module Krikri
  ##
  # Implements SPARQL queries for finding RDF Resources by their PROV-O history.
  module ProvenanceQueryClient
    SPARQL_CLIENT = Repository.query_client

    module_function

    ##
    # Finds all entities generated or revised by the activity whose URI is
    # given.
    #
    # @param activity_uri [#to_uri] the URI of the activity to search
    #
    # @param include_invalidated [Boolean]  Whether to include entities that
    #   have been invalidated with <http://www.w3.org/ns/prov#invalidatedAtTime>
    #
    # @see https://www.w3.org/TR/prov-o/#invalidatedAtTime
    # @see Krikri::LDP::Invalidatable
    #
    # @return [RDF::SPARQL::Query] a query object that, when executed, will
    #   give solutions containing the URIs for the resources in `#record`.
    def find_by_activity(activity_uri, include_invalidated = false)
      raise ArgumentError, 'activity_uri must be an RDF::URI' unless
        activity_uri.respond_to? :to_term
      query = SPARQL_CLIENT.select(:record)
        .where([:record,
                [RDF::PROV.wasGeneratedBy, '|', RDF::DPLA.wasRevisedBy],
                activity_uri.to_term])

      if include_invalidated
        query
      else
        # We need to say "and if RDF::PROV.invalidatedAtTime is not set."
        #
        # The SPARQL query should be:
        #
        # PREFIX prov: <http://www.w3.org/ns/prov#>
        # SELECT * WHERE {
        #   ?subject prov:wasGeneratedBy  <http://xampl.org/ldp/activity/n> .
        #   FILTER NOT EXISTS { ?subject prov:invalidatedAtTime ?x }
        # }
        #
        # ... However there doesn't appear to be a way of constructing
        # 'FILTER NOT EXISTS' with SPARQL::Client.  Instead, we've managed to
        # hack the following solution together.
        #
        # SPARQL::Client#filter is labeled @private in its YARD comment (and
        # has no other documentation) but it's not private, at least for
        # now.
        query.filter \
          'NOT EXISTS ' \
          '{ ?record <http://www.w3.org/ns/prov#invalidatedAtTime> ?x }'
      end
    end
  end
end