X, Y]).
  115
  116node_id(node(_)) :- !.
  117node_id(X) :-
  118    rdf_is_bnode(X)