simplify FPTree and update FPGrowth

Author: mengxr

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

@@ -17,13 +17,15 @@
 
 package org.apache.spark.mllib.fpm
 
-import scala.collection.mutable.ArrayBuffer
+import java.{util => ju}
 
-import org.apache.spark.broadcast.Broadcast
-import org.apache.spark.Logging
-import org.apache.spark.rdd.RDD
+import scala.collection.mutable
 
+import org.apache.spark.{SparkException, HashPartitioner, Logging, Partitioner}
+import org.apache.spark.rdd.RDD
+import org.apache.spark.storage.StorageLevel
 
+class FPGrowthModel(val freqItemsets: RDD[(Array[String], Long)]) extends Serializable
 
 /**
  * This class implements Parallel FP-growth algorithm to do frequent pattern matching on input data.
@@ -34,125 +36,127 @@ import org.apache.spark.rdd.RDD
  *
  * @param minSupport the minimal support level of the frequent pattern, any pattern appears
  *                   more than (minSupport * size-of-the-dataset) times will be output
+ * @param numPartitions number of partitions used by parallel FP-growth
  */
-class FPGrowth private(private var minSupport: Double) extends Logging with Serializable {
+class FPGrowth private (
+    private var minSupport: Double,
+    private var numPartitions: Int) extends Logging with Serializable {
 
   /**
    * Constructs a FPGrowth instance with default parameters:
-   * {minSupport: 0.3}
+   * {minSupport: 0.3, numPartitions: auto}
    */
-  def this() = this(0.3)
+  def this() = this(0.3, -1)
 
   /**
-   * set the minimal support level, default is 0.3
-   * @param minSupport minimal support level
+   * Sets the minimal support level (default: 0.3).
    */
   def setMinSupport(minSupport: Double): this.type = {
     this.minSupport = minSupport
     this
   }
 
   /**
-   * Compute a FPGrowth Model that contains frequent pattern result.
+   * Sets the number of partitions used by parallel FP-growth (default: same as input data).
+   */
+  def setNumPartitions(numPartitions: Int): this.type = {
+    this.numPartitions = numPartitions
+    this
+  }
+
+  /**
+   * Computes an FP-Growth model that contains frequent itemsets.
    * @param data input data set, each element contains a transaction
-   * @return FPGrowth Model
+   * @return an [[FPGrowthModel]]
    */
   def run(data: RDD[Array[String]]): FPGrowthModel = {
+    if (data.getStorageLevel == StorageLevel.NONE) {
+      logWarning("Input data is not cached.")
+    }
     val count = data.count()
-    val minCount = minSupport * count
-    val single = generateSingleItem(data, minCount)
-    val combinations = generateCombinations(data, minCount, single)
-    val all = single.map(v => (Array[String](v._1), v._2)).union(combinations)
-    new FPGrowthModel(all.collect())
+    val minCount = math.ceil(minSupport * count).toLong
+    val numParts = if (numPartitions > 0) numPartitions else data.partitions.length
+    val partitioner = new HashPartitioner(numParts)
+    val freqItems = genFreqItems(data, minCount, partitioner)
+    val freqItemsets = genFreqItemsets(data, minCount, freqItems, partitioner)
+    new FPGrowthModel(freqItemsets)
   }
 
   /**
-   * Generate single item pattern by filtering the input data using minimal support level
-   * @return array of frequent pattern with its count
+   * Generates frequent items by filtering the input data using minimal support level.
+   * @param minCount minimum count for frequent itemsets
+   * @param partitioner partitioner used to distribute items
+   * @return array of frequent pattern ordered by their frequencies
    */
-  private def generateSingleItem(
+  private def genFreqItems(
       data: RDD[Array[String]],
-      minCount: Double): RDD[(String, Long)] = {
-    val single = data.flatMap(v => v.toSet)
-      .map(v => (v, 1L))
-      .reduceByKey(_ + _)
+      minCount: Long,
+      partitioner: Partitioner): Array[String] = {
+    data.flatMap { t =>
+      val uniq = t.toSet
+      if (t.length != uniq.size) {
+        throw new SparkException(s"Items in a transaction must be unique but got ${t.toSeq}.")
+      }
+      t
+    }.map(v => (v, 1L))
+      .reduceByKey(partitioner, _ + _)
       .filter(_._2 >= minCount)
-      .sortBy(_._2)
-    single
+      .collect()
+      .sortBy(-_._2)
+      .map(_._1)
   }
 
   /**
-   * Generate combination of frequent pattern by computing on FPTree,
-   * the computation is done on each FPTree partitions.
-   * @return array of frequent pattern with its count
+   * Generate frequent itemsets by building FP-Trees, the extraction is done on each partition.
+   * @param data transactions
+   * @param minCount minimum count for frequent itemsets
+   * @param freqItems frequent items
+   * @param partitioner partitioner used to distribute transactions
+   * @return an RDD of (frequent itemset, count)
    */
-  private def generateCombinations(
+  private def genFreqItemsets(
       data: RDD[Array[String]],
-      minCount: Double,
-      singleItem: RDD[(String, Long)]): RDD[(Array[String], Long)] = {
-    val single = data.context.broadcast(singleItem.collect())
-    data.flatMap(transaction => createConditionPatternBase(transaction, single))
-      .aggregateByKey(new FPTree)(
-        (aggregator, condPattBase) => aggregator.add(condPattBase),
-        (aggregator1, aggregator2) => aggregator1.merge(aggregator2))
-      .flatMap(partition => partition._2.mine(minCount, partition._1))
+      minCount: Long,
+      freqItems: Array[String],
+      partitioner: Partitioner): RDD[(Array[String], Long)] = {
+    val itemToRank = freqItems.zipWithIndex.toMap
+    data.flatMap { transaction =>
+      genCondTransactions(transaction, itemToRank, partitioner)
+    }.aggregateByKey(new FPTree[Int], partitioner.numPartitions)(
+      (tree, transaction) => tree.add(transaction, 1L),
+      (tree1, tree2) => tree1.merge(tree2))
+    .flatMap { case (part, tree) =>
+      tree.extract(minCount, x => partitioner.getPartition(x) == part)
+    }.map { case (ranks, count) =>
+      (ranks.map(i => freqItems(i)).toArray, count)
+    }
   }
 
   /**
-   * Create FP-Tree partition for the giving basket
-   * @return an array contains a tuple, whose first element is the single
-   *         item (hash key) and second element is its condition pattern base
+   * Generates conditional transactions.
+   * @param transaction a transaction
+   * @param itemToRank map from item to their rank
+   * @param partitioner partitioner used to distribute transactions
+   * @return a map of (target partition, conditional transaction)
    */
-  private def createConditionPatternBase(
+  private def genCondTransactions(
       transaction: Array[String],
-      singleBC: Broadcast[Array[(String, Long)]]): Array[(String, Array[String])] = {
-    var output = ArrayBuffer[(String, Array[String])]()
-    var combination = ArrayBuffer[String]()
-    var items = ArrayBuffer[(String, Long)]()
-    val single = singleBC.value
-    val singleMap = single.toMap
-
-    // Filter the basket by single item pattern and sort
-    // by single item and its count
-    val candidates = transaction
-      .filter(singleMap.contains)
-      .map(item => (item, singleMap(item)))
-      .sortBy(_._1)
-      .sortBy(_._2)
-      .toArray
-
-    val itemIterator = candidates.iterator
-    while (itemIterator.hasNext) {
-      combination.clear()
-      val item = itemIterator.next()
-      val firstNItems = candidates.take(candidates.indexOf(item))
-      if (firstNItems.length > 0) {
-        val iterator = firstNItems.iterator
-        while (iterator.hasNext) {
-          val elem = iterator.next()
-          combination += elem._1
-        }
-        output += ((item._1, combination.toArray))
+      itemToRank: Map[String, Int],
+      partitioner: Partitioner): mutable.Map[Int, Array[Int]] = {
+    val output = mutable.Map.empty[Int, Array[Int]]
+    // Filter the basket by frequent items pattern and sort their ranks.
+    val filtered = transaction.flatMap(itemToRank.get)
+    ju.Arrays.sort(filtered)
+    val n = filtered.length
+    var i = n - 1
+    while (i >= 0) {
+      val item = filtered(i)
+      val part = partitioner.getPartition(item)
+      if (!output.contains(part)) {
+        output(part) = filtered.slice(0, i + 1)
       }
+      i -= 1
     }
-    output.toArray
+    output
   }
-
 }
-
-/**
- * Top-level methods for calling FPGrowth.
- */
-object FPGrowth{
-
-  /**
-   * Generate a FPGrowth Model using the given minimal support level.
-   *
-   * @param data input baskets stored as `RDD[Array[String]]`
-   * @param minSupport minimal support level, for example 0.5
-   */
-  def train(data: RDD[Array[String]], minSupport: Double): FPGrowthModel = {
-    new FPGrowth().setMinSupport(minSupport).run(data)
-  }
-}
-