// <copyright file="KeyedPriorityQueue.cs" company="Microsoft">Copyright (c) Microsoft Corporation.  All rights reserved.</copyright>

using System;
using System.Diagnostics;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;

namespace System.Workflow.Runtime
{
    internal sealed class KeyedPriorityQueueHeadChangedEventArgs<T> : EventArgs where T : class
    {
        private T oldFirstElement;
        private T newFirstElement;

        public KeyedPriorityQueueHeadChangedEventArgs(T oldFirstElement, T newFirstElement)
        {
            this.oldFirstElement = oldFirstElement;
            this.newFirstElement = newFirstElement;
        }

        public T OldFirstElement { get { return oldFirstElement; } }
        public T NewFirstElement { get { return newFirstElement; } }
    }

    /// <summary> Combines the functionality of a dictionary and a heap-sorted priority queue.
    /// Enqueue and Dequeue operations are O(log n), Peek is O(1) and Remove is O(n).
    /// Used by the SchedulerService classes to maintain an ordered list of running timers, etc.
    /// Lesser priority values are higher priority.</summary>
    /// <typeparam name="K">Key</typeparam>
    /// <typeparam name="V">Value</typeparam>
    /// <typeparam name="P">Priority</typeparam>
    [Serializable]
    internal class KeyedPriorityQueue<K, V, P> where V : class
    {
        private List<HeapNode<K, V, P>> heap = new List<HeapNode<K, V, P>>();
        private int size;
        private Comparer<P> priorityComparer = Comparer<P>.Default;
        private HeapNode<K, V, P> placeHolder = default(HeapNode<K, V, P>);

        public event EventHandler<KeyedPriorityQueueHeadChangedEventArgs<V>> FirstElementChanged;

        public KeyedPriorityQueue()
        {
            heap.Add(default(HeapNode<K, V, P>));       // Dummy zeroth element, heap is 1-based
        }

        public void Enqueue(K key, V value, P priority)
        {
            V oldHead = size > 0 ? heap[1].Value : null;
            int i = ++size;
            int parent = i / 2;
            if (i == heap.Count)
                heap.Add(placeHolder);
            while (i > 1 && IsHigher(priority, heap[parent].Priority))
            {
                heap[i] = heap[parent];
                i = parent;
                parent = i / 2;
            }
            heap[i] = new HeapNode<K, V, P>(key, value, priority);
            V newHead = heap[1].Value;
            if (!newHead.Equals(oldHead))
            {
                RaiseHeadChangedEvent(oldHead, newHead);
            }
        }

        public V Dequeue()
        {
            V oldHead = (size < 1) ? null : DequeueImpl();
            V newHead = (size < 1) ? null : heap[1].Value;
            RaiseHeadChangedEvent(null, newHead);
            return oldHead;
        }

        private V DequeueImpl()
        {
            Debug.Assert(size > 0, "Queue Underflow");
            V oldHead = heap[1].Value;
            heap[1] = heap[size];
            heap[size--] = placeHolder;
            Heapify(1);
            return oldHead;
        }


        public V Remove(K key)
        {
            if (size < 1)
                return null;

            V oldHead = heap[1].Value;
            for (int i = 1; i <= size; i++)
            {
                if (heap[i].Key.Equals(key))
                {
                    V retval = heap[i].Value;
                    Swap(i, size);
                    heap[size--] = placeHolder;
                    Heapify(i);
                    V newHead = heap[1].Value;
                    if (!oldHead.Equals(newHead))
                    {
                        RaiseHeadChangedEvent(oldHead, newHead);
                    }
                    return retval;
                }
            }
            return null;
        }

        public V Peek()
        {
            return (size < 1) ? null : heap[1].Value;
        }

        public int Count
        {
            get { return size; }
        }

        public V FindByPriority(P priority, Predicate<V> match)
        {
            return size < 1 ? null : Search(priority, 1, match);
        }

        public ReadOnlyCollection<V> Values
        {
            get
            {
                List<V> values = new List<V>();
                for (int i = 1; i <= size; i++)
                {
                    values.Add(heap[i].Value);
                }
                return new ReadOnlyCollection<V>(values);
            }
        }

        public ReadOnlyCollection<K> Keys
        {
            get
            {
                List<K> keys = new List<K>();
                for (int i = 1; i <= size; i++)
                {
                    keys.Add(heap[i].Key);
                }
                return new ReadOnlyCollection<K>(keys);
            }
        }

        public void Clear()
        {
            heap.Clear();
            size = 0;
        }

        private void RaiseHeadChangedEvent(V oldHead, V newHead)
        {
            if (oldHead != newHead)
            {
                EventHandler<KeyedPriorityQueueHeadChangedEventArgs<V>> fec = FirstElementChanged;
                if (fec != null)
                    fec(this, new KeyedPriorityQueueHeadChangedEventArgs<V>(oldHead, newHead));
            }
        }

        private V Search(P priority, int i, Predicate<V> match)
        {
            Debug.Assert(i >= 1 || i <= size, "Index out of range: i = " + i + ", size = " + size);

            V value = null;
            if (IsHigher(heap[i].Priority, priority))
            {
                if (match(heap[i].Value))
                    value = heap[i].Value;
                int left = 2 * i;
                int right = left + 1;
                if (value == null && left <= size)
                    value = Search(priority, left, match);
                if (value == null && right <= size)
                    value = Search(priority, right, match);
            }
            return value;
        }

        private void Heapify(int i)
        {
            Debug.Assert(i >= 1 || i <= size, "Index out of range: i = " + i + ", size = " + size);

            int left = 2 * i;
            int right = left + 1;
            int highest = i;
            if (left <= size && IsHigher(heap[left].Priority, heap[i].Priority))
                highest = left;
            if (right <= size && IsHigher(heap[right].Priority, heap[highest].Priority))
                highest = right;
            if (highest != i)
            {
                Swap(i, highest);
                Heapify(highest);
            }

        }

        private void Swap(int i, int j)
        {
            Debug.Assert(i >= 1 || j >= 1 || i <= size || j <= size, "Index out of range: i = " + i + ", j = " + j + ", size = " + size);

            HeapNode<K, V, P> temp = heap[i];
            heap[i] = heap[j];
            heap[j] = temp;
        }

        protected virtual bool IsHigher(P p1, P p2)
        {
            return (priorityComparer.Compare(p1, p2) < 1);
        }

        [Serializable]
        private struct HeapNode<KK, VV, PP>
        {
            public KK Key;
            public VV Value;
            public PP Priority;

            public HeapNode(KK key, VV value, PP priority)
            {
                Key = key;
                Value = value;
                Priority = priority;
            }
        }
    }
}