Initial work on union operation.

src/scala/spark/RDD.scala

@@ -72,6 +72,11 @@ abstract class RDD[T: ClassManifest, Split](
   def count(): Long = 
     try { map(x => 1L).reduce(_+_) }
     catch { case e: UnsupportedOperationException => 0L }
+
+  def union[OtherSplit](other: RDD[T, OtherSplit]) =
+    new UnionRDD(sc, this, other)
+
+  def ++[OtherSplit](other: RDD[T, OtherSplit]) = this.union(other)
 }
 
 @serializable
@@ -196,3 +201,35 @@ private object CachedRDD {
   // Remembers which splits are currently being loaded (on workers)
   val loading = new HashSet[String]
 }
+
+@serializable
+abstract class UnionSplit[T: ClassManifest] {
+  def iterator(): Iterator[T]
+  def prefers(offer: SlaveOffer): Boolean
+}
+
+@serializable
+class UnionSplitImpl[T: ClassManifest, Split](
+  rdd: RDD[T, Split], split: Split)
+extends UnionSplit[T] {
+  override def iterator() = rdd.iterator(split)
+  override def prefers(offer: SlaveOffer) = rdd.prefers(split, offer)
+}
+
+@serializable
+class UnionRDD[T: ClassManifest, Split1, Split2](
+  sc: SparkContext, rdd1: RDD[T, Split1], rdd2: RDD[T, Split2])
+extends RDD[T, UnionSplit[T]](sc) {
+
+  @transient val splits_ : Array[UnionSplit[T]] = {
+    val a1 = rdd1.splits.map(s => new UnionSplitImpl(rdd1, s))
+    val a2 = rdd2.splits.map(s => new UnionSplitImpl(rdd2, s))
+    (a1 ++ a2).toArray
+  }
+
+  override def splits = splits_
+
+  override def iterator(s: UnionSplit[T]): Iterator[T] = s.iterator()
+
+  override def prefers(s: UnionSplit[T], o: SlaveOffer) = s.prefers(o)
+}