Add feature: Collect enough frequent prefixes before projection in PrefixSpan.

Author: zhangjiajin

mllib/src/main/scala/org/apache/spark/mllib/fpm/PrefixSpan.scala

@@ -43,6 +43,8 @@ class PrefixSpan private (
     private var minSupport: Double,
     private var maxPatternLength: Int) extends Logging with Serializable {
 
+  private val minPatternsBeforeShuffle: Int = 20
+
   /**
    * Constructs a default instance with default parameters
    * {minSupport: `0.1`, maxPatternLength: `10`}.
@@ -86,16 +88,69 @@ class PrefixSpan private (
       getFreqItemAndCounts(minCount, sequences).collect()
     val prefixAndProjectedDatabase = getPrefixAndProjectedDatabase(
       lengthOnePatternsAndCounts.map(_._1), sequences)
-    val groupedProjectedDatabase = prefixAndProjectedDatabase
-      .map(x => (x._1.toSeq, x._2))
-      .groupByKey()
-      .map(x => (x._1.toArray, x._2.toArray))
-    val nextPatterns = getPatternsInLocal(minCount, groupedProjectedDatabase)
-    val lengthOnePatternsAndCountsRdd =
-      sequences.sparkContext.parallelize(
-        lengthOnePatternsAndCounts.map(x => (Array(x._1), x._2)))
-    val allPatterns = lengthOnePatternsAndCountsRdd ++ nextPatterns
-    allPatterns
+
+    var patternsCount = lengthOnePatternsAndCounts.length
+    var allPatternAndCounts = sequences.sparkContext.parallelize(
+      lengthOnePatternsAndCounts.map(x => (Array(x._1), x._2)))
+    var currentProjectedDatabase = prefixAndProjectedDatabase
+    while (patternsCount <= minPatternsBeforeShuffle &&
+      currentProjectedDatabase.count() != 0) {
+      val (nextPatternAndCounts, nextProjectedDatabase) =
+        getPatternCountsAndProjectedDatabase(minCount, currentProjectedDatabase)
+      patternsCount = nextPatternAndCounts.count().toInt
+      currentProjectedDatabase = nextProjectedDatabase
+      allPatternAndCounts = allPatternAndCounts ++ nextPatternAndCounts
+    }
+    if (patternsCount > 0) {
+      val groupedProjectedDatabase = currentProjectedDatabase
+        .map(x => (x._1.toSeq, x._2))
+        .groupByKey()
+        .map(x => (x._1.toArray, x._2.toArray))
+      val nextPatternAndCounts = getPatternsInLocal(minCount, groupedProjectedDatabase)
+      allPatternAndCounts = allPatternAndCounts ++ nextPatternAndCounts
+    }
+    allPatternAndCounts
+  }
+
+  /**
+   * Get the pattern and counts, and projected database
+   * @param minCount minimum count
+   * @param prefixAndProjectedDatabase prefix and projected database,
+   * @return pattern and counts, and projected database
+   *         (Array[pattern, count], RDD[prefix, projected database ])
+   */
+  private def getPatternCountsAndProjectedDatabase(
+      minCount: Long,
+      prefixAndProjectedDatabase: RDD[(Array[Int], Array[Int])]):
+  (RDD[(Array[Int], Long)], RDD[(Array[Int], Array[Int])]) = {
+    val prefixAndFreqentItemAndCounts = prefixAndProjectedDatabase.flatMap{ x =>
+      x._2.distinct.map(y => ((x._1.toSeq, y), 1L))
+    }.reduceByKey(_+_)
+      .filter(_._2 >= minCount)
+    val patternAndCounts = prefixAndFreqentItemAndCounts
+      .map(x => (x._1._1.toArray ++ Array(x._1._2), x._2))
+    val prefixlength = prefixAndProjectedDatabase.take(1)(0)._1.length
+    if (prefixlength + 1 >= maxPatternLength) {
+      (patternAndCounts, prefixAndProjectedDatabase.filter(x => false))
+    } else {
+      val frequentItemsMap = prefixAndFreqentItemAndCounts
+        .keys.map(x => (x._1, x._2))
+        .groupByKey()
+        .mapValues(_.toSet)
+        .collect
+        .toMap
+      val nextPrefixAndProjectedDatabase = prefixAndProjectedDatabase
+        .filter(x => frequentItemsMap.contains(x._1))
+        .flatMap { x =>
+        val frequentItemSet = frequentItemsMap(x._1)
+        val filteredSequence = x._2.filter(frequentItemSet.contains(_))
+        val subProjectedDabase = frequentItemSet.map{ y =>
+          (y, LocalPrefixSpan.getSuffix(y, filteredSequence))
+        }.filter(_._2.nonEmpty)
+        subProjectedDabase.map(y => (x._1 ++ Array(y._1), y._2))
+      }
+      (patternAndCounts, nextPrefixAndProjectedDatabase)
+    }
   }
 
   /**