Merge pull request #490 from juharris/GraphSailConnection-track_prev_…

…subject

GraphSailConnection: Tracking the previous subject -- yes, this is smart. Nice.

blueprints-graph-sail/src/main/java/com/tinkerpop/blueprints/oupls/sail/GraphSailConnection.java

@@ -59,6 +59,16 @@ public class GraphSailConnection extends NotifyingSailConnectionBase implements
     private boolean statementsAdded;
     private boolean statementsRemoved;
 
+    /**
+     * The subject that was just seen when adding a statement.
+     */
+    private Resource prevSubject = null;
+
+    /**
+     * The vertex corresponding to {@link #prevSubject}.
+     */
+    private Vertex prevOutVertex = null;
+
     public GraphSailConnection(final GraphSail.DataStore store) {
         super(store.sail);
         this.store = store;
@@ -67,6 +77,8 @@ public GraphSailConnection(final GraphSail.DataStore store) {
     protected void startTransactionInternal() throws SailException {
         statementsAdded = false;
         statementsRemoved = false;
+        prevSubject = null;
+        prevOutVertex = null;
     }
 
     public void commitInternal() throws SailException {
@@ -259,7 +271,10 @@ private void addStatementInternal(final boolean inferred,
         for (Resource context : ((0 == contexts.length) ? NULL_CONTEXT_ARRAY : contexts)) {
             String c = null == context ? GraphSail.NULL_CONTEXT_NATIVE : store.resourceToNative(context);
 
-            Vertex out = getOrCreateVertex(subject);
+            // Track the subject since data will often list relations for the same subject consecutively.
+            Vertex out = subject.equals(prevSubject) ? prevOutVertex
+                    : (prevOutVertex = getOrCreateVertex(prevSubject = subject));
+
             // object-level identity of subject and object facilitates creation of self-loop edges in some Graph implementations
             Vertex in = subject.equals(object) ? out : getOrCreateVertex(object);
 
@@ -381,6 +396,8 @@ private void removeStatementsInternal(final boolean inferred,
 
         if (0 < edgesToRemove.size()) {
             statementsRemoved = true;
+            prevSubject = null;
+            prevOutVertex = null;
         }
 
         //System.out.println("\tdone removing");
@@ -446,6 +463,8 @@ private void deleteEdgesInIterator(final boolean inferred,
                 }
 
                 statementsRemoved = true;
+                prevSubject = null;
+                prevOutVertex = null;
             }
         }
     }