A priority queue is somewhat similar to a queue, with an important distinction: each item is added to a priority queue with a priority level, and will be later removed from the queue with the highest priority element first. That is, the items are (conceptually) stored in the queue in priority order instead of in insertion order.



Task

Create a priority queue.   The queue must support at least two operations:

   Insertion.   An element is added to the queue with a priority (a numeric value).
   Top item removal.   Deletes the element or one of the elements with the current top priority and return it.


Optionally, other operations may be defined, such as peeking (find what current top priority/top element is), merging (combining two priority queues into one), etc.



To test your implementation, insert a number of elements into the queue, each with some random priority.

Then dequeue them sequentially; now the elements should be sorted by priority.

You can use the following task/priority items as input data:

        Priority         Task
       ══════════   ════════════════
           3         Clear drains
           4         Feed cat
           5         Make tea
           1         Solve RC tasks
           2         Tax return



The implementation should try to be efficient.   A typical implementation has   O(log n)   insertion and extraction time,   where   n   is the number of items in the queue.

You may choose to impose certain limits such as small range of allowed priority levels, limited capacity, etc.   If so, discuss the reasons behind it.




class Task : Comparable, CustomStringConvertible {
  var priority : Int
  var name: String
  init(priority: Int, name: String) {
    self.priority = priority
    self.name = name
  }
  var description: String {
    return "\(priority), \(name)"
  }
}
func ==(t1: Task, t2: Task) -> Bool {
  return t1.priority == t2.priority
}
func <(t1: Task, t2: Task) -> Bool {
  return t1.priority < t2.priority
}
 
struct TaskPriorityQueue {
  let heap : CFBinaryHeapRef = {
    var callBacks = CFBinaryHeapCallBacks(version: 0, retain: {
      UnsafePointer(Unmanaged<Task>.fromOpaque(COpaquePointer($1)).retain().toOpaque())
      }, release: {
        Unmanaged<Task>.fromOpaque(COpaquePointer($1)).release()
      }, copyDescription: nil, compare: { (ptr1, ptr2, _) in
        let t1 : Task = Unmanaged<Task>.fromOpaque(COpaquePointer(ptr1)).takeUnretainedValue()
        let t2 : Task = Unmanaged<Task>.fromOpaque(COpaquePointer(ptr2)).takeUnretainedValue()
        return t1 == t2 ? CFComparisonResult.CompareEqualTo : t1 < t2 ? CFComparisonResult.CompareLessThan : CFComparisonResult.CompareGreaterThan
    })
    return CFBinaryHeapCreate(nil, 0, &callBacks, nil)
  }()
  var count : Int { return CFBinaryHeapGetCount(heap) }
  mutating func push(t: Task) {
    CFBinaryHeapAddValue(heap, UnsafePointer(Unmanaged.passUnretained(t).toOpaque()))
  }
  func peek() -> Task {
    return Unmanaged<Task>.fromOpaque(COpaquePointer(CFBinaryHeapGetMinimum(heap))).takeUnretainedValue()
  }
  mutating func pop() -> Task {
    let result = Unmanaged<Task>.fromOpaque(COpaquePointer(CFBinaryHeapGetMinimum(heap))).takeUnretainedValue()
    CFBinaryHeapRemoveMinimumValue(heap)
    return result
  }
}
 
var pq = TaskPriorityQueue()
 
pq.push(Task(priority: 3, name: "Clear drains"))
pq.push(Task(priority: 4, name: "Feed cat"))
pq.push(Task(priority: 5, name: "Make tea"))
pq.push(Task(priority: 1, name: "Solve RC tasks"))
pq.push(Task(priority: 2, name: "Tax return"))
 
while pq.count != 0 {
  print(pq.pop())
}