/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.spark.rdd

import java.util.{HashMap => JHashMap}

import scala.collection.mutable.ArrayBuffer
import scala.jdk.CollectionConverters._
import scala.reflect.ClassTag

import org.apache.spark.Dependency
import org.apache.spark.OneToOneDependency
import org.apache.spark.Partition
import org.apache.spark.Partitioner
import org.apache.spark.ShuffleDependency
import org.apache.spark.SparkEnv
import org.apache.spark.TaskContext

/**
 * An optimized version of cogroup for set difference/subtraction.
 *
 * It is possible to implement this operation with just `cogroup`, but
 * that is less efficient because all of the entries from `rdd2`, for
 * both matching and non-matching values in `rdd1`, are kept in the
 * JHashMap until the end.
 *
 * With this implementation, only the entries from `rdd1` are kept in-memory,
 * and the entries from `rdd2` are essentially streamed, as we only need to
 * touch each once to decide if the value needs to be removed.
 *
 * This is particularly helpful when `rdd1` is much smaller than `rdd2`, as
 * you can use `rdd1`'s partitioner/partition size and not worry about running
 * out of memory because of the size of `rdd2`.
 */
private[spark] class SubtractedRDD[K: ClassTag, V: ClassTag, W: ClassTag](
    @transient var rdd1: RDD[_ <: Product2[K, V]],
    @transient var rdd2: RDD[_ <: Product2[K, W]],
    part: Partitioner)
  extends RDD[(K, V)](rdd1.context, Nil) {


  override def getDependencies: Seq[Dependency[_]] = {
    def rddDependency[T1: ClassTag, T2: ClassTag](rdd: RDD[_ <: Product2[T1, T2]])
      : Dependency[_] = {
      if (rdd.partitioner == Some(part)) {
        logDebug("Adding one-to-one dependency with " + rdd)
        new OneToOneDependency(rdd)
      } else {
        logDebug("Adding shuffle dependency with " + rdd)
        new ShuffleDependency[T1, T2, Any](rdd, part)
      }
    }
    Seq(rddDependency[K, V](rdd1), rddDependency[K, W](rdd2))
  }

  override def getPartitions: Array[Partition] = {
    val array = new Array[Partition](part.numPartitions)
    for (i <- array.indices) {
      // Each CoGroupPartition will depend on rdd1 and rdd2
      array(i) = new CoGroupPartition(i, Seq(rdd1, rdd2).zipWithIndex.map { case (rdd, j) =>
        dependencies(j) match {
          case s: ShuffleDependency[_, _, _] =>
            None
          case _ =>
            Some(new NarrowCoGroupSplitDep(rdd, i, rdd.partitions(i)))
        }
      }.toArray)
    }
    array
  }

  override val partitioner = Some(part)

  override def compute(p: Partition, context: TaskContext): Iterator[(K, V)] = {
    val partition = p.asInstanceOf[CoGroupPartition]
    val map = new JHashMap[K, ArrayBuffer[V]]
    def getSeq(k: K): ArrayBuffer[V] = {
      val seq = map.get(k)
      if (seq != null) {
        seq
      } else {
        val seq = new ArrayBuffer[V]()
        map.put(k, seq)
        seq
      }
    }
    def integrate(depNum: Int, op: Product2[K, V] => Unit): Unit = {
      dependencies(depNum) match {
        case oneToOneDependency: OneToOneDependency[_] =>
          val dependencyPartition = partition.narrowDeps(depNum).get.split
          oneToOneDependency.rdd.iterator(dependencyPartition, context)
            .asInstanceOf[Iterator[Product2[K, V]]].foreach(op)

        case shuffleDependency: ShuffleDependency[_, _, _] =>
          val metrics = context.taskMetrics().createTempShuffleReadMetrics()
          val iter = SparkEnv.get.shuffleManager
            .getReader(
              shuffleDependency.shuffleHandle,
              partition.index,
              partition.index + 1,
              context,
              metrics)
            .read()
          iter.foreach(op)
      }
    }

    // the first dep is rdd1; add all values to the map
    integrate(0, t => getSeq(t._1) += t._2)
    // the second dep is rdd2; remove all of its keys
    integrate(1, t => map.remove(t._1))
    map.asScala.iterator.flatMap(t => t._2.iterator.map((t._1, _)))
  }

  override def clearDependencies(): Unit = {
    super.clearDependencies()
    rdd1 = null
    rdd2 = null
  }

}
