[SPARK-7844] [MLLIB] Fix broken tests in KernelDensity

The densities in KernelDensity are scaled down by
(number of parallel processes X number of points). It should be just no.of samples. This results in broken tests in KernelDensitySuite which haven't been tested properly.

Author: MechCoder <[email protected]>

Closes #6383 from MechCoder/spark-7844 and squashes the following commits:

ab81302 [MechCoder] Math->math
9b8ed50 [MechCoder] Make one pass to update count
a92fe50 [MechCoder] [SPARK-7844] Fix broken tests in KernelDensity

Author: MechCoder

mllib/src/main/scala/org/apache/spark/mllib/stat/KernelDensity.scala

@@ -93,7 +93,7 @@ class KernelDensity extends Serializable {
           x._1(i) += normPdf(y, bandwidth, logStandardDeviationPlusHalfLog2Pi, points(i))
           i += 1
         }
-        (x._1, n)
+        (x._1, x._2 + 1)
       },
       (x, y) => {
         blas.daxpy(n, 1.0, y._1, 1, x._1, 1)

mllib/src/test/scala/org/apache/spark/mllib/stat/KernelDensitySuite.scala

@@ -29,8 +29,8 @@ class KernelDensitySuite extends FunSuite with MLlibTestSparkContext {
     val densities = new KernelDensity().setSample(rdd).setBandwidth(3.0).estimate(evaluationPoints)
     val normal = new NormalDistribution(5.0, 3.0)
     val acceptableErr = 1e-6
-    assert(densities(0) - normal.density(5.0) < acceptableErr)
-    assert(densities(0) - normal.density(6.0) < acceptableErr)
+    assert(math.abs(densities(0) - normal.density(5.0)) < acceptableErr)
+    assert(math.abs(densities(1) - normal.density(6.0)) < acceptableErr)
   }
 
   test("kernel density multiple samples") {
@@ -40,7 +40,9 @@ class KernelDensitySuite extends FunSuite with MLlibTestSparkContext {
     val normal1 = new NormalDistribution(5.0, 3.0)
     val normal2 = new NormalDistribution(10.0, 3.0)
     val acceptableErr = 1e-6
-    assert(densities(0) - (normal1.density(5.0) + normal2.density(5.0)) / 2 < acceptableErr)
-    assert(densities(0) - (normal1.density(6.0) + normal2.density(6.0)) / 2 < acceptableErr)
+    assert(math.abs(
+      densities(0) - (normal1.density(5.0) + normal2.density(5.0)) / 2) < acceptableErr)
+    assert(math.abs(
+      densities(1) - (normal1.density(6.0) + normal2.density(6.0)) / 2) < acceptableErr)
   }
 }