#region CPL License
/*
Nuclex Framework
Copyright (C) 2002-2017 Nuclex Development Labs

This library is free software; you can redistribute it and/or
modify it under the terms of the IBM Common Public License as
published by the IBM Corporation; either version 1.0 of the
License, or (at your option) any later version.

This library 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
IBM Common Public License for more details.

You should have received a copy of the IBM Common Public
License along with this library
*/
#endregion

using System;
using System.Collections.Generic;

namespace Nuclex.Support.Collections {

  /// <summary>Pool that recycles objects in order to avoid garbage build-up</summary>
  /// <typeparam name="TItem">Type of objects being pooled</typeparam>
  /// <remarks>
  ///   <para>
  ///     Use this class to recycle objects instead of letting them become garbage,
  ///     creating new instances each time. The Pool class is designed to either be
  ///     used on its own or as a building block for a static class that wraps it.
  ///   </para>
  ///   <para>
  ///     Special care has to be taken to revert the entire state of a recycled
  ///     object when it is returned to the pool. For example, events will need to
  ///     have their subscriber lists emptied to avoid sending out events to the
  ///     wrong subscribers and accumulating more and more subscribers each time
  ///     they are reused.
  ///   </para>
  ///   <para>
  ///     To simplify such cleanup, pooled objects can implement the IRecyclable
  ///     interface. When an object is returned to the pool, the pool will
  ///     automatically call its IRecyclable.Recycle() method.
  ///   </para>
  /// </remarks>
  public class Pool<TItem> {

    /// <summary>Default number of recyclable objects the pool will store</summary>
    public const int DefaultPoolSize = 64;

    /// <summary>Initializes a new pool using the default capacity</summary>
    public Pool() : this(DefaultPoolSize, null, null) { }

    /// <summary>Initializes a new pool using the default capacity</summary>
    /// <param name="createNewDelegate">Delegate that will be used to create new items</param>
    public Pool(Func<TItem> createNewDelegate) :
      this(DefaultPoolSize, createNewDelegate, null) { }

    /// <summary>Initializes a new pool using the default capacity</summary>
    /// <param name="createNewDelegate">Delegate that will be used to create new items</param>
    /// <param name="recycleDelegate">Delegate that will be used to recycle items</param>
    public Pool(Func<TItem> createNewDelegate, Action<TItem> recycleDelegate) :
      this(DefaultPoolSize, createNewDelegate, recycleDelegate) { }

    /// <summary>Initializes a new pool using a user-specified capacity</summary>
    /// <param name="capacity">Capacity of the pool</param>
    public Pool(int capacity) :
      this(capacity, null, null) { }

    /// <summary>Initializes a new pool using a user-specified capacity</summary>
    /// <param name="capacity">Capacity of the pool</param>
    /// <param name="createNewDelegate">Delegate that will be used to create new items</param>
    public Pool(int capacity, Func<TItem> createNewDelegate) :
      this(capacity, createNewDelegate, null) { }

    /// <summary>Initializes a new pool using a user-specified capacity</summary>
    /// <param name="capacity">Capacity of the pool</param>
    /// <param name="createNewDelegate">Delegate that will be used to create new items</param>
    /// <param name="recycleDelegate">Delegate that will be used to recycle items</param>
    public Pool(int capacity, Func<TItem> createNewDelegate, Action<TItem> recycleDelegate) {
      Capacity = capacity;

      if(createNewDelegate == null) {
        if(!typeof(TItem).HasDefaultConstructor()) {
          throw new ArgumentException(
            "Type " + typeof(TItem).Name + " has no default constructor and " +
            "requires a custom 'create instance' delegate"
          );
        }
        createNewDelegate = new Func<TItem>(Activator.CreateInstance<TItem>);
      }
      if(recycleDelegate == null) {
        recycleDelegate = new Action<TItem>(callRecycleIfSupported);
      }

      this.createNewDelegate = createNewDelegate;
      this.recycleDelegate = recycleDelegate;
    }

    /// <summary>
    ///   Returns a new or recycled instance of the types managed by the pool
    /// </summary>
    /// <returns>A new or recycled instance</returns>
    public TItem Get() {
      lock(this) {
        if(this.items.Count > 0) {
          return this.items.Dequeue();
        } else {
          return this.createNewDelegate();
        }
      }
    }

    /// <summary>
    ///   Redeems an instance that is no longer used to be recycled by the pool
    /// </summary>
    /// <param name="item">The instance that will be redeemed</param>
    public void Redeem(TItem item) {

      // Call Recycle() when the object is redeemed (instead of when it leaves
      // the pool again) in order to eliminate any references the object may hold
      // to other objects.
      this.recycleDelegate(item);

      lock(this) {
        if(this.items.Count < this.capacity) {
          this.items.Enqueue(item);
        }
      }

    }

    /// <summary>Number of objects the pool can retain</summary>
    /// <remarks>
    ///   Changing this value causes the pool to be emtpied. It is recommended that
    ///   you only read the pool's capacity, never change it.
    /// </remarks>
    public int Capacity {
      get { return this.capacity; }
      set {
        this.capacity = value;
        this.items = new Queue<TItem>(value);
      }
    }

    /// <summary>
    ///   Calls the Recycle() method on an objects if it implements
    ///   the IRecyclable interface
    /// </summary>
    /// <param name="item">
    ///   Object whose Recycle() method will be called if supported by the object
    /// </param>
    private static void callRecycleIfSupported(TItem item) {
      IRecyclable recycleable = item as IRecyclable;
      if(recycleable != null) {
        recycleable.Recycle();
      }
    }

    /// <summary>Objects being retained for recycling</summary>
    private Queue<TItem> items;
    /// <summary>Capacity of the pool</summary>
    /// <remarks>
    ///   Required because the Queue class doesn't allow this value to be retrieved
    /// </remarks>
    private int capacity;
    /// <summary>Delegate used to create new instances of the pool's type</summary>
    private Func<TItem> createNewDelegate;
    /// <summary>Delegate used to recycle instances</summary>
    private Action<TItem> recycleDelegate;

  }

} // namespace Nuclex.Support.Collections