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