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/*
* Scala (https://www.scala-lang.org)
*
* Copyright EPFL and Lightbend, Inc.
*
* Licensed under Apache License 2.0
* (http://www.apache.org/licenses/LICENSE-2.0).
*
* See the NOTICE file distributed with this work for
* additional information regarding copyright ownership.
*/
package scala.collection
package mutable
import scala.collection.generic.DefaultSerializationProxy
import scala.math.Ordering
/** This class implements priority queues using a heap.
* To prioritize elements of type A there must be an implicit
* Ordering[A] available at creation.
*
* If multiple elements have the same priority in the ordering of this
* PriorityQueue, no guarantees are made regarding the order in which elements
* are returned by `dequeue` or `dequeueAll`. In particular, that means this
* class does not guarantee first in first out behaviour that may be
* incorrectly inferred from the Queue part of the name of this class.
*
* Only the `dequeue` and `dequeueAll` methods will return elements in priority
* order (while removing elements from the heap). Standard collection methods
* including `drop`, `iterator`, and `toString` will remove or traverse the heap
* in whichever order seems most convenient.
*
* Therefore, printing a `PriorityQueue` will not reveal the priority order of
* the elements, though the highest-priority element will be printed first. To
* print the elements in order, one must duplicate the `PriorityQueue` (by using
* `clone`, for instance) and then dequeue them:
*
* @example {{{
* val pq = collection.mutable.PriorityQueue(1, 2, 5, 3, 7)
* println(pq) // elements probably not in order
* println(pq.clone.dequeueAll) // prints ArraySeq(7, 5, 3, 2, 1)
* }}}
*
* @tparam A type of the elements in this priority queue.
* @param ord implicit ordering used to compare the elements of type `A`.
*
* @define Coll PriorityQueue
* @define coll priority queue
* @define orderDependent
* @define orderDependentFold
* @define mayNotTerminateInf
* @define willNotTerminateInf
*/
sealed class PriorityQueue[A](implicit val ord: Ordering[A])
extends AbstractIterable[A]
with Iterable[A]
with IterableOps[A, Iterable, PriorityQueue[A]]
with StrictOptimizedIterableOps[A, Iterable, PriorityQueue[A]]
with Builder[A, PriorityQueue[A]]
with Cloneable[PriorityQueue[A]]
with Growable[A]
with Serializable
{
private class ResizableArrayAccess[A0] extends ArrayBuffer[A0] {
override def mapInPlace(f: A0 => A0): this.type = {
var i = 1 // see "we do not use array(0)" comment below (???)
val siz = this.size
while (i < siz) { this(i) = f(this(i)); i += 1 }
this
}
def p_size0 = size0
def p_size0_=(s: Int) = size0 = s
def p_array = array
def p_ensureSize(n: Int) = super.ensureSize(n)
def p_swap(a: Int, b: Int): Unit = {
val h = array(a)
array(a) = array(b)
array(b) = h
}
}
private val resarr = new ResizableArrayAccess[A]
resarr.p_size0 += 1 // we do not use array(0) TODO: explain -- what is the first element even for?
def length: Int = resarr.length - 1 // adjust length accordingly
override def size: Int = length
override def knownSize: Int = length
override def isEmpty: Boolean = resarr.p_size0 < 2
// not eligible for EvidenceIterableFactoryDefaults since C != CC[A] (PriorityQueue[A] != Iterable[A])
override protected def fromSpecific(coll: scala.collection.IterableOnce[A]): PriorityQueue[A] = PriorityQueue.from(coll)
override protected def newSpecificBuilder: Builder[A, PriorityQueue[A]] = PriorityQueue.newBuilder
override def empty: PriorityQueue[A] = PriorityQueue.empty
def mapInPlace(f: A => A): this.type = {
resarr.mapInPlace(f)
heapify(1)
this
}
def result() = this
private def toA(x: AnyRef): A = x.asInstanceOf[A]
protected def fixUp(as: Array[AnyRef], m: Int): Unit = {
var k: Int = m
// use `ord` directly to avoid allocating `OrderingOps`
while (k > 1 && ord.lt(toA(as(k / 2)), toA(as(k)))) {
resarr.p_swap(k, k / 2)
k = k / 2
}
}
protected def fixDown(as: Array[AnyRef], m: Int, n: Int): Boolean = {
// returns true if any swaps were done (used in heapify)
var k: Int = m
while (n >= 2 * k) {
var j = 2 * k
// use `ord` directly to avoid allocating `OrderingOps`
if (j < n && ord.lt(toA(as(j)), toA(as(j + 1))))
j += 1
if (ord.gteq(toA(as(k)), toA(as(j))))
return k != m
else {
val h = as(k)
as(k) = as(j)
as(j) = h
k = j
}
}
k != m
}
/** Inserts a single element into the priority queue.
*
* @param elem the element to insert.
* @return this $coll.
*/
def addOne(elem: A): this.type = {
resarr.p_ensureSize(resarr.p_size0 + 1)
resarr.p_array(resarr.p_size0) = elem.asInstanceOf[AnyRef]
fixUp(resarr.p_array, resarr.p_size0)
resarr.p_size0 += 1
this
}
override def addAll(xs: IterableOnce[A]): this.type = {
val from = resarr.p_size0
for (x <- xs.iterator) unsafeAdd(x)
heapify(from)
this
}
private def unsafeAdd(elem: A): Unit = {
// like += but skips fixUp, which breaks the ordering invariant
// a series of unsafeAdds MUST be followed by heapify
resarr.p_ensureSize(resarr.p_size0 + 1)
resarr.p_array(resarr.p_size0) = elem.asInstanceOf[AnyRef]
resarr.p_size0 += 1
}
private def heapify(from: Int): Unit = {
// elements at indices 1..from-1 were already in heap order before any adds
// elements at indices from..n are newly added, their order must be fixed
val n = length
if (from <= 2) {
// no pre-existing order to maintain, do the textbook heapify algorithm
for (i <- n/2 to 1 by -1) fixDown(resarr.p_array, i, n)
}
else if (n - from < 4) {
// for very small adds, doing the simplest fix is faster
for (i <- from to n) fixUp(resarr.p_array, i)
}
else {
var min = from/2 // tracks the minimum element in the queue
val queue = scala.collection.mutable.Queue[Int](min)
// do fixDown on the parents of all the new elements
// except the parent of the first new element, which is in the queue
// (that parent is treated specially because it might be the root)
for (i <- n/2 until min by -1) {
if (fixDown(resarr.p_array, i, n)) {
// there was a swap, so also need to fixDown i's parent
val parent = i/2
if (parent < min) { // make sure same parent isn't added twice
min = parent
queue += parent
}
}
}
while (queue.nonEmpty) {
val i = queue.dequeue()
if (fixDown(resarr.p_array, i, n)) {
val parent = i/2
if (parent < min && parent > 0) {
// the "parent > 0" is to avoid adding the parent of the root
min = parent
queue += parent
}
}
}
}
}
/** Adds all elements provided by a `IterableOnce` object
* into the priority queue.
*
* @param xs a iterable object.
* @return a new priority queue containing elements of both `xs` and `this`.
*/
def ++(xs: IterableOnce[A]): PriorityQueue[A] = { this.clone() ++= xs }
/** Adds all elements to the queue.
*
* @param elems the elements to add.
*/
def enqueue(elems: A*): Unit = { this ++= elems }
/** Returns the element with the highest priority in the queue,
* and removes this element from the queue.
*
* @throws NoSuchElementException
* @return the element with the highest priority.
*/
def dequeue(): A =
if (resarr.p_size0 > 1) {
resarr.p_size0 = resarr.p_size0 - 1
val result = resarr.p_array(1)
resarr.p_array(1) = resarr.p_array(resarr.p_size0)
resarr.p_array(resarr.p_size0) = null // erase reference from array
fixDown(resarr.p_array, 1, resarr.p_size0 - 1)
toA(result)
} else
throw new NoSuchElementException("no element to remove from heap")
def dequeueAll[A1 >: A]: immutable.Seq[A1] = {
val b = ArrayBuilder.make[Any]
b.sizeHint(size)
while (nonEmpty) {
b += dequeue()
}
immutable.ArraySeq.unsafeWrapArray(b.result()).asInstanceOf[immutable.ArraySeq[A1]]
}
/** Returns the element with the highest priority in the queue,
* or throws an error if there is no element contained in the queue.
*
* @return the element with the highest priority.
*/
override def head: A = if (resarr.p_size0 > 1) toA(resarr.p_array(1)) else throw new NoSuchElementException("queue is empty")
/** Removes all elements from the queue. After this operation is completed,
* the queue will be empty.
*/
def clear(): Unit = {
resarr.clear()
resarr.p_size0 = 1
}
/** Returns an iterator which yields all the elements.
*
* Note: The order of elements returned is undefined.
* If you want to traverse the elements in priority queue
* order, use `clone().dequeueAll.iterator`.
*
* @return an iterator over all the elements.
*/
override def iterator: Iterator[A] = resarr.iterator.drop(1)
/** Returns the reverse of this priority queue. The new priority queue has
* the same elements as the original, but the opposite ordering.
*
* For example, the element with the highest priority in `pq` has the lowest
* priority in `pq.reverse`, and vice versa.
*
* Ties are handled arbitrarily. Elements with equal priority may or
* may not be reversed with respect to each other.
*
* @return the reversed priority queue.
*/
def reverse: PriorityQueue[A] = {
val revq = new PriorityQueue[A]()(ord.reverse)
// copy the existing data into the new array backwards
// this won't put it exactly into the correct order,
// but will require less fixing than copying it in
// the original order
val n = resarr.p_size0
revq.resarr.p_ensureSize(n)
revq.resarr.p_size0 = n
val from = resarr.p_array
val to = revq.resarr.p_array
for (i <- 1 until n) to(i) = from(n-i)
revq.heapify(1)
revq
}
/** Returns an iterator which yields all the elements in the reverse order
* than that returned by the method `iterator`.
*
* Note: The order of elements returned is undefined.
*
* @return an iterator over all elements sorted in descending order.
*/
def reverseIterator: Iterator[A] = new AbstractIterator[A] {
private[this] var i = resarr.p_size0 - 1
def hasNext: Boolean = i >= 1
def next(): A = {
val n = resarr.p_array(i)
i -= 1
toA(n)
}
}
/** Returns a regular queue containing the same elements.
*
* Note: the order of elements is undefined.
*/
def toQueue: Queue[A] = new Queue[A] ++= this.iterator
/** Returns a textual representation of a queue as a string.
*
* @return the string representation of this queue.
*/
override def toString() = toList.mkString("PriorityQueue(", ", ", ")")
/** Converts this $coll to a list.
*
* Note: the order of elements is undefined.
*
* @return a list containing all elements of this $coll.
*/
override def toList: immutable.List[A] = immutable.List.from(this.iterator)
/** This method clones the priority queue.
*
* @return a priority queue with the same elements.
*/
override def clone(): PriorityQueue[A] = {
val pq = new PriorityQueue[A]
val n = resarr.p_size0
pq.resarr.p_ensureSize(n)
java.lang.System.arraycopy(resarr.p_array, 1, pq.resarr.p_array, 1, n-1)
pq.resarr.p_size0 = n
pq
}
override def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Int = {
val copied = IterableOnce.elemsToCopyToArray(length, xs.length, start, len)
if (copied > 0) {
Array.copy(resarr.p_array, 1, xs, start, copied)
}
copied
}
@deprecated("Use `PriorityQueue` instead", "2.13.0")
def orderedCompanion: PriorityQueue.type = PriorityQueue
protected[this] def writeReplace(): AnyRef = new DefaultSerializationProxy(PriorityQueue.evidenceIterableFactory[A], this)
override protected[this] def className = "PriorityQueue"
}
@SerialVersionUID(3L)
object PriorityQueue extends SortedIterableFactory[PriorityQueue] {
def newBuilder[A : Ordering]: Builder[A, PriorityQueue[A]] = {
new Builder[A, PriorityQueue[A]] {
val pq = new PriorityQueue[A]
def addOne(elem: A): this.type = { pq.unsafeAdd(elem); this }
def result(): PriorityQueue[A] = { pq.heapify(1); pq }
def clear(): Unit = pq.clear()
}
}
def empty[A : Ordering]: PriorityQueue[A] = new PriorityQueue[A]
def from[E : Ordering](it: IterableOnce[E]): PriorityQueue[E] = {
val b = newBuilder[E]
b ++= it
b.result()
}
}
