Added fold() and aggregate() operations that reuse an object to

merge results into rather than requiring a new object allocation
for each element merged. Fixes #95.

core/src/main/scala/spark/RDD.scala

@@ -133,7 +133,7 @@ abstract class RDD[T: ClassManifest](@transient sc: SparkContext) extends Serial
     val cleanF = sc.clean(f)
     val reducePartition: Iterator[T] => Option[T] = iter => {
       if (iter.hasNext)
-        Some(iter.reduceLeft(f))
+        Some(iter.reduceLeft(cleanF))
       else
         None
     }
@@ -144,7 +144,36 @@ abstract class RDD[T: ClassManifest](@transient sc: SparkContext) extends Serial
     if (results.size == 0)
       throw new UnsupportedOperationException("empty collection")
     else
-      return results.reduceLeft(f)
+      return results.reduceLeft(cleanF)
+  }
+
+  /**
+   * Aggregate the elements of each partition, and then the results for all the
+   * partitions, using a given associative function and a neutral "zero value".
+   * The function op(t1, t2) is allowed to modify t1 and return it as its result
+   * value to avoid object allocation; however, it should not modify t2.
+   */
+  def fold(zeroValue: T)(op: (T, T) => T): T = {
+    val cleanOp = sc.clean(op)
+    val results = sc.runJob(this, (iter: Iterator[T]) => iter.fold(zeroValue)(cleanOp))
+    return results.fold(zeroValue)(cleanOp)
+  }
+
+  /**
+   * Aggregate the elements of each partition, and then the results for all the
+   * partitions, using given combine functions and a neutral "zero value". This
+   * function can return a different result type, U, than the type of this RDD, T.
+   * Thus, we need one operation for merging a T into an U and one operation for
+   * merging two U's, as in scala.TraversableOnce. Both of these functions are
+   * allowed to modify and return their first argument instead of creating a new U
+   * to avoid memory allocation.
+   */
+  def aggregate[U: ClassManifest](zeroValue: U)(seqOp: (U, T) => U, combOp: (U, U) => U): U = {
+    val cleanSeqOp = sc.clean(seqOp)
+    val cleanCombOp = sc.clean(combOp)
+    val results = sc.runJob(this,
+      (iter: Iterator[T]) => iter.aggregate(zeroValue)(cleanSeqOp, cleanCombOp))
+    return results.fold(zeroValue)(cleanCombOp)
   }
   
   def count(): Long = {

core/src/test/scala/spark/RDDSuite.scala

@@ -1,5 +1,6 @@
 package spark
 
+import scala.collection.mutable.HashMap
 import org.scalatest.FunSuite
 import SparkContext._
 
@@ -9,6 +10,7 @@ class RDDSuite extends FunSuite {
     val nums = sc.makeRDD(Array(1, 2, 3, 4), 2)
     assert(nums.collect().toList === List(1, 2, 3, 4))
     assert(nums.reduce(_ + _) === 10)
+    assert(nums.fold(0)(_ + _) === 10)
     assert(nums.map(_.toString).collect().toList === List("1", "2", "3", "4"))
     assert(nums.filter(_ > 2).collect().toList === List(3, 4))
     assert(nums.flatMap(x => 1 to x).collect().toList === List(1, 1, 2, 1, 2, 3, 1, 2, 3, 4))
@@ -18,4 +20,26 @@ class RDDSuite extends FunSuite {
     assert(partitionSums.collect().toList === List(3, 7))
     sc.stop()
   }
+
+  test("aggregate") {
+    val sc = new SparkContext("local", "test")
+    val pairs = sc.makeRDD(Array(("a", 1), ("b", 2), ("a", 2), ("c", 5), ("a", 3)))
+    type StringMap = HashMap[String, Int]
+    val emptyMap = new StringMap {
+      override def default(key: String): Int = 0
+    }
+    val mergeElement: (StringMap, (String, Int)) => StringMap = (map, pair) => {
+      map(pair._1) += pair._2
+      map
+    }
+    val mergeMaps: (StringMap, StringMap) => StringMap = (map1, map2) => {
+      for ((key, value) <- map2) {
+        map1(key) += value
+      }
+      map1
+    }
+    val result = pairs.aggregate(emptyMap)(mergeElement, mergeMaps)
+    assert(result.toSet === Set(("a", 6), ("b", 2), ("c", 5)))
+    sc.stop()
+  }
 }