Added a serializable wrapper for HyperLogLog

core/src/main/scala/org/apache/spark/util/SerializableHyperLogLog.scala

@@ -0,0 +1,44 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.util
+
+import java.io.{ObjectOutputStream, ObjectInputStream}
+import com.clearspring.analytics.stream.cardinality.{ICardinality, HyperLogLog}
+
+/**
+ * A wrapper around com.clearspring.analytics.stream.cardinality.HyperLogLog that is serializable.
+ */
+private[spark]
+class SerializableHyperLogLog(@transient var value: ICardinality) extends Serializable {
+
+
+  def merge(other: SerializableHyperLogLog) = new SerializableHyperLogLog(value.merge(other.value))
+
+  private def readObject(in: ObjectInputStream) {
+    val byteLength = in.readInt()
+    val bytes = new Array[Byte](byteLength)
+    in.readFully(bytes)
+    value = HyperLogLog.Builder.build(bytes)
+  }
+
+  private def writeObject(out: ObjectOutputStream) {
+    val bytes = value.getBytes()
+    out.writeInt(bytes.length)
+    out.write(bytes)
+  }
+}