Return count metadata.

Author: RubenVerborgh

src/org/linkeddatafragments/datasource/BasicLinkedDataFragment.java

@@ -14,8 +14,8 @@ public interface BasicLinkedDataFragment {
 	public Model getTriples();
 
 	/**
-	 * Gets the total number of triples in the fragment.
+	 * Gets the total number of triples in the fragment (can be an estimate).
 	 * @return the total number of triples
 	 */
-	public int getTotalSize();
+	public long getTotalSize();
 }

src/org/linkeddatafragments/datasource/HdtDataSource.java

@@ -2,51 +2,75 @@
 
 import java.io.IOException;
 
+import org.rdfhdt.hdt.enums.TripleComponentRole;
 import org.rdfhdt.hdt.hdt.HDT;
 import org.rdfhdt.hdt.hdt.HDTManager;
-import org.rdfhdt.hdtjena.HDTGraph;
+import org.rdfhdt.hdt.triples.IteratorTripleID;
+import org.rdfhdt.hdt.triples.TripleID;
+import org.rdfhdt.hdtjena.NodeDictionary;
 
+import com.hp.hpl.jena.graph.Triple;
 import com.hp.hpl.jena.rdf.model.Model;
 import com.hp.hpl.jena.rdf.model.ModelFactory;
 import com.hp.hpl.jena.rdf.model.Property;
 import com.hp.hpl.jena.rdf.model.RDFNode;
 import com.hp.hpl.jena.rdf.model.Resource;
-import com.hp.hpl.jena.rdf.model.StmtIterator;
 
 /**
  * An HDT data source of Basic Linked Data Fragments.
  * @author Ruben Verborgh
  */
 public class HdtDataSource implements DataSource {
 	private final static int TRIPLES_LIMIT = 100;
-	private Model data;
+	private final HDT datasource;
+	private final NodeDictionary dictionary;
 
 	/**
 	 * Creates a new HdtDataSource.
 	 * @param hdtFile the HDT datafile
 	 * @throws IOException if the file cannot be loaded
 	 */
 	public HdtDataSource(String hdtFile) throws IOException {
-		final HDT hdt = HDTManager.mapIndexedHDT(hdtFile, null);
-		data = ModelFactory.createModelForGraph(new HDTGraph(hdt));
+		datasource = HDTManager.mapIndexedHDT(hdtFile, null);
+		dictionary = new NodeDictionary(datasource.getDictionary());
 	}
 
 	@Override
-	public BasicLinkedDataFragment getFragment(final Resource subject, final Property predicate, final RDFNode object) {
+	public BasicLinkedDataFragment getFragment(Resource subject, Property predicate, RDFNode object) {
+		// look up the result from the HDT datasource
+        final int subjectId = subject == null ? 0 : dictionary.getIntID(subject.asNode(), TripleComponentRole.SUBJECT);
+        final int predicateId = predicate == null ? 0 : dictionary.getIntID(predicate.asNode(), TripleComponentRole.PREDICATE);
+        final int objectId = object == null ? 0 : dictionary.getIntID(object.asNode(), TripleComponentRole.OBJECT);
+		final IteratorTripleID result = datasource.getTriples().search(new TripleID(subjectId, predicateId, objectId));
+		
+		// create the fragment
 		return new BasicLinkedDataFragment() {
 			@Override
 			public Model getTriples() {
 				final Model triples = ModelFactory.createDefaultModel();
-				final StmtIterator statements = data.listStatements(subject, predicate, object);
-				for (int i = 0; i < TRIPLES_LIMIT && statements.hasNext(); i++)
-					triples.add(statements.next());
+				result.goToStart();
+				for (int i = 0; i < TRIPLES_LIMIT && result.hasNext(); i++)
+					triples.add(triples.asStatement(toTriple(result.next())));
 				return triples;
 			}
 
 			@Override
-			public int getTotalSize() {
-				return 0;
+			public long getTotalSize() {
+				return result.estimatedNumResults();
 			}
 		};
 	}
+
+	/**
+	 * Converts the HDT triple to a Jena Triple.
+	 * @param tripleId the HDT triple
+	 * @return the Jena triple
+	 */
+	private Triple toTriple(TripleID tripleId) {
+		return new Triple(
+			dictionary.getNode(tripleId.getSubject(), TripleComponentRole.SUBJECT),
+			dictionary.getNode(tripleId.getPredicate(), TripleComponentRole.PREDICATE),
+			dictionary.getNode(tripleId.getObject(), TripleComponentRole.OBJECT)
+		);
+	}
 }

src/org/linkeddatafragments/servlet/BasicLdfServlet.java

@@ -16,7 +16,10 @@
 import org.linkeddatafragments.datasource.DataSource;
 import org.linkeddatafragments.datasource.HdtDataSource;
 
+import static org.linkeddatafragments.util.CommonResources.*;
+
 import com.hp.hpl.jena.datatypes.TypeMapper;
+import com.hp.hpl.jena.datatypes.xsd.XSDDatatype;
 import com.hp.hpl.jena.rdf.model.Model;
 import com.hp.hpl.jena.rdf.model.Property;
 import com.hp.hpl.jena.rdf.model.RDFNode;
@@ -52,6 +55,7 @@ public void doGet(HttpServletRequest request, HttpServletResponse response) thro
 		try {
 			// find the data source
 			final String path = request.getRequestURI().substring(request.getContextPath().length());
+			final String query = request.getQueryString();
 			final String dataSourceName = path.substring(1);
 			final DataSource dataSource = dataSources.get(dataSourceName);
 			if (dataSource == null)
@@ -67,6 +71,13 @@ public void doGet(HttpServletRequest request, HttpServletResponse response) thro
 			final Model output = fragment.getTriples();
 			output.setNsPrefixes(config.getPrefixes());
 
+			// add metadata
+			final String datasetUrl = request.getScheme() + "://" + request.getServerName() + path;
+			final String fragmentUrl = query == null ? datasetUrl : (datasetUrl + "?" + query);
+			final Resource fragmentId = output.createResource(fragmentUrl);
+			output.add(fragmentId, VOID_TRIPLES,
+						output.createTypedLiteral(fragment.getTotalSize(), XSDDatatype.XSDinteger));
+			
 			// serialize the output as Turtle
 			response.setHeader("Server", "Linked Data Fragments Server");
 			response.setHeader("Content-Type", "text/turtle");

src/org/linkeddatafragments/util/CommonResources.java

@@ -0,0 +1,17 @@
+package org.linkeddatafragments.util;
+
+import com.hp.hpl.jena.rdf.model.Property;
+import com.hp.hpl.jena.rdf.model.ResourceFactory;
+
+@SuppressWarnings("javadoc")
+public class CommonResources {
+	public final static String RDF = "http://www.w3.org/1999/02/22-rdf-syntax-ns#";
+	public final static Property RDF_TYPE = createProperty(RDF + "type");
+	
+	public final static String VOID = "http://rdfs.org/ns/void#";
+	public final static Property VOID_TRIPLES = createProperty(VOID + "triples");
+	
+	private final static Property createProperty(String uri) {
+		return ResourceFactory.createProperty(uri);
+	}
+}