#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="IDictionaryEnumerator"/>.
/// </summary>
/// <remarks>
/// <para>
/// A singleton implementation of the <see cref="IDictionaryEnumerator"/> over a collection
/// that is empty and not modifiable.
/// </para>
/// </remarks>
/// <author>Nicko Cadell</author>
/// <author>Gert Driesen</author>
public sealed class NullDictionaryEnumerator : IDictionaryEnumerator
{
  /// <summary>
  /// Initializes a new instance of the <see cref="NullDictionaryEnumerator" /> class. 
  /// </summary>
  /// <remarks>
  /// <para>
  /// Uses a private access modifier to enforce the singleton pattern.
  /// </para>
  /// </remarks>
  private NullDictionaryEnumerator()
  {
  }

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

  /// <summary>
  /// Gets the current object from the enumerator.
  /// </summary>
  /// <remarks>
  /// Throws an <see cref="InvalidOperationException" /> because the 
  /// <see cref="NullDictionaryEnumerator" /> never has a current value.
  /// </remarks>
  /// <remarks>
  /// <para>
  /// As the enumerator is over an empty collection its <see cref="Current"/>
  /// value cannot be moved over a valid position, therefore <see cref="Current"/>
  /// will throw an <see cref="InvalidOperationException"/>.
  /// </para>
  /// </remarks>
  /// <exception cref="InvalidOperationException">The collection is empty and <see cref="Current"/> 
  /// cannot be positioned over a valid location.</exception>
  public object Current => throw new InvalidOperationException();

  /// <summary>
  /// Test if the enumerator can advance, if so advance.
  /// </summary>
  /// <returns><see langword="false"/> as the <see cref="NullDictionaryEnumerator" /> cannot advance.</returns>
  /// <remarks>
  /// <para>
  /// As the enumerator is over an empty collection its <see cref="Current"/>
  /// value cannot be moved over a valid position, therefore <see cref="MoveNext"/>
  /// will always return <see langword="false"/>.
  /// </para>
  /// </remarks>
  public bool MoveNext() => false;

  /// <summary>
  /// Resets the enumerator back to the start.
  /// </summary>
  /// <remarks>
  /// <para>
  /// As the enumerator is over an empty collection <see cref="Reset"/> does nothing.
  /// </para>
  /// </remarks>
  public void Reset()
  {
  }

  /// <summary>
  /// Gets the current key from the enumerator.
  /// </summary>
  /// <remarks>
  /// Throws an exception because the <see cref="NullDictionaryEnumerator" />
  /// never has a current value.
  /// </remarks>
  /// <remarks>
  /// <para>
  /// As the enumerator is over an empty collection its <see cref="Current"/>
  /// value cannot be moved over a valid position, therefore <see cref="Key"/>
  /// will throw an <see cref="InvalidOperationException"/>.
  /// </para>
  /// </remarks>
  /// <exception cref="InvalidOperationException">The collection is empty and <see cref="Current"/> 
  /// cannot be positioned over a valid location.</exception>
  public object Key => throw new InvalidOperationException();

  /// <summary>
  /// Gets the current value from the enumerator.
  /// </summary>
  /// <value>The current value from the enumerator.</value>
  /// <remarks>
  /// Throws an <see cref="InvalidOperationException" /> because the 
  /// <see cref="NullDictionaryEnumerator" /> never has a current value.
  /// </remarks>
  /// <remarks>
  /// <para>
  /// As the enumerator is over an empty collection its <see cref="Current"/>
  /// value cannot be moved over a valid position, therefore <see cref="Value"/>
  /// will throw an <see cref="InvalidOperationException"/>.
  /// </para>
  /// </remarks>
  /// <exception cref="InvalidOperationException">The collection is empty and <see cref="Current"/> 
  /// cannot be positioned over a valid location.</exception>
  public object Value => throw new InvalidOperationException();

  /// <summary>
  /// Gets the current entry from the enumerator.
  /// </summary>
  /// <remarks>
  /// Throws an <see cref="InvalidOperationException" /> because the 
  /// <see cref="NullDictionaryEnumerator" /> never has a current entry.
  /// </remarks>
  /// <remarks>
  /// <para>
  /// As the enumerator is over an empty collection its <see cref="Current"/>
  /// value cannot be moved over a valid position, therefore <see cref="Entry"/>
  /// will throw an <see cref="InvalidOperationException"/>.
  /// </para>
  /// </remarks>
  /// <exception cref="InvalidOperationException">The collection is empty and <see cref="Current"/> 
  /// cannot be positioned over a valid location.</exception>
  public DictionaryEntry Entry => throw new InvalidOperationException();
}