/*************************************************************************
 * ModernUO                                                              *
 * Copyright (C) 2019-2020 - ModernUO Development Team                   *
 * Email: hi@modernuo.com                                                *
 * File: RefPool.cs - Created: 2020/02/20 - Updated: 2020/07/30        *
 *                                                                       *
 * This program is free software: you can redistribute it and/or modify  *
 * it under the terms of the GNU General Public License as published by  *
 * the Free Software Foundation, either version 3 of the License, or     *
 * (at your option) any later version.                                   *
 *                                                                       *
 * This program is distributed in the hope that it will be useful,       *
 * but WITHOUT ANY WARRANTY; without even the implied warranty of        *
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
 * GNU General Public License for more details.                          *
 *                                                                       *
 * You should have received a copy of the GNU General Public License     *
 * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
 *************************************************************************/

using System;
using System.Collections.Generic;

namespace Server.Utilities
{
  /// <summary> A resource reference object that can be disposed. </summary>
  /// <remarks>
  /// Disposing the reference is expected to return itself back into the
  /// original pool that created it.
  /// </remarks>
  public interface IRef : IDisposable
  {
  }

  /// <summary>
  /// Base implementation of the <see cref="IRef"/> interface.
  /// </summary>
  /// <remarks>
  /// New implementations of <see cref="IRef"/> should either derive from, or mirror
  /// the functionality of this base implementation.
  /// </remarks>
  /// <typeparam name="TDerived"></typeparam>
  public abstract class BaseRef<TDerived> : IRef where TDerived : IRef
  {
    private readonly RefPool<TDerived> m_Pool;
    public BaseRef(RefPool<TDerived> pool) => m_Pool = pool;
    protected abstract void OnDispose();

    public void Dispose()
    {
      OnDispose();
      m_Pool.Return((TDerived)(object)this);
    }
  }

  /// <summary>
  /// A resource reference pool that manages a collection of reusable resources.
  /// </summary>
  /// <typeparam name="TRef">The <see cref="IRef"/> resource type the pool will contain.</typeparam>
  public class RefPool<TRef> where TRef : IRef
  {
    public delegate TRef Generator(RefPool<TRef> targetPool);

    public const int DEFAULT_RESOURCE_RETENTION = 10;

    private readonly Stack<TRef> m_Resources = new Stack<TRef>();
    private readonly Generator m_Generator;
    private int m_MaxRefrenceRetention;

    /// <summary>
    /// The maximum number of unused resources to hold in the pool.
    /// </summary>
    public int MaxRefrenceRetention
    {
      get => m_MaxRefrenceRetention;
      set
      {
        m_MaxRefrenceRetention = value;
        while (m_Resources.Count > value) m_Resources.Pop();
      }
    }

    /// <param name="generator">The generator function for creating new resources.</param>
    /// <param name="preGenerateCount">An amount of resources that should be pre-generated during initialization of the resource pool.</param>
    public RefPool(Generator generator, int preGenerateCount = 0, int maxRefrenceRetention = DEFAULT_RESOURCE_RETENTION)
    {
      if (generator == null)
        throw new ArgumentNullException(nameof(generator));
      if (preGenerateCount > maxRefrenceRetention)
        throw new IndexOutOfRangeException($"{nameof(preGenerateCount)} greater than {nameof(maxRefrenceRetention)}");
      m_Generator = generator;
      m_MaxRefrenceRetention = maxRefrenceRetention;
      while (--preGenerateCount >= 0) m_Resources.Push(generator(this));
    }

    /// <summary>
    /// Retrieves a resource reference that is managed by this <see cref="RefPool{TRef}"/>. If the pool is has unused resources,
    /// it will remove one from the pool and return it; otherwise, a new resource will be generated.
    /// </summary>
    /// <returns>Unused resource, or a new resource if no unused resources available.</returns>
    public TRef Get() => m_Resources.TryPop(out var item) ? item : m_Generator(this);

    /// <summary>
    /// Returns a resource reference to the pool of unused resources.
    /// </summary>
    /// <param name="queueRef">Resource to be returned.</param>
    public void Return(TRef queueRef)
    {
      if (m_Resources.Count < MaxRefrenceRetention)
        m_Resources.Push(queueRef);
    }
  }

  public class QueueRef<T> : Queue<T>, IRef
  {
    private readonly RefPool<QueueRef<T>> m_Pool;
    private QueueRef(RefPool<QueueRef<T>> pool) => m_Pool = pool;

    /// <summary>Clears the queue and returns this resource to its parent resource pool.</summary>
    public void Dispose()
    {
      Clear();
      m_Pool.Return(this);
    }

    /// <summary>
    /// Generator function for creating instances of the <see cref="QueueRef{T}"/> resource.
    /// </summary>
    public static RefPool<QueueRef<T>>.Generator Generate = targetPool => new QueueRef<T>(targetPool);
  }

  public class StackRef<T> : Stack<T>, IRef
  {
    private readonly RefPool<StackRef<T>> m_Pool;
    private StackRef(RefPool<StackRef<T>> pool) => m_Pool = pool;

    /// <summary>Clears the stack and returns this resource to its parent resource pool.</summary>
    public void Dispose()
    {
      Clear();
      m_Pool.Return(this);
    }

    /// <summary>
    /// Generator function for creating instances of the <see cref="StackRef{T}"/> resource.
    /// </summary>
    public static RefPool<StackRef<T>>.Generator Generate = (targetPool) => new StackRef<T>(targetPool);
  }
}