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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.util
import java.io.Serializable
import java.util.{PriorityQueue => JPriorityQueue}
import scala.collection.generic.Growable
import scala.collection.JavaConverters._
/**
* Bounded priority queue. This class wraps the original PriorityQueue
* class and modifies it such that only the top K elements are retained.
* The top K elements are defined by an implicit Ordering[A].
*/
class BoundedPriorityQueue[A](maxSize: Int)(implicit ord: Ordering[A])
extends Iterable[A] with Growable[A] with Serializable {
private val underlying = new JPriorityQueue[A](maxSize, ord)
override def iterator: Iterator[A] = underlying.iterator.asScala
override def ++=(xs: TraversableOnce[A]): this.type = {
xs.foreach { this += _ }
this
}
override def +=(elem: A): this.type = {
if (size < maxSize) underlying.offer(elem)
else maybeReplaceLowest(elem)
this
}
override def +=(elem1: A, elem2: A, elems: A*): this.type = {
this += elem1 += elem2 ++= elems
}
override def clear() { underlying.clear() }
private def maybeReplaceLowest(a: A): Boolean = {
val head = underlying.peek()
if (head != null && ord.gt(a, head)) {
underlying.poll()
underlying.offer(a)
} else false
}
}
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