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/*
This source file is part of the Swift.org open source project
Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
Licensed under Apache License v2.0 with Runtime Library Exception
See http://swift.org/LICENSE.txt for license information
See http://swift.org/CONTRIBUTORS.txt for Swift project authors
*/
/// This class can be used as a shared queue between multiple threads providing
/// thread safe APIs.
public final class SynchronizedQueue<Element> {
/// Linked list node.
private final class Node {
var value: Element
var next: Node?
init(_ value: Element) {
self.value = value
}
}
/// Head node of the queue.
private var head: Node? = nil
/// Tail node of the queue.
private weak var tail: Node? = nil
/// Condition variable to block the thread trying dequeue and queue is empty.
private var notEmptyCondition: Condition
/// Create a default instance of queue.
public init() {
notEmptyCondition = Condition()
}
/// Safely enqueue an element to end of the queue and signals a thread blocked on dequeue.
///
/// - Parameters:
/// - element: The element to be enqueued.
public func enqueue(_ element: Element) {
notEmptyCondition.whileLocked {
let node = Node(element)
if head == nil {
head = node
} else {
tail?.next = node
}
// Update the tail node.
tail = node
// Signal a thread blocked on dequeue.
notEmptyCondition.signal()
}
}
/// Dequeue an element from front of the queue. Blocks the calling thread until an element is available.
///
/// - Returns: First element in the queue.
public func dequeue() -> Element {
return notEmptyCondition.whileLocked {
// Wait until we have an element available in the queue.
while head == nil {
notEmptyCondition.wait()
}
// There are elements in the queue, `head` is not nil.
let element = head!.value
// Remove the first node.
head = head!.next
return element
}
}
}
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