Added initial groupEdges code. Still a prototype, I haven't figured out quite how it should all work yet.

graph/src/main/scala/org/apache/spark/graph/Graph.scala

@@ -188,6 +188,8 @@ abstract class Graph[VD: ClassManifest, ED: ClassManifest] {
   //  */
   // def combineEdges(reduce: (ED, ED) => ED): Graph[VD, ED]
 
+  def groupEdges[ED2: ClassManifest](f: Iter[EdgeTriplet[ED,VD]] => ED2 ): Graph[VD,ED2]
+
 
 
 

graph/src/main/scala/org/apache/spark/graph/impl/GraphImpl.scala

@@ -118,6 +118,59 @@ class GraphImpl[VD: ClassManifest, ED: ClassManifest] protected (
     new GraphImpl(newVertices, newEdges)
   }
 
+
+  // Because of the edgepartitioner, we know that all edges with the same src and dst
+  // will be in the same partition
+
+  // We will want to keep the same partitioning scheme. Use newGraph() rather than
+  // new GraphImpl()
+  override def groupEdges[ED2: ClassManifest](f: Iterator[EdgeTriplet[ED,VD]] => ED2 ):
+    Graph[VD,ED2] = {
+
+      // I think that
+      // myRDD.mapPartitions { part => 
+      //  val (vmap, edges) = part.next()
+      // gives me access to the vertex map and the set of
+      // edges within that partition
+
+      // This is what happens during mapPartitions
+      // The iterator iterates over all partitions
+      // val result: RDD[U] = new RDD[T]().mapPartitions(f: Iterator[T] => Iterator[U])
+
+      // TODO(crankshaw) figure out how to actually get the new Edge RDD and what
+      // type that should have
+      val newEdges: RDD[Edge[ED2]] = triplets.mapPartitions { partIterator =>
+        // toList lets us operate on all EdgeTriplets in a single partition at once
+        partIterator.toList
+        // groups all ETs in this partition that have the same src and dst
+        // Because all ETs with the same src and dst will live on the same
+        // partition due to the EdgePartitioner, this guarantees that these
+        // ET groups will be complete.
+        .groupBy { t => (t.src.id, t.dst.id) }
+        // Apply the user supplied supplied edge group function to
+        // each group of edges
+        // The result of this line is Map[(Long, Long, ED2]
+        .mapValues { ts => f(ts.toIterator) }
+        // convert the resulting map back to a list of tuples
+        .toList
+        // map over those tuples that contain src and dst info plus the
+        // new edge data to make my new edges
+        .map { case ((src, dst), data) => Edge(src, dst, data) }
+
+        // How do I convert from a scala map to a list?
+        // I want to be able to apply a function like:
+        // f: (key, value): (K, V) => result: [R]
+        // so that I can transfrom a Map[K, V] to List[R]
+
+        // Maybe look at collections.breakOut
+        // see http://stackoverflow.com/questions/1715681/scala-2-8-breakout
+        // and http://stackoverflow.com/questions/6998676/converting-a-scala-map-to-a-list
+
+      }
+      newGraph(vertices, newEdges)
+
+  }
+
   //////////////////////////////////////////////////////////////////////////////////////////////////
   // Lower level transformation methods
   //////////////////////////////////////////////////////////////////////////////////////////////////