#region Apache License
//
// Licensed to the Apache Software Foundation (ASF) under one or more 
// contributor license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright ownership. 
// The ASF licenses this file to you under the Apache License, Version 2.0
// (the "License"); you may not use this file except in compliance with 
// the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
#endregion

using System;
using System.Collections;

namespace log4net.Util
{
  /// <summary>
  /// An always empty <see cref="IDictionary"/>.
  /// </summary>
  /// <remarks>
  /// <para>
  /// A singleton implementation of the <see cref="IDictionary"/>
  /// interface that always represents an empty collection.
  /// </para>
  /// </remarks>
  /// <author>Nicko Cadell</author>
  /// <author>Gert Driesen</author>
  [Log4NetSerializable]
  public sealed class EmptyDictionary : IDictionary
  {
    /// <summary>
    /// Initializes a new instance of the <see cref="EmptyDictionary" /> class. 
    /// </summary>
    /// <remarks>
    /// <para>
    /// Uses a private access modifier to enforce the singleton pattern.
    /// </para>
    /// </remarks>
    private EmptyDictionary()
    {
    }

    /// <summary>
    /// Gets the singleton instance of the <see cref="EmptyDictionary" />.
    /// </summary>
    /// <returns>The singleton instance of the <see cref="EmptyDictionary" />.</returns>
    /// <remarks>
    /// <para>
    /// Gets the singleton instance of the <see cref="EmptyDictionary" />.
    /// </para>
    /// </remarks>
    public static EmptyDictionary Instance { get; } = new();

    /// <summary>
    /// Copies the elements of the <see cref="ICollection"/> to an 
    /// <see cref="Array"/>, starting at a particular Array index.
    /// </summary>
    /// <param name="array">The one-dimensional <see cref="Array"/> 
    /// that is the destination of the elements copied from 
    /// <see cref="ICollection"/>. The Array must have zero-based 
    /// indexing.</param>
    /// <param name="index">The zero-based index in array at which 
    /// copying begins.</param>
    /// <remarks>
    /// <para>
    /// As the collection is empty no values are copied into the array.
    /// </para>
    /// </remarks>
    public void CopyTo(System.Array array, int index)
    {
      // copy nothing
    }

    /// <summary>
    /// Gets a value indicating if access to the <see cref="ICollection"/> is synchronized (thread-safe).
    /// </summary>
    /// <value>
    /// <b>true</b> if access to the <see cref="ICollection"/> is synchronized (thread-safe); otherwise, <b>false</b>.
    /// </value>
    /// <remarks>
    /// <para>
    /// For the <see cref="EmptyCollection"/> this property is always <b>true</b>.
    /// </para>
    /// </remarks>
    public bool IsSynchronized => true;

    /// <summary>
    /// Gets the number of elements contained in the <see cref="ICollection"/>
    /// </summary>
    /// <value>
    /// The number of elements contained in the <see cref="ICollection"/>.
    /// </value>
    /// <remarks>
    /// <para>
    /// As the collection is empty the <see cref="Count"/> is always <c>0</c>.
    /// </para>
    /// </remarks>
    public int Count => 0;

    /// <summary>
    /// Gets an object that can be used to synchronize access to the <see cref="ICollection"/>.
    /// </summary>
    /// <value>
    /// An object that can be used to synchronize access to the <see cref="ICollection"/>.
    /// </value>
    /// <remarks>
    /// <para>
    /// As the collection is empty and thread safe and synchronized this instance is also
    /// the <see cref="SyncRoot"/> object.
    /// </para>
    /// </remarks>
    public object SyncRoot => this;

    /// <summary>
    /// Returns an enumerator that can iterate through a collection.
    /// </summary>
    /// <returns>
    /// An <see cref="IEnumerator"/> that can be used to 
    /// iterate through the collection.
    /// </returns>
    /// <remarks>
    /// <para>
    /// As the collection is empty a <see cref="NullEnumerator"/> is returned.
    /// </para>
    /// </remarks>
    IEnumerator IEnumerable.GetEnumerator() => NullEnumerator.Instance;

    /// <summary>
    /// Adds an element with the provided key and value to the 
    /// <see cref="EmptyDictionary" />.
    /// </summary>
    /// <param name="key">The <see cref="object" /> to use as the key of the element to add.</param>
    /// <param name="value">The <see cref="object" /> to use as the value of the element to add.</param>
    /// <remarks>
    /// <para>
    /// As the collection is empty no new values can be added. A <see cref="InvalidOperationException"/>
    /// is thrown if this method is called.
    /// </para>
    /// </remarks>
    /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
    public void Add(object key, object? value) => throw new InvalidOperationException();

    /// <summary>
    /// Removes all elements from the <see cref="EmptyDictionary" />.
    /// </summary>
    /// <remarks>
    /// <para>
    /// As the collection is empty no values can be removed. A <see cref="InvalidOperationException"/>
    /// is thrown if this method is called.
    /// </para>
    /// </remarks>
    /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
    public void Clear() => throw new InvalidOperationException();

    /// <summary>
    /// Determines whether the <see cref="EmptyDictionary" /> contains an element 
    /// with the specified key.
    /// </summary>
    /// <param name="key">The key to locate in the <see cref="EmptyDictionary" />.</param>
    /// <returns><c>false</c></returns>
    /// <remarks>
    /// <para>
    /// As the collection is empty the <see cref="Contains"/> method always returns <c>false</c>.
    /// </para>
    /// </remarks>
    public bool Contains(object key) => false;

    /// <summary>
    /// Returns an enumerator that can iterate through a collection.
    /// </summary>
    /// <returns>
    /// An <see cref="IEnumerator"/> that can be used to 
    /// iterate through the collection.
    /// </returns>
    /// <remarks>
    /// <para>
    /// As the collection is empty a <see cref="NullEnumerator"/> is returned.
    /// </para>
    /// </remarks>
    public IDictionaryEnumerator GetEnumerator() => NullDictionaryEnumerator.Instance;

    /// <summary>
    /// Removes the element with the specified key from the <see cref="EmptyDictionary" />.
    /// </summary>
    /// <param name="key">The key of the element to remove.</param>
    /// <remarks>
    /// <para>
    /// As the collection is empty no values can be removed. A <see cref="InvalidOperationException"/>
    /// is thrown if this method is called.
    /// </para>
    /// </remarks>
    /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
    public void Remove(object key) => throw new InvalidOperationException();

    /// <summary>
    /// Gets a value indicating whether the <see cref="EmptyDictionary" /> has a fixed size.
    /// </summary>
    /// <value><c>true</c></value>
    /// <remarks>
    /// <para>
    /// As the collection is empty <see cref="IsFixedSize"/> always returns <c>true</c>.
    /// </para>
    /// </remarks>
    public bool IsFixedSize => true;

    /// <summary>
    /// Gets a value indicating whether the <see cref="EmptyDictionary" /> is read-only.
    /// </summary>
    /// <value><c>true</c></value>
    /// <remarks>
    /// <para>
    /// As the collection is empty <see cref="IsReadOnly"/> always returns <c>true</c>.
    /// </para>
    /// </remarks>
    public bool IsReadOnly => true;

    /// <summary>
    /// Gets an <see cref="ICollection" /> containing the keys of the <see cref="EmptyDictionary" />.
    /// </summary>
    /// <value>An <see cref="ICollection" /> containing the keys of the <see cref="EmptyDictionary" />.</value>
    /// <remarks>
    /// <para>
    /// As the collection is empty a <see cref="EmptyCollection"/> is returned.
    /// </para>
    /// </remarks>
    public ICollection Keys => EmptyCollection.Instance;

    /// <summary>
    /// Gets an <see cref="ICollection" /> containing the values of the <see cref="EmptyDictionary" />.
    /// </summary>
    /// <value>An <see cref="ICollection" /> containing the values of the <see cref="EmptyDictionary" />.</value>
    /// <remarks>
    /// <para>
    /// As the collection is empty a <see cref="EmptyCollection"/> is returned.
    /// </para>
    /// </remarks>
    public ICollection Values => EmptyCollection.Instance;

    /// <summary>
    /// Gets or sets the element with the specified key.
    /// </summary>
    /// <param name="key">The key of the element to get or set.</param>
    /// <value><c>null</c></value>
    /// <remarks>
    /// <para>
    /// As the collection is empty no values can be looked up or stored. 
    /// If the index getter is called then <c>null</c> is returned.
    /// A <see cref="InvalidOperationException"/> is thrown if the setter is called.
    /// </para>
    /// </remarks>
    /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
    public object? this[object key]
    {
      get => null;
      set => throw new InvalidOperationException();
    }
  }
}