[SPARK-8018][MLlib]KMeans should accept initial cluster centers as param

mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeans.scala

@@ -156,6 +156,20 @@ class KMeans private (
     this
   }
 
+  // Initial cluster centers can be provided as a KMeansModel object rather than using the
+  // random or k-means|| initializationMode
+  private var initialModel: Option[KMeansModel] = None
+
+  /** Set the initial starting point, bypassing the random initialization or k-means||
+    * The condition (model.k == this.k) must be met; failure will result in an
+    * IllegalArgumentException.
+    */
+  def setInitialModel(model: KMeansModel): this.type = {
+    require(model.k==k, "mismatched cluster count")
+    initialModel = Some(model)
+    this
+  }
+
   /**
    * Train a K-means model on the given set of points; `data` should be cached for high
    * performance, because this is an iterative algorithm.
@@ -193,12 +207,19 @@ class KMeans private (
 
     val initStartTime = System.nanoTime()
 
-    val centers = if (initializationMode == KMeans.RANDOM) {
-      initRandom(data)
-    } else {
-      initKMeansParallel(data)
+    val centers = initialModel match {
+      case Some(kMeansCenters) => {
+        Array.tabulate(runs)(r => kMeansCenters.clusterCenters
+          .map(s => new VectorWithNorm(s, Vectors.norm(s, 2.0))))
+      }
+      case None => {
+        if (initializationMode == KMeans.RANDOM) {
+          initRandom(data)
+        } else {
+          initKMeansParallel(data)
+        }
+      }
     }
-
     val initTimeInSeconds = (System.nanoTime() - initStartTime) / 1e9
     logInfo(s"Initialization with $initializationMode took " + "%.3f".format(initTimeInSeconds) +
       " seconds.")
@@ -478,6 +499,25 @@ object KMeans {
     train(data, k, maxIterations, runs, K_MEANS_PARALLEL)
   }
 
+  /**
+   * Trains a k-means model using the given set of parameters and initial cluster centers
+   *
+   * @param data training points stored as `RDD[Vector]`
+   * @param k number of clusters
+   * @param maxIterations max number of iterations
+   * @param initialModel an existing set of cluster centers.
+   */
+  def train(
+       data: RDD[Vector],
+       k: Int,
+       maxIterations: Int,
+       initialModel: KMeansModel): KMeansModel = {
+    new KMeans().setK(k)
+      .setMaxIterations(maxIterations)
+      .setInitialModel(initialModel)
+      .run(data)
+  }
+
   /**
    * Returns the index of the closest center to the given point, as well as the squared distance.
    */

mllib/src/test/scala/org/apache/spark/mllib/clustering/KMeansSuite.scala

@@ -278,6 +278,34 @@ class KMeansSuite extends SparkFunSuite with MLlibTestSparkContext {
       }
     }
   }
+
+  test("Initialize using given cluster centers") {
+    val points = Seq(
+      Vectors.dense(0.0, 0.0),
+      Vectors.dense(0.0, 0.1),
+      Vectors.dense(0.1, 0.0),
+      Vectors.dense(9.0, 0.0),
+      Vectors.dense(9.0, 0.2),
+      Vectors.dense(9.2, 0.0)
+    )
+    val rdd = sc.parallelize(points, 3)
+    val model = KMeans.train(rdd, k = 2, maxIterations = 2, runs = 1)
+
+    val tempDir = Utils.createTempDir()
+    val path = tempDir.toURI.toString
+    model.save(sc, path)
+    val loadedModel = KMeansModel.load(sc, path)
+
+    val newModel = KMeans.train(rdd, k = 2, maxIterations = 2, initialModel = loadedModel)
+    val predicts = newModel.predict(rdd).collect()
+
+      assert(predicts(0) === predicts(1))
+      assert(predicts(0) === predicts(2))
+      assert(predicts(3) === predicts(4))
+      assert(predicts(3) === predicts(5))
+      assert(predicts(0) != predicts(3))
+  }
+
 }
 
 object KMeansSuite extends SparkFunSuite {