// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
 
using System;
using System.Collections.Generic;
using System.Text;
 
namespace Microsoft.ML.Tokenizers
{
    internal class PriorityQueue<T> where T : IComparable<T>
    {
        private readonly List<T> _data;
 
        public PriorityQueue() : this(0)
        {
        }
 
        public PriorityQueue(int capacity)
        {
            _data = new List<T>(capacity);
        }
 
        public void Enqueue(T item)
        {
            _data.Add(item);
            int ci = _data.Count - 1; // child index; start at end
            while (ci > 0)
            {
                int pi = (ci - 1) / 2; // parent index
                if (_data[ci].CompareTo(_data[pi]) >= 0) break; // child item is larger than (or equal) parent so we're done
                T tmp = _data[ci]; _data[ci] = _data[pi]; _data[pi] = tmp;
                ci = pi;
            }
        }
 
        public T Dequeue()
        {
            // assumes pq is not empty; up to calling code
            int li = _data.Count - 1; // last index (before removal)
            T frontItem = _data[0];   // fetch the front
            _data[0] = _data[li];
            _data.RemoveAt(li);
 
            --li; // last index (after removal)
            int pi = 0; // parent index. start at front of pq
            while (true)
            {
                int ci = pi * 2 + 1; // left child index of parent
                if (ci > li) break;  // no children so done
                int rc = ci + 1;     // right child
                if (rc <= li && _data[rc].CompareTo(_data[ci]) < 0) // if there is a rc (ci + 1), and it is smaller than left child, use the rc instead
                    ci = rc;
                if (_data[pi].CompareTo(_data[ci]) <= 0) break; // parent is smaller than (or equal to) smallest child so done
                T tmp = _data[pi]; _data[pi] = _data[ci]; _data[ci] = tmp; // swap parent and child
                pi = ci;
            }
            return frontItem;
        }
 
        public T Peek()
        {
            T frontItem = _data[0];
            return frontItem;
        }
 
        public int Count => _data.Count;
 
        public override string ToString()
        {
            string s = "";
            for (int i = 0; i < _data.Count; ++i)
                s += _data[i].ToString() + " ";
            s += "count = " + _data.Count;
            return s;
        }
 
        public void Clear() => _data.Clear();
 
        public bool IsConsistent()
        {
            // is the heap property true for all data?
            if (_data.Count == 0) return true;
            int li = _data.Count - 1; // last index
            for (int pi = 0; pi < _data.Count; ++pi) // each parent index
            {
                int lci = 2 * pi + 1; // left child index
                int rci = 2 * pi + 2; // right child index
 
                if (lci <= li && _data[pi].CompareTo(_data[lci]) > 0) return false; // if lc exists and it's greater than parent then bad.
                if (rci <= li && _data[pi].CompareTo(_data[rci]) > 0) return false; // check the right child too.
            }
            return true; // passed all checks
        } // IsConsistent
    } // PriorityQueue
}