#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;
using System.Collections.Generic;
using System.Collections.ObjectModel;

namespace Nuclex.Support.Collections {

  /// <summary>Collection of weakly referenced objects</summary>
  /// <remarks>
  ///   This collection tries to expose the interface of a normal collection, but stores
  ///   objects as weak references. When an object is accessed, it can return null.
  ///   when the collection detects that one of its items was garbage collected, it
  ///   will silently remove that item.
  /// </remarks>
  public partial class WeakCollection<TItem> : IList<TItem>, IList
    where TItem : class {

    #region class UnpackingEnumerator

    /// <summary>
    ///   An enumerator that unpacks the items returned by an enumerator of the
    ///   weak reference collection into the actual item type on-the-fly.
    /// </summary>
    private class UnpackingEnumerator : IEnumerator<TItem> {

      /// <summary>Initializes a new unpacking enumerator</summary>
      /// <param name="containedTypeEnumerator">
      ///   Enumerator of the weak reference collection
      /// </param>
      public UnpackingEnumerator(
        IEnumerator<WeakReference<TItem>> containedTypeEnumerator
      ) {
        this.containedTypeEnumerator = containedTypeEnumerator;
      }

      /// <summary>Immediately releases all resources used by the instance</summary>
      public void Dispose() {
        this.containedTypeEnumerator.Dispose();
      }

      /// <summary>
      ///   The element in the collection at the current position of the enumerator.
      /// </summary>
      public TItem Current {
        get { return this.containedTypeEnumerator.Current.Target; }
      }

      /// <summary>Gets the current element in the collection.</summary>
      /// <returns>The current element in the collection.</returns>
      /// <exception cref="System.InvalidOperationException">
      ///   The enumerator is positioned before the first element of the collection
      ///   or after the last element.
      /// </exception>
      public bool MoveNext() {
        return this.containedTypeEnumerator.MoveNext();
      }

      /// <summary>
      ///   Sets the enumerator to its initial position, which is before the first element
      ///   in the collection.
      /// </summary>
      /// <exception cref="System.InvalidOperationException">
      ///   The collection was modified after the enumerator was created.
      /// </exception>
      public void Reset() {
        this.containedTypeEnumerator.Reset();
      }

      /// <summary>The current element in the collection.</summary>
      /// <exception cref="System.InvalidOperationException">
      ///   The enumerator is positioned before the first element of the collection
      ///   or after the last element.
      /// </exception>
      object IEnumerator.Current {
        get { return Current; }
      }

      /// <summary>An enumerator from the wrapped collection</summary>
      private IEnumerator<WeakReference<TItem>> containedTypeEnumerator;

    }

    #endregion // class UnpackingEnumerator

    /// <summary>Initializes a new weak reference collection</summary>
    /// <param name="items">
    ///   Internal list of weak references that are unpacking when accessed through
    ///   the WeakCollection's interface.
    /// </param>
    public WeakCollection(IList<WeakReference<TItem>> items) :
      this(items, EqualityComparer<TItem>.Default) { }

    /// <summary>Initializes a new weak reference collection</summary>
    /// <param name="items">
    ///   Internal list of weak references that are unpacking when accessed through
    ///   the WeakCollection's interface.
    /// </param>
    /// <param name="comparer">
    ///   Comparer used to identify and compare items to each other
    /// </param>
    public WeakCollection(
      IList<WeakReference<TItem>> items, IEqualityComparer<TItem> comparer
    ) {
      this.items = items;
      this.comparer = comparer;
    }

    /// <summary>
    ///   Determines whether an element is in the WeakCollection
    /// </summary>
    /// <param name="item">
    ///   The object to locate in the WeakCollection. The value can be null.
    /// </param>
    /// <returns>
    ///   True if value is found in the WeakCollection; otherwise, false.
    /// </returns>
    /// <remarks>
    ///   The default implementation of this method is very unoptimized and will
    ///   enumerate all the items in the collection, transforming one after another
    ///   to check whether the transformed item matches the item the user was
    ///   looking for. It is recommended to provide a custom implementation of
    ///   this method, if possible.
    /// </remarks>
    public virtual bool Contains(TItem item) {
      return (IndexOf(item) != -1);
    }

    /// <summary>
    ///   Copies the entire WeakCollection to a compatible one-dimensional
    ///   System.Array, starting at the specified index of the target array.
    /// </summary>
    /// <param name="array">
    ///   The one-dimensional System.Array that is the destination of the elements copied
    ///   from the WeakCollection. The System.Array must have zero-based indexing.
    /// </param>
    /// <param name="index">
    ///   The zero-based index in array at which copying begins.
    /// </param>
    /// <exception cref="System.ArgumentException">
    ///   Index is equal to or greater than the length of array or the number of elements
    ///   in the source WeakCollection is greater than the available space from index to
    ///   the end of the destination array.
    /// </exception>
    /// <exception cref="System.ArgumentOutOfRangeException">
    ///   Index is less than zero.
    /// </exception>
    /// <exception cref="System.ArgumentNullException">
    ///   Array is null.
    /// </exception>
    public void CopyTo(TItem[] array, int index) {
      if(this.items.Count > (array.Length - index)) {
        throw new ArgumentException(
          "Array too small to fit the collection items starting at the specified index"
        );
      }

      for(int itemIndex = 0; itemIndex < this.items.Count; ++itemIndex) {
        array[itemIndex + index] = this.items[itemIndex].Target;
      }
    }

    /// <summary>Removes all items from the WeakCollection</summary>
    public void Clear() {
      this.items.Clear();
    }

    /// <summary>
    ///   Returns an enumerator that iterates through the WeakCollection.
    /// </summary>
    /// <returns>An enumerator or the WeakCollection.</returns>
    public IEnumerator<TItem> GetEnumerator() {
      return new UnpackingEnumerator(this.items.GetEnumerator());
    }

    /// <summary>
    ///   Searches for the specified object and returns the zero-based index of the
    ///   first occurrence within the entire WeakCollection.
    /// </summary>
    /// <param name="item">
    ///   The object to locate in the WeakCollection. The value can
    ///   be null for reference types.
    /// </param>
    /// <returns>
    ///   The zero-based index of the first occurrence of item within the entire
    ///   WeakCollection, if found; otherwise, -1.
    /// </returns>
    /// <remarks>
    ///   The default implementation of this method is very unoptimized and will
    ///   enumerate all the items in the collection, transforming one after another
    ///   to check whether the transformed item matches the item the user was
    ///   looking for. It is recommended to provide a custom implementation of
    ///   this method, if possible.
    /// </remarks>
    public int IndexOf(TItem item) {
      for(int index = 0; index < this.items.Count; ++index) {
        TItem itemAtIndex = this.items[index].Target;
        if((itemAtIndex == null) || (item == null)) {
          if(ReferenceEquals(item, itemAtIndex)) {
            return index;
          }
        } else {
          if(this.comparer.Equals(itemAtIndex, item)) {
            return index;
          }
        }
      }

      return -1;
    }

    /// <summary>
    ///   The number of elements contained in the WeakCollection instance
    /// </summary>
    public int Count {
      get { return this.items.Count; }
    }

    /// <summary>Gets the element at the specified index.</summary>
    /// <param name="index">The zero-based index of the element to get.</param>
    /// <returns>The element at the specified index.</returns>
    /// <exception cref="System.ArgumentOutOfRangeException">
    ///    Index is less than zero or index is equal to or greater than
    ///    WeakCollection.Count.
    /// </exception>
    public TItem this[int index] {
      get { return this.items[index].Target; }
      set { this.items[index] = new WeakReference<TItem>(value); }
    }

    /// <summary>
    ///   Removes the first occurrence of a specific object from the WeakCollection.
    /// </summary>
    /// <param name="item">The object to remove from the WeakCollection</param>
    /// <returns>
    ///   True if item was successfully removed from the WeakCollection; otherwise, false.
    /// </returns>
    public bool Remove(TItem item) {
      for(int index = 0; index < this.items.Count; ++index) {
        TItem itemAtIndex = this.items[index].Target;
        if((itemAtIndex == null) || (item == null)) {
          if(ReferenceEquals(item, itemAtIndex)) {
            this.items.RemoveAt(index);
            return true;
          }
        } else {
          if(this.comparer.Equals(item, itemAtIndex)) {
            this.items.RemoveAt(index);
            return true;
          }
        }
      }

      return false;
    }

    /// <summary>Adds an item to the WeakCollection.</summary>
    /// <param name="item">The object to add to the WeakCollection</param>
    public void Add(TItem item) {
      this.items.Add(new WeakReference<TItem>(item));
    }

    /// <summary>Inserts an item to the WeakCollection at the specified index.</summary>
    /// <param name="index">
    ///   The zero-based index at which item should be inserted.
    /// </param>
    /// <param name="item">The object to insert into the WeakCollection</param>
    /// <exception cref="System.ArgumentOutOfRangeException">
    ///   index is not a valid index in the WeakCollection.
    /// </exception>
    public void Insert(int index, TItem item) {
      this.items.Insert(index, new WeakReference<TItem>(item));
    }

    /// <summary>
    ///   Removes the WeakCollection item at the specified index.
    /// </summary>
    /// <param name="index">The zero-based index of the item to remove.</param>
    /// <exception cref="System.ArgumentOutOfRangeException">
    ///   Index is not a valid index in the WeakCollection.
    /// </exception>
    public void RemoveAt(int index) {
      this.items.RemoveAt(index);
    }

    /// <summary>Whether the List is write-protected</summary>
    public bool IsReadOnly {
      get { return this.items.IsReadOnly; }
    }

    /// <summary>
    ///   Removes the items that have been garbage collected from the collection
    /// </summary>
    public void RemoveDeadItems() {
      int newCount = 0;

      // Eliminate all items that have been garbage collected by shifting
      for(int index = 0; index < this.items.Count; ++index) {
        if(this.items[index].IsAlive) {
          this.items[newCount] = this.items[index];
          ++newCount;
        }
      }

      // If any garbage collected items were found, resize the collection so
      // the space that became empty in the previous shifting process will be freed
      while(this.items.Count > newCount) {
        this.items.RemoveAt(this.items.Count - 1);
      }
    }

    /// <summary>Weak references to the items contained in the collection</summary>
    private IList<WeakReference<TItem>> items;
    /// <summary>Used to identify and compare items in the collection</summary>
    private IEqualityComparer<TItem> comparer;
    /// <summary>Synchronization root for threaded accesses to this collection</summary>
    private object syncRoot;

  }

} // namespace Nuclex.Support.Collections