[SPARK-21680][ML][MLLIB] optimize Vector compress

## What changes were proposed in this pull request?

When use Vector.compressed to change a Vector to SparseVector, the performance is very low comparing with Vector.toSparse.
This is because you have to scan the value three times using Vector.compressed, but you just need two times when use Vector.toSparse.
When the length of the vector is large, there is significant performance difference between this two method.

## How was this patch tested?

The existing UT

Author: Peng Meng <[email protected]>

Closes apache#18899 from mpjlu/optVectorCompress.

Author: Peng Meng

mllib-local/src/main/scala/org/apache/spark/ml/linalg/Vectors.scala

@@ -136,7 +136,21 @@ sealed trait Vector extends Serializable {
    * Converts this vector to a sparse vector with all explicit zeros removed.
    */
   @Since("2.0.0")
-  def toSparse: SparseVector
+  def toSparse: SparseVector = toSparseWithSize(numNonzeros)
+
+  /**
+   * Converts this vector to a sparse vector with all explicit zeros removed when the size is known.
+   * This method is used to avoid re-computing the number of non-zero elements when it is
+   * already known. This method should only be called after computing the number of non-zero
+   * elements via [[numNonzeros]]. e.g.
+   * {{{
+   *   val nnz = numNonzeros
+   *   val sv = toSparse(nnz)
+   * }}}
+   *
+   * If `nnz` is under-specified, a [[java.lang.ArrayIndexOutOfBoundsException]] is thrown.
+   */
+  private[linalg] def toSparseWithSize(nnz: Int): SparseVector
 
   /**
    * Converts this vector to a dense vector.
@@ -152,7 +166,7 @@ sealed trait Vector extends Serializable {
     val nnz = numNonzeros
     // A dense vector needs 8 * size + 8 bytes, while a sparse vector needs 12 * nnz + 20 bytes.
     if (1.5 * (nnz + 1.0) < size) {
-      toSparse
+      toSparseWithSize(nnz)
     } else {
       toDense
     }
@@ -495,8 +509,7 @@ class DenseVector @Since("2.0.0") ( @Since("2.0.0") val values: Array[Double]) e
     nnz
   }
 
-  override def toSparse: SparseVector = {
-    val nnz = numNonzeros
+  private[linalg] override def toSparseWithSize(nnz: Int): SparseVector = {
     val ii = new Array[Int](nnz)
     val vv = new Array[Double](nnz)
     var k = 0
@@ -635,8 +648,7 @@ class SparseVector @Since("2.0.0") (
     nnz
   }
 
-  override def toSparse: SparseVector = {
-    val nnz = numNonzeros
+  private[linalg] override def toSparseWithSize(nnz: Int): SparseVector = {
     if (nnz == numActives) {
       this
     } else {

mllib-local/src/test/scala/org/apache/spark/ml/linalg/VectorsSuite.scala

@@ -318,11 +318,21 @@ class VectorsSuite extends SparkMLFunSuite {
     assert(dv0s.numActives === 2)
     assert(dv0s === dv0)
 
+    assert(dv0.toSparseWithSize(dv0.numNonzeros) === dv0)
+    val dv0s2 = dv0.toSparseWithSize(dv0.numNonzeros)
+    assert(dv0s2.numActives === 2)
+    assert(dv0s2 === dv0s)
+
     val sv0 = Vectors.sparse(4, Array(0, 1, 2), Array(0.0, 2.0, 3.0))
     assert(sv0.toDense === sv0)
     val sv0s = sv0.toSparse
     assert(sv0s.numActives === 2)
     assert(sv0s === sv0)
+
+    assert(sv0.toSparseWithSize(sv0.numNonzeros) === sv0)
+    val sv0s2 = sv0.toSparseWithSize(sv0.numNonzeros)
+    assert(sv0s2.numActives === 2)
+    assert(sv0s2 === sv0s)
   }
 
   test("Vector.compressed") {

mllib/src/main/scala/org/apache/spark/mllib/linalg/Vectors.scala

@@ -149,7 +149,21 @@ sealed trait Vector extends Serializable {
    * Converts this vector to a sparse vector with all explicit zeros removed.
    */
   @Since("1.4.0")
-  def toSparse: SparseVector
+  def toSparse: SparseVector = toSparseWithSize(numNonzeros)
+
+  /**
+   * Converts this vector to a sparse vector with all explicit zeros removed when the size is known.
+   * This method is used to avoid re-computing the number of non-zero elements when it is
+   * already known. This method should only be called after computing the number of non-zero
+   * elements via [[numNonzeros]]. e.g.
+   * {{{
+   *   val nnz = numNonzeros
+   *   val sv = toSparse(nnz)
+   * }}}
+   *
+   * If `nnz` is under-specified, a [[java.lang.ArrayIndexOutOfBoundsException]] is thrown.
+   */
+  private[linalg] def toSparseWithSize(nnz: Int): SparseVector
 
   /**
    * Converts this vector to a dense vector.
@@ -165,7 +179,7 @@ sealed trait Vector extends Serializable {
     val nnz = numNonzeros
     // A dense vector needs 8 * size + 8 bytes, while a sparse vector needs 12 * nnz + 20 bytes.
     if (1.5 * (nnz + 1.0) < size) {
-      toSparse
+      toSparseWithSize(nnz)
     } else {
       toDense
     }
@@ -669,9 +683,7 @@ class DenseVector @Since("1.0.0") (
     nnz
   }
 
-  @Since("1.4.0")
-  override def toSparse: SparseVector = {
-    val nnz = numNonzeros
+  private[linalg] override def toSparseWithSize(nnz: Int): SparseVector = {
     val ii = new Array[Int](nnz)
     val vv = new Array[Double](nnz)
     var k = 0
@@ -822,9 +834,7 @@ class SparseVector @Since("1.0.0") (
     nnz
   }
 
-  @Since("1.4.0")
-  override def toSparse: SparseVector = {
-    val nnz = numNonzeros
+  private[linalg] override def toSparseWithSize(nnz: Int): SparseVector = {
     if (nnz == numActives) {
       this
     } else {

mllib/src/test/scala/org/apache/spark/mllib/linalg/VectorsSuite.scala

@@ -351,11 +351,21 @@ class VectorsSuite extends SparkFunSuite with Logging {
     assert(dv0s.numActives === 2)
     assert(dv0s === dv0)
 
+    assert(dv0.toSparseWithSize(dv0.numNonzeros) === dv0)
+    val dv0s2 = dv0.toSparseWithSize(dv0.numNonzeros)
+    assert(dv0s2.numActives === 2)
+    assert(dv0s2 === dv0s)
+
     val sv0 = Vectors.sparse(4, Array(0, 1, 2), Array(0.0, 2.0, 3.0))
     assert(sv0.toDense === sv0)
     val sv0s = sv0.toSparse
     assert(sv0s.numActives === 2)
     assert(sv0s === sv0)
+
+    assert(sv0.toSparseWithSize(sv0.numNonzeros) === sv0)
+    val sv0s2 = sv0.toSparseWithSize(sv0.numNonzeros)
+    assert(sv0s2.numActives === 2)
+    assert(sv0s2 === sv0s)
   }
 
   test("Vector.compressed") {

project/MimaExcludes.scala

@@ -1015,6 +1015,10 @@ object MimaExcludes {
       ProblemFilters.exclude[IncompatibleResultTypeProblem]("org.apache.spark.ml.classification.RandomForestClassificationModel.setFeatureSubsetStrategy"),
       ProblemFilters.exclude[IncompatibleResultTypeProblem]("org.apache.spark.ml.regression.RandomForestRegressionModel.numTrees"),
       ProblemFilters.exclude[IncompatibleResultTypeProblem]("org.apache.spark.ml.regression.RandomForestRegressionModel.setFeatureSubsetStrategy")
+    ) ++ Seq(
+      // [SPARK-21680][ML][MLLIB]optimzie Vector coompress
+      ProblemFilters.exclude[ReversedMissingMethodProblem]("org.apache.spark.mllib.linalg.Vector.toSparseWithSize"),
+      ProblemFilters.exclude[ReversedMissingMethodProblem]("org.apache.spark.ml.linalg.Vector.toSparseWithSize")
     )
   }