apache
diff --git a/‎core/src/main/scala/org/apache/spark/rdd/SortedMergeCoalescedRDD.scala‎
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diff --git a/‎core/src/test/scala/org/apache/spark/rdd/SortedMergeCoalescedRDDSuite.scala‎
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+/*
+ * 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.io.{IOException, ObjectOutputStream}
+
+import scala.collection.mutable
+import scala.reflect.ClassTag
+
+import org.apache.spark.{Partition, TaskContext}
+import org.apache.spark.util.Utils
+
+/**
+ * An RDD that coalesces partitions while preserving ordering through k-way merge.
+ *
+ * Unlike CoalescedRDD which simply concatenates partitions, this RDD performs a sorted
+ * merge of multiple input partitions to maintain ordering. This is useful when input
+ * partitions are locally sorted and we want to preserve that ordering after coalescing.
+ *
+ * The merge is performed using a priority queue (min-heap) which provides O(n log k)
+ * time complexity, where n is the total number of elements and k is the number of
+ * partitions being merged.
+ *
+ * @param prev The parent RDD
+ * @param numPartitions The number of output partitions after coalescing
+ * @param ordering The ordering to maintain during merge
+ * @param partitionCoalescer The coalescer defining how to group input partitions
+ * @tparam T The element type
+ */
+private[spark] class SortedMergeCoalescedRDD[T: ClassTag](
+    @transient var prev: RDD[T],
+    numPartitions: Int,
+    partitionCoalescer: PartitionCoalescer,
+    ordering: Ordering[T])
+  extends RDD[T](prev.context, Nil) {
+
+  override def getPartitions: Array[Partition] = {
+    partitionCoalescer.coalesce(numPartitions, prev).zipWithIndex.map {
+      case (pg, i) =>
+        val parentIndices = pg.partitions.map(_.index).toSeq
+        new SortedMergePartition(i, prev, parentIndices, pg.prefLoc)
+    }
+  }
+
+  override def compute(partition: Partition, context: TaskContext): Iterator[T] = {
+    val mergePartition = partition.asInstanceOf[SortedMergePartition]
+    val parentPartitions = mergePartition.parents
+
+    if (parentPartitions.isEmpty) {
+      Iterator.empty
+    } else if (parentPartitions.length == 1) {
+      // No merge needed for single partition
+      firstParent[T].iterator(parentPartitions.head, context)
+    } else {
+      // Perform k-way merge
+      new SortedMergeIterator[T](
+        parentPartitions.map(p => firstParent[T].iterator(p, context)),
+        ordering
+      )
+    }
+  }
+
+  override def getDependencies: Seq[org.apache.spark.Dependency[_]] = {
+    Seq(new org.apache.spark.NarrowDependency(prev) {
+      def getParents(id: Int): Seq[Int] =
+        partitions(id).asInstanceOf[SortedMergePartition].parentsIndices
+    })
+  }
+
+  override def getPreferredLocations(partition: Partition): Seq[String] = {
+    partition.asInstanceOf[SortedMergePartition].prefLoc.toSeq
+  }
+
+  override def clearDependencies(): Unit = {
+    super.clearDependencies()
+    prev = null
+  }
+}
+
+/**
+ * Partition for SortedMergeCoalescedRDD that tracks which parent partitions to merge.
+ * @param index of this coalesced partition
+ * @param rdd which it belongs to
+ * @param parentsIndices list of indices in the parent that have been coalesced into this partition
+ * @param prefLoc the preferred location for this partition
+ */
+private[spark] class SortedMergePartition(
+    idx: Int,
+    @transient private val rdd: RDD[_],
+    val parentsIndices: Seq[Int],
+    val prefLoc: Option[String] = None) extends Partition {
+  override val index: Int = idx
+  var parents: Seq[Partition] = parentsIndices.map(rdd.partitions(_))
+
+  @throws(classOf[IOException])
+  private def writeObject(oos: ObjectOutputStream): Unit = Utils.tryOrIOException {
+    // Update the reference to parent partition at the time of task serialization
+    parents = parentsIndices.map(rdd.partitions(_))
+    oos.defaultWriteObject()
+  }
+}
+
+/**
+ * Iterator that performs k-way merge of sorted iterators.
+ *
+ * Uses a priority queue (min-heap) to efficiently find the next smallest element
+ * across all input iterators according to the specified ordering. This provides
+ * O(n log k) time complexity where n is the total number of elements and k is
+ * the number of iterators being merged.
+ *
+ * @param iterators The sequence of sorted iterators to merge
+ * @param ordering The ordering to use for comparison
+ * @tparam T The element type
+ */
+private[spark] class SortedMergeIterator[T](
+    iterators: Seq[Iterator[T]],
+    ordering: Ordering[T]) extends Iterator[T] {
+
+  // Priority queue entry: (current element, iterator index)
+  private case class QueueEntry(element: T, iteratorIdx: Int)
+
+  // Min-heap ordered by element according to the provided ordering
+  private implicit val queueOrdering: Ordering[QueueEntry] = new Ordering[QueueEntry] {
+    override def compare(x: QueueEntry, y: QueueEntry): Int = {
+      // Reverse for min-heap (PriorityQueue is max-heap by default)
+      ordering.compare(y.element, x.element)
+    }
+  }
+
+  private val queue = mutable.PriorityQueue.empty[QueueEntry]
+
+  // Initialize queue with first element from each non-empty iterator
+  iterators.zipWithIndex.foreach { case (iter, idx) =>
+    if (iter.hasNext) {
+      queue.enqueue(QueueEntry(iter.next(), idx))
+    }
+  }
+
+  override def hasNext: Boolean = queue.nonEmpty
+
+  override def next(): T = {
+    if (!hasNext) {
+      throw new NoSuchElementException("next on empty iterator")
+    }
+
+    val entry = queue.dequeue()
+    val result = entry.element
+
+    // If the iterator has more elements, add the next one to the queue
+    val iter = iterators(entry.iteratorIdx)
+    if (iter.hasNext) {
+      queue.enqueue(QueueEntry(iter.next(), entry.iteratorIdx))
+    }
+
+    result
+  }
+}
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+/*
+ * 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 org.apache.spark.{SharedSparkContext, SparkFunSuite}
+
+class SortedMergeCoalescedRDDSuite extends SparkFunSuite with SharedSparkContext {
+
+  test("SPARK-55715: k-way merge maintains ordering - integers") {
+    // Create RDD with 4 partitions, each sorted
+    val data = Seq(
+      Seq(1, 5, 9, 13),  // partition 0
+      Seq(2, 6, 10, 14), // partition 1
+      Seq(3, 7, 11, 15), // partition 2
+      Seq(4, 8, 12, 16)  // partition 3
+    )
+
+    val rdd = sc.parallelize(data, data.size).flatMap(identity)
+
+    // Coalesce to 2 partitions using sorted merge
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0, 1), Seq(2, 3)))
+    val merged = new SortedMergeCoalescedRDD[Int](rdd, 2, coalescer, Ordering.Int)
+
+    // Verify per-partition contents: group (0,1) merges elements from partitions 0+1,
+    // group (2,3) merges elements from partitions 2+3
+    val partitionData = merged
+      .mapPartitionsWithIndex { (idx, iter) => Iterator.single((idx, iter.toSeq)) }
+      .collect().toMap
+
+    assert(partitionData(0) === Seq(1, 2, 5, 6, 9, 10, 13, 14))
+    assert(partitionData(1) === Seq(3, 4, 7, 8, 11, 12, 15, 16))
+  }
+
+  test("SPARK-55715: k-way merge handles empty partitions") {
+    val data = Seq(
+      Seq(1, 5, 9),   // partition 0
+      Seq.empty[Int], // partition 1 - empty
+      Seq(3, 7, 11),  // partition 2
+      Seq.empty[Int]  // partition 3 - empty
+    )
+
+    val rdd = sc.parallelize(data, data.size).flatMap(identity)
+
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0, 1, 2, 3)))
+    val merged = new SortedMergeCoalescedRDD[Int]( rdd, 1, coalescer, Ordering.Int)
+
+    val result = merged.collect()
+    assert(result === Seq(1, 3, 5, 7, 9, 11))
+  }
+
+  test("SPARK-55715: k-way merge handles all-empty partitions in a group") {
+    val data = Seq(
+      Seq.empty[Int], // partition 0 - empty
+      Seq.empty[Int], // partition 1 - empty
+      Seq(1, 2, 3)    // partition 2 - non-empty (different group)
+    )
+
+    val rdd = sc.parallelize(data, data.size).flatMap(identity)
+
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0, 1), Seq(2)))
+    val merged = new SortedMergeCoalescedRDD[Int]( rdd, 2, coalescer, Ordering.Int)
+
+    val partitionData = merged
+      .mapPartitionsWithIndex { (idx, iter) => Iterator.single((idx, iter.toSeq)) }
+      .collect().toMap
+
+    assert(partitionData(0) === Seq.empty)
+    assert(partitionData(1) === Seq(1, 2, 3))
+  }
+
+  test("SPARK-55715: k-way merge with single partition per group - no merge needed") {
+    val data = Seq(1, 2, 3, 4, 5, 6)
+    val rdd = sc.parallelize(data, 3)
+
+    // Each group has only 1 partition - should just pass through
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0), Seq(1), Seq(2)))
+    val merged = new SortedMergeCoalescedRDD[Int](rdd, 3, coalescer, Ordering.Int)
+
+    assert(merged.collect() === data)
+  }
+
+  test("SPARK-55715: k-way merge with reverse ordering") {
+    val data = Seq(
+      Seq(13, 9, 5, 1),  // partition 0 - descending
+      Seq(14, 10, 6, 2), // partition 1 - descending
+      Seq(15, 11, 7, 3), // partition 2 - descending
+      Seq(16, 12, 8, 4)  // partition 3 - descending
+    )
+
+    val rdd = sc.parallelize(data, data.size).flatMap(identity)
+
+    // Use reverse ordering
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0, 1, 2, 3)))
+    val merged = new SortedMergeCoalescedRDD[Int](
+      rdd, 1, coalescer, Ordering.Int.reverse)
+
+    val result = merged.collect()
+    assert(result === (1 to 16).reverse)
+  }
+
+  test("SPARK-55715: k-way merge with many partitions") {
+    val numPartitions = 20
+    val rowsPerPartition = 50
+
+    // Create sorted data where partition i starts at i and increments by numPartitions
+    val data = (0 until numPartitions).map { partIdx =>
+      (0 until rowsPerPartition).map(i => partIdx + i * numPartitions)
+    }
+
+    val rdd = sc.parallelize(data, numPartitions).flatMap(identity)
+
+    // Coalesce all partitions into one
+    val coalescer = new TestPartitionCoalescer(Seq((0 until numPartitions)))
+    val merged = new SortedMergeCoalescedRDD[Int](rdd, 1, coalescer, Ordering.Int)
+
+    val result = merged.collect()
+    assert(result.length === numPartitions * rowsPerPartition)
+    assert(result === result.sorted)
+  }
+
+  test("SPARK-55715: k-way merge preserves duplicate elements across partitions") {
+    val data = Seq(
+      Seq(1, 2, 3), // partition 0
+      Seq(1, 2, 3), // partition 1 - identical to partition 0
+      Seq(2, 2, 4)  // partition 2 - contains repeated value within partition
+    )
+
+    val rdd = sc.parallelize(data, data.size).flatMap(identity)
+
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0, 1, 2)))
+    val merged = new SortedMergeCoalescedRDD[Int](rdd, 1, coalescer, Ordering.Int)
+
+    val result = merged.collect()
+    assert(result === Seq(1, 1, 2, 2, 2, 2, 3, 3, 4))
+  }
+
+  test("SPARK-55715: k-way merge with strings") {
+    val data = Seq(
+      Seq("apple", "cherry", "grape"),
+      Seq("banana", "date", "kiwi"),
+      Seq("apricot", "fig", "mango")
+    )
+
+    val rdd = sc.parallelize(data, data.size).flatMap(identity)
+
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0, 1, 2)))
+    val merged = new SortedMergeCoalescedRDD[String](rdd, 1, coalescer, Ordering.String)
+
+    val result = merged.collect()
+    assert(result === result.sorted)
+  }
+
+  test("SPARK-55715: k-way merge with tuples") {
+    val data = Seq(
+      Seq((1, "a"), (3, "c"), (5, "e")),
+      Seq((2, "b"), (4, "d"), (6, "f")),
+      Seq((1, "z"), (4, "y"), (7, "x"))
+    )
+
+    val rdd = sc.parallelize(data, data.size).flatMap(identity)
+
+    implicit val tupleOrdering: Ordering[(Int, String)] = Ordering.by[(Int, String), Int](_._1)
+
+    val coalescer = new TestPartitionCoalescer(Seq(Seq(0, 1, 2)))
+    val merged = new SortedMergeCoalescedRDD[(Int, String)](rdd, 1, coalescer, tupleOrdering)
+
+    val result = merged.collect()
+    val expected = data.flatten.sortBy(_._1)
+    assert(result === expected)
+  }
+
+  test("SPARK-55715: SortedMergeIterator - next() on empty iterator throws " +
+      "NoSuchElementException") {
+    val iter = new SortedMergeIterator[Int](Seq.empty, Ordering.Int)
+    assert(!iter.hasNext)
+    intercept[NoSuchElementException] { iter.next() }
+  }
+
+  test("SPARK-55715: SortedMergeIterator - empty iterators list") {
+    val iter = new SortedMergeIterator[Int](Seq(Iterator.empty, Iterator.empty), Ordering.Int)
+    assert(!iter.hasNext)
+    assert(iter.toSeq === Seq.empty)
+  }
+
+  test("SPARK-55715: SortedMergeIterator - single iterator passes through unchanged") {
+    val iter = new SortedMergeIterator[Int](Seq(Iterator(3, 1, 2)), Ordering.Int)
+    assert(iter.toSeq === Seq(3, 1, 2))
+  }
+}
+
+/**
+ * Test partition coalescer that groups partitions according to a predefined plan.
+ */
+class TestPartitionCoalescer(grouping: Seq[Seq[Int]]) extends PartitionCoalescer with Serializable {
+  override def coalesce(maxPartitions: Int, parent: RDD[_]): Array[PartitionGroup] = {
+    grouping.map { partitionIndices =>
+      val pg = new PartitionGroup(None)
+      partitionIndices.foreach { idx =>
+        pg.partitions += parent.partitions(idx)
+      }
+      pg
+    }.toArray
+  }
+}