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Pods/RxSwift/Platform/DataStructures/Queue.swift

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+//
+//  Queue.swift
+//  Platform
+//
+//  Created by Krunoslav Zaher on 3/21/15.
+//  Copyright © 2015 Krunoslav Zaher. All rights reserved.
+//
+
+/**
+Data structure that represents queue.
+
+Complexity of `enqueue`, `dequeue` is O(1) when number of operations is
+averaged over N operations.
+
+Complexity of `peek` is O(1).
+*/
+struct Queue<T>: Sequence {
+    /// Type of generator.
+    typealias Generator = AnyIterator<T>
+
+    private let resizeFactor = 2
+    
+    private var storage: ContiguousArray<T?>
+    private var innerCount = 0
+    private var pushNextIndex = 0
+    private let initialCapacity: Int
+
+    /**
+    Creates new queue.
+    
+    - parameter capacity: Capacity of newly created queue.
+    */
+    init(capacity: Int) {
+        initialCapacity = capacity
+
+        storage = ContiguousArray<T?>(repeating: nil, count: capacity)
+    }
+    
+    private var dequeueIndex: Int {
+        let index = pushNextIndex - count
+        return index < 0 ? index + storage.count : index
+    }
+    
+    /// - returns: Is queue empty.
+    var isEmpty: Bool { count == 0 }
+    
+    /// - returns: Number of elements inside queue.
+    var count: Int { innerCount }
+    
+    /// - returns: Element in front of a list of elements to `dequeue`.
+    func peek() -> T {
+        precondition(count > 0)
+        
+        return storage[dequeueIndex]!
+    }
+    
+    mutating private func resizeTo(_ size: Int) {
+        var newStorage = ContiguousArray<T?>(repeating: nil, count: size)
+        
+        let count = self.count
+        
+        let dequeueIndex = self.dequeueIndex
+        let spaceToEndOfQueue = storage.count - dequeueIndex
+        
+        // first batch is from dequeue index to end of array
+        let countElementsInFirstBatch = Swift.min(count, spaceToEndOfQueue)
+        // second batch is wrapped from start of array to end of queue
+        let numberOfElementsInSecondBatch = count - countElementsInFirstBatch
+        
+        newStorage[0 ..< countElementsInFirstBatch] = storage[dequeueIndex ..< (dequeueIndex + countElementsInFirstBatch)]
+        newStorage[countElementsInFirstBatch ..< (countElementsInFirstBatch + numberOfElementsInSecondBatch)] = storage[0 ..< numberOfElementsInSecondBatch]
+        
+        self.innerCount = count
+        pushNextIndex = count
+        storage = newStorage
+    }
+    
+    /// Enqueues `element`.
+    ///
+    /// - parameter element: Element to enqueue.
+    mutating func enqueue(_ element: T) {
+        if count == storage.count {
+            resizeTo(Swift.max(storage.count, 1) * resizeFactor)
+        }
+        
+        storage[pushNextIndex] = element
+        pushNextIndex += 1
+        innerCount += 1
+        
+        if pushNextIndex >= storage.count {
+            pushNextIndex -= storage.count
+        }
+    }
+    
+    private mutating func dequeueElementOnly() -> T {
+        precondition(count > 0)
+        
+        let index = dequeueIndex
+
+        defer {
+            storage[index] = nil
+            innerCount -= 1
+        }
+
+        return storage[index]!
+    }
+
+    /// Dequeues element or throws an exception in case queue is empty.
+    ///
+    /// - returns: Dequeued element.
+    mutating func dequeue() -> T? {
+        if self.count == 0 {
+            return nil
+        }
+
+        defer {
+            let downsizeLimit = storage.count / (resizeFactor * resizeFactor)
+            if count < downsizeLimit && downsizeLimit >= initialCapacity {
+                resizeTo(storage.count / resizeFactor)
+            }
+        }
+
+        return dequeueElementOnly()
+    }
+    
+    /// - returns: Generator of contained elements.
+    func makeIterator() -> AnyIterator<T> {
+        var i = dequeueIndex
+        var innerCount = count
+
+        return AnyIterator {
+            if innerCount == 0 {
+                return nil
+            }
+
+            defer {
+                innerCount -= 1
+                i += 1
+            }
+
+            if i >= self.storage.count {
+                i -= self.storage.count
+            }
+
+            return self.storage[i]
+        }
+    }
+}