using System;
using System.Collections.Generic;

namespace Skyiv.Util
{
   class PriorityQueue
  {
    IComparer comparer;
    T[] heap;
    public int Count { get; private set; }
    public PriorityQueue() : this(null) { }
    public PriorityQueue(int capacity) : this(capacity, null) { }
    public PriorityQueue(IComparer comparer) : this(16, comparer) { }
    public PriorityQueue(int capacity, IComparer comparer)
    {
      this.comparer = (comparer == null) ? Comparer.Default : comparer;
      this.heap = new T[capacity];
    }
    public void Push(T v)
    {
      if (Count >= heap.Length) Array.Resize(ref heap, Count * 2);
      heap[Count] = v;
      SiftUp(Count++);
    }
    public T Pop()
    {
      var v = Top();
      heap[0] = heap[--Count];
      if (Count > 0) SiftDown(0);
      return v;
    }
    public T Top()
    {
      if (Count > 0) return heap[0];
      throw new InvalidOperationException("優先隊列為空");
    }
    void SiftUp(int n)
    {
      var v = heap[n];
      for (var n2 = n / 2; n > 0 && comparer.Compare(v, heap[n2]) > 0; n = n2, n2 /= 2)
        heap[n] = heap[n2];
      heap[n] = v;
    }
    void SiftDown(int n)
    {
      var v = heap[n];
      for (var n2 = n * 2; n2 < Count; n = n2, n2 *= 2)
      {
        if (n2 + 1 < Count && comparer.Compare(heap[n2 + 1], heap[n2]) > 0) n2++;
        if (comparer.Compare(v, heap[n2]) >= 0) break;
        heap[n] = heap[n2];
      }
      heap[n] = v;
    }
  }
}