[MLLIB] minor update to word2vec

very minor update Ishiihara

Author: Xiangrui Meng <meng@databricks.com>

Closes #2043 from mengxr/minor-w2v and squashes the following commits:

be649fd [Xiangrui Meng] remove map because we only need append
eccefcc [Xiangrui Meng] minor updates to word2vec

mllib/src/main/scala/org/apache/spark/mllib/feature/Word2Vec.scala

@@ -30,11 +30,9 @@ import org.apache.spark.SparkContext._
 import org.apache.spark.annotation.Experimental
 import org.apache.spark.api.java.JavaRDD
 import org.apache.spark.mllib.linalg.{Vector, Vectors}
-import org.apache.spark.mllib.rdd.RDDFunctions._
 import org.apache.spark.rdd._
 import org.apache.spark.util.Utils
 import org.apache.spark.util.random.XORShiftRandom
-import org.apache.spark.util.collection.PrimitiveKeyOpenHashMap
 
 /**
  *  Entry in vocabulary 
@@ -285,9 +283,9 @@ class Word2Vec extends Serializable with Logging {
     
     val newSentences = sentences.repartition(numPartitions).cache()
     val initRandom = new XORShiftRandom(seed)
-    var syn0Global =
+    val syn0Global =
       Array.fill[Float](vocabSize * vectorSize)((initRandom.nextFloat() - 0.5f) / vectorSize)
-    var syn1Global = new Array[Float](vocabSize * vectorSize)
+    val syn1Global = new Array[Float](vocabSize * vectorSize)
     var alpha = startingAlpha
     for (k <- 1 to numIterations) {
       val partial = newSentences.mapPartitionsWithIndex { case (idx, iter) =>
@@ -349,21 +347,21 @@ class Word2Vec extends Serializable with Logging {
         }
         val syn0Local = model._1
         val syn1Local = model._2
-        val synOut = new PrimitiveKeyOpenHashMap[Int, Array[Float]](vocabSize * 2)
+        val synOut = mutable.ListBuffer.empty[(Int, Array[Float])]
         var index = 0
         while(index < vocabSize) {
           if (syn0Modify(index) != 0) {
-            synOut.update(index, syn0Local.slice(index * vectorSize, (index + 1) * vectorSize))
+            synOut += ((index, syn0Local.slice(index * vectorSize, (index + 1) * vectorSize)))
           }
           if (syn1Modify(index) != 0) {
-            synOut.update(index + vocabSize,
-              syn1Local.slice(index * vectorSize, (index + 1) * vectorSize))
+            synOut += ((index + vocabSize,
+              syn1Local.slice(index * vectorSize, (index + 1) * vectorSize)))
           }
           index += 1
         }
-        Iterator(synOut)
+        synOut.toIterator
       }
-      val synAgg = partial.flatMap(x => x).reduceByKey { case (v1, v2) =>
+      val synAgg = partial.reduceByKey { case (v1, v2) =>
           blas.saxpy(vectorSize, 1.0f, v2, 1, v1, 1)
           v1
       }.collect()