[MLlib] SPARK-5954: Top by key

This PR implements two functions
  - `topByKey(num: Int): RDD[(K, Array[V])]` finds the top-k values for each key in a pair RDD. This can be used, e.g., in computing top recommendations.

- `takeOrderedByKey(num: Int): RDD[(K, Array[V])] ` does the opposite of `topByKey`

The `sorted` is used here as the `toArray` method of the PriorityQueue does not return a necessarily sorted array.

Author: Shuo Xiang <shuoxiangpub@gmail.com>

Closes #5075 from coderxiang/topByKey and squashes the following commits:

1611c37 [Shuo Xiang] code clean up
6f565c0 [Shuo Xiang] naming
a80e0ec [Shuo Xiang] typo and warning
82dded9 [Shuo Xiang] Merge remote-tracking branch 'upstream/master' into topByKey
d202745 [Shuo Xiang] move to MLPairRDDFunctions
901b0af [Shuo Xiang] style check
70c6e35 [Shuo Xiang] remove takeOrderedByKey, update doc and test
0895c17 [Shuo Xiang] Merge remote-tracking branch 'upstream/master' into topByKey
b10e325 [Shuo Xiang] Merge remote-tracking branch 'upstream/master' into topByKey
debccad [Shuo Xiang] topByKey

1 files changed, 36 insertions, 0 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/mllib/rdd/MLPairRDDFunctionsSuite.scala b/mllib/src/test/scala/org/apache/spark/mllib/rdd/MLPairRDDFunctionsSuite.scala
new file mode 100644
index 0000000000..1ac7c12c4e
--- /dev/null
+++ b/mllib/src/test/scala/org/apache/spark/mllib/rdd/MLPairRDDFunctionsSuite.scala
@@ -0,0 +1,36 @@
+/*
+ * 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.mllib.rdd
+
+import org.scalatest.FunSuite
+
+import org.apache.spark.mllib.util.MLlibTestSparkContext
+import org.apache.spark.mllib.rdd.MLPairRDDFunctions._
+
+class MLPairRDDFunctionsSuite extends FunSuite with MLlibTestSparkContext {
+  test("topByKey") {
+    val topMap = sc.parallelize(Array((1, 1), (1, 2), (3, 2), (3, 7), (3, 5), (5, 1), (5, 3)), 2)
+      .topByKey(2)
+      .collectAsMap()
+
+    assert(topMap.size === 3)
+    assert(topMap(1) === Array(2, 1))
+    assert(topMap(3) === Array(7, 5))
+    assert(topMap(5) === Array(3, 1))
+  }
+}