Add paging support.

pom.xml

@@ -21,6 +21,11 @@
 			<artifactId>jena-arq</artifactId>
 			<version>2.11.1</version>
 		</dependency>
+		<dependency>
+			<groupId>org.apache.httpcomponents</groupId>
+			<artifactId>httpclient</artifactId>
+			<version>4.3.3</version>
+		</dependency>
 		<dependency>
 			<groupId>com.google.code.gson</groupId>
 			<artifactId>gson</artifactId>
@@ -30,6 +35,7 @@
 			<groupId>javax.servlet</groupId>
 			<artifactId>javax.servlet-api</artifactId>
 			<version>3.0.1</version>
+			<scope>provided</scope>
 		</dependency>
 	</dependencies>
 	<build>

src/org/linkeddatafragments/datasource/DataSource.java

@@ -10,11 +10,14 @@
  */
 public interface DataSource {
 	/**
-	 * Gets the Basic Linked Data Fragment matching the specified triple pattern.
+	 * Gets a page of the Basic Linked Data Fragment matching the specified triple pattern.
 	 * @param subject the subject (null to match any subject)
 	 * @param predicate the predicate (null to match any predicate)
 	 * @param object the object (null to match any object)
+	 * @param offset the triple index at which to start the page
+	 * @param limit the number of triples on the page
 	 * @return the first page of the fragment
 	 */
-	public BasicLinkedDataFragment getFragment(Resource subject, Property predicate, RDFNode object);
+	public BasicLinkedDataFragment getFragment(Resource subject, Property predicate, RDFNode object,
+											   long offset, long limit);
 }

src/org/linkeddatafragments/datasource/HdtDataSource.java

@@ -21,7 +21,6 @@
  * @author Ruben Verborgh
  */
 public class HdtDataSource implements DataSource {
-	private final static int TRIPLES_LIMIT = 100;
 	private final HDT datasource;
 	private final NodeDictionary dictionary;
 
@@ -36,7 +35,10 @@ public HdtDataSource(String hdtFile) throws IOException {
 	}
 
 	@Override
-	public BasicLinkedDataFragment getFragment(Resource subject, Property predicate, RDFNode object) {
+	public BasicLinkedDataFragment getFragment(Resource subject, Property predicate, RDFNode object, final long offset, final long limit) {
+		if (offset < 0) throw new IndexOutOfBoundsException("offset");
+		if (limit  < 1) throw new IllegalArgumentException("limit");
+
 		// 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);
@@ -50,9 +52,29 @@ public BasicLinkedDataFragment getFragment(Resource subject, Property predicate,
 			@Override
 			public Model getTriples() {
 				final Model triples = ModelFactory.createDefaultModel();
-				result.goToStart();
-				for (int i = 0; i < TRIPLES_LIMIT && result.hasNext(); i++)
-					triples.add(triples.asStatement(toTriple(result.next())));
+
+				// try to jump directly to the offset
+				boolean atOffset;
+				if (result.canGoTo()) {
+					try {
+						result.goTo(offset);
+						atOffset = true;
+					}
+					// if the offset is outside the bounds, this page has no matches
+					catch (IndexOutOfBoundsException exception) { atOffset = false; }
+				}
+				// if not possible, advance to the offset iteratively
+				else {
+					result.goToStart();
+					for (int i = 0; !(atOffset = i == offset) && result.hasNext(); i++)
+						result.next();
+				}
+
+				// add `limit` triples to the result model
+				if (atOffset) {
+					for (int i = 0; i < limit && result.hasNext(); i++)
+						triples.add(triples.asStatement(toTriple(result.next())));
+				}
 				return triples;
 			}
 

src/org/linkeddatafragments/servlet/BasicLdfServlet.java

@@ -13,6 +13,7 @@
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
+import org.apache.http.client.utils.URIBuilder;
 import org.linkeddatafragments.config.ConfigReader;
 import org.linkeddatafragments.datasource.BasicLinkedDataFragment;
 import org.linkeddatafragments.datasource.DataSource;
@@ -37,6 +38,7 @@ public class BasicLdfServlet extends HttpServlet {
 	private final static long serialVersionUID = 1L;
 	private final static Pattern STRINGPATTERN = Pattern.compile("^\"(.*)\"(?:@(.*)|\\^\\^<(.*)>)?$");
 	private final static TypeMapper types = TypeMapper.getInstance();
+	private final static long TRIPLESPERPAGE = 100;
 
 	private ConfigReader config;
 	private HashMap<String, DataSource> dataSources = new HashMap<String, DataSource>();
@@ -76,18 +78,21 @@ public void doGet(HttpServletRequest request, HttpServletResponse response) thro
 			final Resource subject = parseAsResource(request.getParameter("subject"));
 			final Property predicate = parseAsProperty(request.getParameter("predicate"));
 			final RDFNode object = parseAsNode(request.getParameter("object"));
-			final BasicLinkedDataFragment fragment = dataSource.getFragment(subject, predicate, object);
+			final long page = Math.max(1, parseAsInteger(request.getParameter("page")));
+			final long limit = TRIPLESPERPAGE, offset = limit * (page - 1);
+			final BasicLinkedDataFragment fragment = dataSource.getFragment(subject, predicate, object, offset, limit);
 
 			// fill the output model
 			final Model output = fragment.getTriples();
+			final boolean isEmpty = output.size() == 0;
 			output.setNsPrefixes(config.getPrefixes());
 
 			// add dataset metadata
 			final String hostName = request.getHeader("Host");
 			final String datasetUrl = request.getScheme() + "://" +
 								      (hostName == null ? request.getServerName() : hostName) + request.getRequestURI();
 			final String fragmentUrl = query == null ? datasetUrl : (datasetUrl + "?" + query);
-			final Resource datasetId = output.createResource(datasetUrl);
+			final Resource datasetId = output.createResource(datasetUrl + "#dataset");
 			final Resource fragmentId = output.createResource(fragmentUrl);
 			output.add(datasetId, RDF_TYPE, VOID_DATASET);
 			output.add(datasetId, RDF_TYPE, HYDRA_COLLECTION);
@@ -99,6 +104,20 @@ public void doGet(HttpServletRequest request, HttpServletResponse response) thro
 			final Literal total = output.createTypedLiteral(fragment.getTotalSize(), XSDDatatype.XSDinteger);
 			output.add(fragmentId, VOID_TRIPLES, total);
 			output.add(fragmentId, HYDRA_TOTALITEMS, total);
+			output.add(fragmentId, HYDRA_ITEMSPERPAGE, output.createTypedLiteral(limit, XSDDatatype.XSDinteger));
+
+			// add pages
+			final URIBuilder pagedUrl = new URIBuilder(fragmentUrl);
+			pagedUrl.setParameter("page", "1");
+			output.add(fragmentId, HYDRA_FIRSTPAGE, output.createResource(pagedUrl.toString()));
+			if (offset > 0) {
+				pagedUrl.setParameter("page", Long.toString(page - 1));
+				output.add(fragmentId, HYDRA_PREVIOUSPAGE, output.createResource(pagedUrl.toString()));
+			}
+			if (offset + limit < fragment.getTotalSize()) {
+				pagedUrl.setParameter("page", Long.toString(page + 1));
+				output.add(fragmentId, HYDRA_NEXTPAGE, output.createResource(pagedUrl.toString()));
+			}
 
 			// add controls
 			final Resource triplePattern    = output.createResource();
@@ -118,17 +137,27 @@ public void doGet(HttpServletRequest request, HttpServletResponse response) thro
 			output.add(objectMapping,    HYDRA_PROPERTY, RDF_OBJECT);
 
 			// serialize the output as Turtle
-			response.setStatus(fragment.getTotalSize() == 0 ? 404 : 200);
+			response.setStatus(isEmpty ? 404 : 200);
 			response.setHeader("Server", "Linked Data Fragments Server");
 			response.setContentType("text/turtle");
 			response.setCharacterEncoding("utf-8");
-			output.write(response.getWriter(), "Turtle");
+			output.write(response.getWriter(), "Turtle", fragmentUrl);
 		}
 		catch (Exception e) {
 			throw new ServletException(e);
 		}
 	}
 
+	/**
+	 * Parses the given value as an integer.
+	 * @param value the value
+	 * @return the parsed value
+	 */
+	private int parseAsInteger(String value) {
+		try { return Integer.parseInt(value); }
+		catch (NumberFormatException ex) { return 0; }
+	}
+
 	/**
 	 * Parses the given value as an RDF resource.
 	 * @param value the value

src/org/linkeddatafragments/util/CommonResources.java

@@ -26,6 +26,10 @@ public class CommonResources {
 	public final static Property HYDRA_PROPERTY = createProperty(HYDRA + "property");
 	public final static Property HYDRA_COLLECTION = createProperty(HYDRA + "Collection");
 	public final static Property HYDRA_PAGEDCOLLECTION = createProperty(HYDRA + "PagedCollection");
+	public final static Property HYDRA_FIRSTPAGE = createProperty(HYDRA + "firstPage");
+	public final static Property HYDRA_LASTPAGE = createProperty(HYDRA + "lastPage");
+	public final static Property HYDRA_NEXTPAGE = createProperty(HYDRA + "nextPage");
+	public final static Property HYDRA_PREVIOUSPAGE = createProperty(HYDRA + "previousPage");
 
 	private final static Property createProperty(String uri) {
 		return ResourceFactory.createProperty(uri);