// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Diagnostics;
using System.Linq;

namespace CommunityToolkit.Common.Collections;

/// <summary>
/// A read-only list of groups.
/// </summary>
/// <typeparam name="TKey">The type of the group key.</typeparam>
/// <typeparam name = "TValue" > The type of the items in the collection.</typeparam>
public sealed class ReadOnlyObservableGroupedCollection<TKey, TValue> : ReadOnlyObservableCollection<ReadOnlyObservableGroup<TKey, TValue>>
    where TKey : notnull
{
    /// <summary>
    /// Initializes a new instance of the <see cref="ReadOnlyObservableGroupedCollection{TKey, TValue}"/> class.
    /// </summary>
    /// <param name="collection">The source collection to wrap.</param>
    public ReadOnlyObservableGroupedCollection(ObservableGroupedCollection<TKey, TValue> collection)
        : this(collection.Select(static g => new ReadOnlyObservableGroup<TKey, TValue>(g)))
    {
        ((INotifyCollectionChanged)collection).CollectionChanged += OnSourceCollectionChanged;
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="ReadOnlyObservableGroupedCollection{TKey, TValue}"/> class.
    /// </summary>
    /// <param name="collection">The initial data to add in the grouped collection.</param>
    public ReadOnlyObservableGroupedCollection(IEnumerable<ReadOnlyObservableGroup<TKey, TValue>> collection)
        : base(new ObservableCollection<ReadOnlyObservableGroup<TKey, TValue>>(collection))
    {
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="ReadOnlyObservableGroupedCollection{TKey, TValue}"/> class.
    /// </summary>
    /// <param name="collection">The initial data to add in the grouped collection.</param>
    public ReadOnlyObservableGroupedCollection(IEnumerable<IGrouping<TKey, TValue>> collection)
        : this(collection.Select(static g => new ReadOnlyObservableGroup<TKey, TValue>(g.Key, g)))
    {
    }

    private void OnSourceCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
    {
        // Even if NotifyCollectionChangedEventArgs allows multiple items, the actual implementation
        // is only reporting the changes one by one. We consider only this case for now.
        if (e.OldItems?.Count > 1 || e.NewItems?.Count > 1)
        {
            static void ThrowNotSupportedException()
            {
                throw new NotSupportedException(
                    "ReadOnlyObservableGroupedCollection<TKey, TValue> doesn't support operations on multiple items at once.\n" +
                    "If this exception was thrown, it likely means support for batched item updates has been added to the " +
                    "underlying ObservableCollection<T> type, and this implementation doesn't support that feature yet.\n" +
                    "Please consider opening an issue in https://aka.ms/windowstoolkit to report this.");
            }

            ThrowNotSupportedException();
        }

        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Add or NotifyCollectionChangedAction.Replace:

                // We only need to find the new item if the operation is either add or remove. In this
                // case we just directly find the first item that was modified, or throw if it's not present.
                // This normally never happens anyway - add and replace should always have a target element.
                ObservableGroup<TKey, TValue> newItem = e.NewItems!.Cast<ObservableGroup<TKey, TValue>>().First();

                if (e.Action == NotifyCollectionChangedAction.Add)
                {
                    Items.Insert(e.NewStartingIndex, new ReadOnlyObservableGroup<TKey, TValue>(newItem));
                }
                else
                {
                    Items[e.OldStartingIndex] = new ReadOnlyObservableGroup<TKey, TValue>(newItem);
                }

                break;
            case NotifyCollectionChangedAction.Move:

                // Our inner Items list is our own ObservableCollection<ReadOnlyObservableGroup<TKey, TValue>> so we can safely cast Items to its concrete type here.
                ((ObservableCollection<ReadOnlyObservableGroup<TKey, TValue>>)Items).Move(e.OldStartingIndex, e.NewStartingIndex);
                break;
            case NotifyCollectionChangedAction.Remove:
                Items.RemoveAt(e.OldStartingIndex);
                break;
            case NotifyCollectionChangedAction.Reset:
                Items.Clear();
                break;
            default:
                Debug.Fail("unsupported value");
                break;
        }
    }
}