using System;
using System.Collections.Generic;

namespace Clones
{
    public class CloneVersionSystem : ICloneVersionSystem
    {
        List<Clone> clones;
        public string Execute(string query)
        {
            string[] commands = query.Split();
            clones = new List<Clone>();
                switch (commands[0])
                {
                    case "learn":
                    {
                        Learn(Int32.Parse(commands[1])-1, commands[2]);
                        break;
                    }
                    case "rollback":
                    {
                        RollBack(Int32.Parse(commands[1])-1);
                        break;
                    }
                    case "relearn":
                    {
                        Relearn(Int32.Parse(commands[1])-1);
                        break;
                    }
                    case "clone":
                    {
                        MakeClone(Int32.Parse(commands[1])-1);
                        break;
                    }
                    case "check":
                    {
                        return Check(Int32.Parse(commands[1])-1);
                    }
                }
                return null;
        }

        private void Learn(int index, string command){
            if(index >= clones.Count){
                clones.Insert(index, new Clone(index));
            }
            clones[index].Learned.Push(command);
        }

        private void RollBack(int index){
            var lastProgramm = clones[index].Learned.Pop();
            clones[index].Canceled.Push(lastProgramm);
        }

        private void Relearn(int index){
            var lastProgramm = clones[index].Canceled.Pop();
            clones[index].Learned.Push(lastProgramm);
        }

        private void MakeClone(int index){
            clones.Add(new Clone(clones.Count));
            clones[clones.Count].Learned.Head = clones[index].Learned.Head;
            clones[clones.Count].Canceled.Head = clones[index].Canceled.Head;
        }

        private string Check(int index){
            if(clones[index].Learned.Head == null)
                return "basic";
            return clones[index].Learned.Head.Value;
        }
    }

    public class Clone{
        public CloneStack Learned;
        public CloneStack Canceled;
        int Number;

        public Clone(int number){
            Number = number;
            Learned = new CloneStack();
            Canceled = new CloneStack();
        }
    }

    public class StackItem{
        public string Value {get; set; }
        public StackItem Next{get; set;}
    }

    public class CloneStack{
        public StackItem Head;

        public void Push(string value){
            if(Head == null){
                Head = new StackItem { Value = value, Next = null };
            }
            else
            {
                Head = new StackItem { Value = value, Next = Head };
            }
        }

        public string Pop(){
            if (Head == null) throw new InvalidOperationException();
            var result = Head.Value;
            Head = Head.Next;
            return result;
        }

    }
}