[SPARK-11302][MLLIB] 2) Multivariate Gaussian Model with Covariance matrix returns incorrect answer in some cases

Fix computation of root-sigma-inverse in multivariate Gaussian; add a test and fix related Python mixture model test.

Supersedes https://github.com/apache/spark/pull/9293

Author: Sean Owen <sowen@cloudera.com>

Closes #9309 from srowen/SPARK-11302.2.

2 files changed, 19 insertions, 4 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussian.scala b/mllib/src/main/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussian.scala
index 92a5af708d..0724af9308 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussian.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussian.scala
@@ -56,7 +56,7 @@ class MultivariateGaussian @Since("1.3.0") (
 
   /**
    * Compute distribution dependent constants:
-   *    rootSigmaInv = D^(-1/2)^ * U, where sigma = U * D * U.t
+   *    rootSigmaInv = D^(-1/2)^ * U.t, where sigma = U * D * U.t
    *    u = log((2*pi)^(-k/2)^ * det(sigma)^(-1/2)^)
    */
   private val (rootSigmaInv: DBM[Double], u: Double) = calculateCovarianceConstants
@@ -104,11 +104,11 @@ class MultivariateGaussian @Since("1.3.0") (
    *
    *    sigma = U * D * U.t
    *    inv(Sigma) = U * inv(D) * U.t
-   *               = (D^{-1/2}^ * U).t * (D^{-1/2}^ * U)
+   *               = (D^{-1/2}^ * U.t).t * (D^{-1/2}^ * U.t)
    *
    * and thus
    *
-   *    -0.5 * (x-mu).t * inv(Sigma) * (x-mu) = -0.5 * norm(D^{-1/2}^ * U  * (x-mu))^2^
+   *    -0.5 * (x-mu).t * inv(Sigma) * (x-mu) = -0.5 * norm(D^{-1/2}^ * U.t  * (x-mu))^2^
    *
    * To guard against singular covariance matrices, this method computes both the
    * pseudo-determinant and the pseudo-inverse (Moore-Penrose).  Singular values are considered
@@ -130,7 +130,7 @@ class MultivariateGaussian @Since("1.3.0") (
       // by inverting the square root of all non-zero values
       val pinvS = diag(new DBV(d.map(v => if (v > tol) math.sqrt(1.0 / v) else 0.0).toArray))
 
-      (pinvS * u, -0.5 * (mu.size * math.log(2.0 * math.Pi) + logPseudoDetSigma))
+      (pinvS * u.t, -0.5 * (mu.size * math.log(2.0 * math.Pi) + logPseudoDetSigma))
     } catch {
       case uex: UnsupportedOperationException =>
         throw new IllegalArgumentException("Covariance matrix has no non-zero singular values")
diff --git a/mllib/src/test/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussianSuite.scala b/mllib/src/test/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussianSuite.scala
index aa60deb665..6e7a003475 100644
--- a/mllib/src/test/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussianSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/mllib/stat/distribution/MultivariateGaussianSuite.scala
@@ -65,4 +65,19 @@ class MultivariateGaussianSuite extends SparkFunSuite with MLlibTestSparkContext
     assert(dist.pdf(x1) ~== 0.11254 absTol 1E-5)
     assert(dist.pdf(x2) ~== 0.068259 absTol 1E-5)
   }
+
+  test("SPARK-11302") {
+    val x = Vectors.dense(629, 640, 1.7188, 618.19)
+    val mu = Vectors.dense(
+      1055.3910505836575, 1070.489299610895, 1.39020554474708, 1040.5907503867697)
+    val sigma = Matrices.dense(4, 4, Array(
+      166769.00466698944, 169336.6705268059, 12.820670788921873, 164243.93314092053,
+      169336.6705268059, 172041.5670061245, 21.62590020524533, 166678.01075856484,
+      12.820670788921873, 21.62590020524533, 0.872524191943962, 4.283255814732373,
+      164243.93314092053, 166678.01075856484, 4.283255814732373, 161848.9196719207))
+    val dist = new MultivariateGaussian(mu, sigma)
+    // Agrees with R's dmvnorm: 7.154782e-05
+    assert(dist.pdf(x) ~== 7.154782224045512E-5 absTol 1E-9)
+  }
+
 }