[SPARK-20043][ML] DecisionTreeModel: ImpurityCalculator builder fails for uppercase impurity type Gini

mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Impurity.scala

@@ -184,7 +184,7 @@ private[spark] object ImpurityCalculator {
    * the given stats.
    */
   def getCalculator(impurity: String, stats: Array[Double]): ImpurityCalculator = {
-    impurity match {
+    impurity.toLowerCase match {
       case "gini" => new GiniCalculator(stats)
       case "entropy" => new EntropyCalculator(stats)
       case "variance" => new VarianceCalculator(stats)

mllib/src/test/scala/org/apache/spark/ml/classification/DecisionTreeClassifierSuite.scala

@@ -385,6 +385,20 @@ class DecisionTreeClassifierSuite
     testEstimatorAndModelReadWrite(dt, continuousData, allParamSettings ++ Map("maxDepth" -> 0),
       allParamSettings ++ Map("maxDepth" -> 0), checkModelData)
   }
+
+  test("read/write: ImpurityCalculator builder did not recognize impurity type: Gini") {
+    val rdd = TreeTests.getTreeReadWriteData(sc)
+
+    val categoricalData: DataFrame =
+      TreeTests.setMetadata(rdd, Map(0 -> 2, 1 -> 3), numClasses = 2)
+
+    // BUG: see SPARK-20043
+    val dt = new DecisionTreeClassifier().setImpurity("Gini")
+
+    val model = dt.fit(categoricalData)
+
+    testDefaultReadWrite(model, false)
+  }
 }
 
 private[ml] object DecisionTreeClassifierSuite extends SparkFunSuite {

mllib/src/test/scala/org/apache/spark/ml/regression/DecisionTreeRegressorSuite.scala

@@ -178,6 +178,20 @@ class DecisionTreeRegressorSuite
       TreeTests.allParamSettings ++ Map("maxDepth" -> 0),
       TreeTests.allParamSettings ++ Map("maxDepth" -> 0), checkModelData)
   }
+
+  test("read/write: ImpurityCalculator builder did not recognize impurity type: Variance") {
+    val rdd = TreeTests.getTreeReadWriteData(sc)
+
+    val continuousData: DataFrame =
+      TreeTests.setMetadata(rdd, Map.empty[Int, Int], numClasses = 0)
+
+    // BUG: see SPARK-20043
+    val dt = new DecisionTreeRegressor().setImpurity("Variance")
+
+    val model = dt.fit(continuousData)
+
+    testDefaultReadWrite(model, false)
+  }
 }
 
 private[ml] object DecisionTreeRegressorSuite extends SparkFunSuite {