Swift Queue

Queue.swift

struct Queue<Element> {
  private var frontArray: [Element] = []
  private var backArray: [Element] = []
  
  mutating func append(_ element: Element) {
    backArray.append(element)
  }
  
  mutating func popFirst() -> Element? {
    if !frontArray.isEmpty { return frontArray.popLast() }
    frontArray = backArray.reversed()
    backArray = []
    return frontArray.popLast()
  }
}

extension Queue: Collection {
  
  typealias Element = Element
  
  var startIndex: Int {
    0
  }
  
  var endIndex: Int {
    frontArray.count + backArray.count
  }
  
  subscript(position: Int) -> Element {
    precondition(startIndex..<endIndex ~= position, "Index out of bounds")
    if position < frontArray.count {
      return frontArray[frontArray.count - position - 1]
    }
    return backArray[position - frontArray.count]
  }
  
  func index(after i: Int) -> Int {
    precondition(i < endIndex, "Can't advance beyond endIndex")
    return i + 1
  }
}

extension Queue: ExpressibleByArrayLiteral {
  init(arrayLiteral elements: Element...) {
    backArray = elements
  }
}