// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Collections;
using System.ComponentModel;
 
namespace System.Windows.Forms;
 
public partial class ListBox
{
    public partial class SelectedIndexCollection : IList
    {
        private readonly ListBox _owner;
 
        public SelectedIndexCollection(ListBox owner)
        {
            _owner = owner.OrThrowIfNull();
        }
 
        /// <summary>
        ///  Number of current selected items.
        /// </summary>
        [Browsable(false)]
        public int Count
        {
            get
            {
                return _owner.SelectedItems.Count;
            }
        }
 
        object ICollection.SyncRoot
        {
            get
            {
                return this;
            }
        }
 
        bool ICollection.IsSynchronized
        {
            get
            {
                return false;
            }
        }
 
        bool IList.IsFixedSize
        {
            get
            {
                return true;
            }
        }
 
        public bool IsReadOnly
        {
            get
            {
                return true;
            }
        }
 
        public bool Contains(int selectedIndex)
        {
            return IndexOf(selectedIndex) != -1;
        }
 
        bool IList.Contains(object? selectedIndex)
        {
            if (selectedIndex is int selectedIndexAsInt)
            {
                return Contains(selectedIndexAsInt);
            }
            else
            {
                return false;
            }
        }
 
        public int IndexOf(int selectedIndex)
        {
            // Just what does this do?  The selectedIndex parameter above is the index into the
            // main object collection. We look at the state of that item, and if the state indicates
            // that it is selected, we get back the virtualized index into this collection. Indexes on
            // this collection match those on the SelectedObjectCollection.
            if (selectedIndex >= 0 &&
                selectedIndex < InnerArray.Count &&
                InnerArray.GetState(selectedIndex, SelectedObjectCollection.SelectedObjectMask))
            {
                return InnerArray.IndexOf(InnerArray.GetItem(selectedIndex), SelectedObjectCollection.SelectedObjectMask);
            }
 
            return -1;
        }
 
        int IList.IndexOf(object? selectedIndex)
        {
            if (selectedIndex is int selectedIndexAsInt)
            {
                return IndexOf(selectedIndexAsInt);
            }
            else
            {
                return -1;
            }
        }
 
        int IList.Add(object? value)
        {
            throw new NotSupportedException(SR.ListBoxSelectedIndexCollectionIsReadOnly);
        }
 
        void IList.Clear()
        {
            throw new NotSupportedException(SR.ListBoxSelectedIndexCollectionIsReadOnly);
        }
 
        void IList.Insert(int index, object? value)
        {
            throw new NotSupportedException(SR.ListBoxSelectedIndexCollectionIsReadOnly);
        }
 
        void IList.Remove(object? value)
        {
            throw new NotSupportedException(SR.ListBoxSelectedIndexCollectionIsReadOnly);
        }
 
        void IList.RemoveAt(int index)
        {
            throw new NotSupportedException(SR.ListBoxSelectedIndexCollectionIsReadOnly);
        }
 
        /// <summary>
        ///  Retrieves the specified selected item.
        /// </summary>
        public int this[int index]
        {
            get
            {
                object identifier = InnerArray.GetEntryObject(index, SelectedObjectCollection.SelectedObjectMask);
                return InnerArray.IndexOf(identifier);
            }
        }
 
        object? IList.this[int index]
        {
            get
            {
                return this[index];
            }
            set
            {
                throw new NotSupportedException(SR.ListBoxSelectedIndexCollectionIsReadOnly);
            }
        }
 
        /// <summary>
        ///  This is the item array that stores our data. We share this backing store
        ///  with the main object collection.
        /// </summary>
        private ItemArray InnerArray
        {
            get
            {
                _owner.SelectedItems.EnsureUpToDate();
                return _owner.Items.InnerArray;
            }
        }
 
        public void CopyTo(Array destination, int index)
        {
            int cnt = Count;
            for (int i = 0; i < cnt; i++)
            {
                destination.SetValue(this[i], i + index);
            }
        }
 
        public void Clear()
        {
            _owner?.ClearSelected();
        }
 
        public void Add(int index)
        {
            if (_owner is not null)
            {
                ObjectCollection items = _owner.Items;
                if (items is not null)
                {
                    if (index != -1 && !Contains(index))
                    {
                        _owner.SetSelected(index, true);
                    }
                }
            }
        }
 
        public void Remove(int index)
        {
            if (_owner is not null)
            {
                ObjectCollection items = _owner.Items;
                if (items is not null)
                {
                    if (index != -1 && Contains(index))
                    {
                        _owner.SetSelected(index, false);
                    }
                }
            }
        }
 
        public IEnumerator GetEnumerator()
        {
            return new SelectedIndexEnumerator(this);
        }
    }
}