[SPARK-3614][MLLIB] Add minimumOccurence filtering to IDF

This PR for [SPARK-3614](https://issues.apache.org/jira/browse/SPARK-3614) adds functionality for filtering out terms which do not appear in at least a minimum number of documents.

This is implemented using a minimumOccurence parameter (default 0).  When terms' document frequencies are less than minimumOccurence, their IDFs are set to 0, just like when the DF is 0.  As a result, the TF-IDFs for the terms are found to be 0, as if the terms were not present in the documents.

This PR makes the following changes:
* Add a minimumOccurence parameter to the IDF and DocumentFrequencyAggregator classes.
* Create a parameter-less constructor for IDF with a default minimumOccurence value of 0 to remain backwards-compatibility with the original IDF API.
* Sets the IDFs to 0 for terms which DFs are less than minimumOccurence
* Add tests to the Spark IDFSuite and Java JavaTfIdfSuite test suites
* Updated the MLLib Feature Extraction programming guide to describe the new feature

Author: RJ Nowling <rnowling@gmail.com>

Closes #2494 from rnowling/spark-3614-idf-filter and squashes the following commits:

0aa3c63 [RJ Nowling] Fix identation
e6523a8 [RJ Nowling] Remove unnecessary toDouble's from IDFSuite
bfa82ec [RJ Nowling] Add space after if
30d20b3 [RJ Nowling] Add spaces around equals signs
9013447 [RJ Nowling] Add space before division operator
79978fc [RJ Nowling] Remove unnecessary semi-colon
40fd70c [RJ Nowling] Change minimumOccurence to minDocFreq in code and docs
47850ab [RJ Nowling] Changed minimumOccurence to Int from Long
9fb4093 [RJ Nowling] Remove unnecessary lines from IDF class docs
1fc09d8 [RJ Nowling] Add backwards-compatible constructor to DocumentFrequencyAggregator
1801fd2 [RJ Nowling] Fix style errors in IDF.scala
6897252 [RJ Nowling] Preface minimumOccurence members with val to make them final and immutable
a200bab [RJ Nowling] Remove unnecessary else statement
4b974f5 [RJ Nowling] Remove accidentally-added import from testing
c0cc643 [RJ Nowling] Add minimumOccurence filtering to IDF

4 files changed, 103 insertions, 5 deletions

diff --git a/docs/mllib-feature-extraction.md b/docs/mllib-feature-extraction.md
index 41a27f6208..1511ae6dda 100644
--- a/docs/mllib-feature-extraction.md
+++ b/docs/mllib-feature-extraction.md
@@ -82,6 +82,21 @@ tf.cache()
 val idf = new IDF().fit(tf)
 val tfidf: RDD[Vector] = idf.transform(tf)
 {% endhighlight %}
+
+MLLib's IDF implementation provides an option for ignoring terms which occur in less than a
+minimum number of documents.  In such cases, the IDF for these terms is set to 0.  This feature
+can be used by passing the `minDocFreq` value to the IDF constructor.
+
+{% highlight scala %}
+import org.apache.spark.mllib.feature.IDF
+
+// ... continue from the previous example
+tf.cache()
+val idf = new IDF(minDocFreq = 2).fit(tf)
+val tfidf: RDD[Vector] = idf.transform(tf)
+{% endhighlight %}
+
+
 </div>
 </div>
 
diff --git a/mllib/src/main/scala/org/apache/spark/mllib/feature/IDF.scala b/mllib/src/main/scala/org/apache/spark/mllib/feature/IDF.scala
index d40d5553c1..720bb70b08 100644
--- a/mllib/src/main/scala/org/apache/spark/mllib/feature/IDF.scala
+++ b/mllib/src/main/scala/org/apache/spark/mllib/feature/IDF.scala
@@ -30,9 +30,18 @@ import org.apache.spark.rdd.RDD
  * Inverse document frequency (IDF).
  * The standard formulation is used: `idf = log((m + 1) / (d(t) + 1))`, where `m` is the total
  * number of documents and `d(t)` is the number of documents that contain term `t`.
+ *
+ * This implementation supports filtering out terms which do not appear in a minimum number
+ * of documents (controlled by the variable `minDocFreq`). For terms that are not in
+ * at least `minDocFreq` documents, the IDF is found as 0, resulting in TF-IDFs of 0.
+ *
+ * @param minDocFreq minimum of documents in which a term
+ *                   should appear for filtering
  */
 @Experimental
-class IDF {
+class IDF(val minDocFreq: Int) {
+
+  def this() = this(0)
 
   // TODO: Allow different IDF formulations.
 
@@ -41,7 +50,8 @@ class IDF {
    * @param dataset an RDD of term frequency vectors
    */
   def fit(dataset: RDD[Vector]): IDFModel = {
-    val idf = dataset.treeAggregate(new IDF.DocumentFrequencyAggregator)(
+    val idf = dataset.treeAggregate(new IDF.DocumentFrequencyAggregator(
+          minDocFreq = minDocFreq))(
       seqOp = (df, v) => df.add(v),
       combOp = (df1, df2) => df1.merge(df2)
     ).idf()
@@ -60,13 +70,16 @@ class IDF {
 private object IDF {
 
   /** Document frequency aggregator. */
-  class DocumentFrequencyAggregator extends Serializable {
+  class DocumentFrequencyAggregator(val minDocFreq: Int) extends Serializable {
 
     /** number of documents */
     private var m = 0L
     /** document frequency vector */
     private var df: BDV[Long] = _
 
+
+    def this() = this(0)
+
     /** Adds a new document. */
     def add(doc: Vector): this.type = {
       if (isEmpty) {
@@ -123,7 +136,18 @@ private object IDF {
       val inv = new Array[Double](n)
       var j = 0
       while (j < n) {
-        inv(j) = math.log((m + 1.0)/ (df(j) + 1.0))
+        /*
+         * If the term is not present in the minimum
+         * number of documents, set IDF to 0. This
+         * will cause multiplication in IDFModel to
+         * set TF-IDF to 0.
+         *
+         * Since arrays are initialized to 0 by default,
+         * we just omit changing those entries.
+         */
+        if(df(j) >= minDocFreq) {
+          inv(j) = math.log((m + 1.0) / (df(j) + 1.0))
+        }
         j += 1
       }
       Vectors.dense(inv)
@@ -140,6 +164,11 @@ class IDFModel private[mllib] (val idf: Vector) extends Serializable {
 
   /**
    * Transforms term frequency (TF) vectors to TF-IDF vectors.
+   *
+   * If `minDocFreq` was set for the IDF calculation,
+   * the terms which occur in fewer than `minDocFreq`
+   * documents will have an entry of 0.
+   *
    * @param dataset an RDD of term frequency vectors
    * @return an RDD of TF-IDF vectors
    */
diff --git a/mllib/src/test/java/org/apache/spark/mllib/feature/JavaTfIdfSuite.java b/mllib/src/test/java/org/apache/spark/mllib/feature/JavaTfIdfSuite.java
index e8d99f4ae4..064263e02c 100644
--- a/mllib/src/test/java/org/apache/spark/mllib/feature/JavaTfIdfSuite.java
+++ b/mllib/src/test/java/org/apache/spark/mllib/feature/JavaTfIdfSuite.java
@@ -63,4 +63,24 @@ public class JavaTfIdfSuite implements Serializable {
       Assert.assertEquals(0.0, v.apply(indexOfThis), 1e-15);
     }
   }
+
+  @Test
+  public void tfIdfMinimumDocumentFrequency() {
+    // The tests are to check Java compatibility.
+    HashingTF tf = new HashingTF();
+    JavaRDD<ArrayList<String>> documents = sc.parallelize(Lists.newArrayList(
+      Lists.newArrayList("this is a sentence".split(" ")),
+      Lists.newArrayList("this is another sentence".split(" ")),
+      Lists.newArrayList("this is still a sentence".split(" "))), 2);
+    JavaRDD<Vector> termFreqs = tf.transform(documents);
+    termFreqs.collect();
+    IDF idf = new IDF(2);
+    JavaRDD<Vector> tfIdfs = idf.fit(termFreqs).transform(termFreqs);
+    List<Vector> localTfIdfs = tfIdfs.collect();
+    int indexOfThis = tf.indexOf("this");
+    for (Vector v: localTfIdfs) {
+      Assert.assertEquals(0.0, v.apply(indexOfThis), 1e-15);
+    }
+  }
+
 }
diff --git a/mllib/src/test/scala/org/apache/spark/mllib/feature/IDFSuite.scala b/mllib/src/test/scala/org/apache/spark/mllib/feature/IDFSuite.scala
index 53d9c0c640..43974f84e3 100644
--- a/mllib/src/test/scala/org/apache/spark/mllib/feature/IDFSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/mllib/feature/IDFSuite.scala
@@ -38,7 +38,7 @@ class IDFSuite extends FunSuite with LocalSparkContext {
     val idf = new IDF
     val model = idf.fit(termFrequencies)
     val expected = Vectors.dense(Array(0, 3, 1, 2).map { x =>
-      math.log((m.toDouble + 1.0) / (x + 1.0))
+      math.log((m + 1.0) / (x + 1.0))
     })
     assert(model.idf ~== expected absTol 1e-12)
     val tfidf = model.transform(termFrequencies).cache().zipWithIndex().map(_.swap).collectAsMap()
@@ -54,4 +54,38 @@ class IDFSuite extends FunSuite with LocalSparkContext {
     assert(tfidf2.indices === Array(1))
     assert(tfidf2.values(0) ~== (1.0 * expected(1)) absTol 1e-12)
   }
+
+  test("idf minimum document frequency filtering") {
+    val n = 4
+    val localTermFrequencies = Seq(
+      Vectors.sparse(n, Array(1, 3), Array(1.0, 2.0)),
+      Vectors.dense(0.0, 1.0, 2.0, 3.0),
+      Vectors.sparse(n, Array(1), Array(1.0))
+    )
+    val m = localTermFrequencies.size
+    val termFrequencies = sc.parallelize(localTermFrequencies, 2)
+    val idf = new IDF(minDocFreq = 1)
+    val model = idf.fit(termFrequencies)
+    val expected = Vectors.dense(Array(0, 3, 1, 2).map { x =>
+      if (x > 0) {
+        math.log((m + 1.0) / (x + 1.0))
+      } else {
+        0
+      }
+    })
+    assert(model.idf ~== expected absTol 1e-12)
+    val tfidf = model.transform(termFrequencies).cache().zipWithIndex().map(_.swap).collectAsMap()
+    assert(tfidf.size === 3)
+    val tfidf0 = tfidf(0L).asInstanceOf[SparseVector]
+    assert(tfidf0.indices === Array(1, 3))
+    assert(Vectors.dense(tfidf0.values) ~==
+      Vectors.dense(1.0 * expected(1), 2.0 * expected(3)) absTol 1e-12)
+    val tfidf1 = tfidf(1L).asInstanceOf[DenseVector]
+    assert(Vectors.dense(tfidf1.values) ~==
+      Vectors.dense(0.0, 1.0 * expected(1), 2.0 * expected(2), 3.0 * expected(3)) absTol 1e-12)
+    val tfidf2 = tfidf(2L).asInstanceOf[SparseVector]
+    assert(tfidf2.indices === Array(1))
+    assert(tfidf2.values(0) ~== (1.0 * expected(1)) absTol 1e-12)
+  }
+
 }