using System;
using System.Collections.Generic;

namespace TodoApplication
{
    class History<TItem>
    {
        public History(Operation operation, int index, TItem item)
        {
            Operation = operation;
            Index = index;
            Item = item;
        }

        public Operation Operation;
        public int Index;
        public TItem Item;
    }

    public enum Operation
    {
        Add,
        Remove
    }

    public class ListModel<TItem>
    {
        public List<TItem> Items { get; }
        public readonly int Limit;
        private readonly LimitedSizeStack<History<TItem>> operationStack;

        public ListModel(int limit)
        {
            Items = new List<TItem>();
            Limit = limit;
            operationStack = new LimitedSizeStack<History<TItem>>(limit);
        }

        public void AddItem(TItem item)
        {
            var newAddCommand = new History<TItem>(Operation.Add, Items.Count, item);
            operationStack.Push(newAddCommand);
            Items.Add(item);
        }

        public void RemoveItem(int index)
        {
            var newRemoveCommand = new History<TItem>(Operation.Remove, index, Items[index]);
            operationStack.Push(newRemoveCommand);
            Items.RemoveAt(index);
        }

        public bool CanUndo()
        {
            return operationStack.Count > 0;
        }

        public void Undo()
        {
            History<TItem> deepestCommand = operationStack.Pop();

            switch (deepestCommand.Operation)
            {
                case Operation.Add:
                    Items.RemoveAt(deepestCommand.Index);
                    break;
                case Operation.Remove:
                    Items.Insert(deepestCommand.Index, deepestCommand.Item);
                    break;
            }
        }
    }
}