CVs

using System;
using System.Collections.Generic;
using NUnit.Framework;

namespace Clones
{
    public class CloneVersionSystem : ICloneVersionSystem
    {
        public string Execute(string query)
        {
            var segments = query.Split(' ');
            var command = segments[0];
            var ci = int.Parse(segments[1]);
            switch (command)
            {
                case "learn":
                    var pi = int.Parse(segments[2]);
                    Trooper.Learn(ci, pi);
                    return null;
                case "relearn":
                    Trooper.Relearn(ci);
                    return null;
                case "rollback":
                    Trooper.Rollback(ci);
                    return null;
                case "clone":
                    Trooper.CloneIt(ci);
                    return null;
                case "check":
                    return Trooper.CheckIt(ci);

                default: throw new ArgumentException();
            }

        }
    }
    public class Trooper
    {
        public static List<Trooper> SoldiersList = new List<Trooper>();
        public Stack<int> ActivatedPrograms = new Stack<int>();
        public Stack<int> CanceledPrograms = new Stack<int>();

        public static void Learn(int number, int program)
        {
            CaseOfNull(SoldiersList, number);
            var soldier = SoldiersList[number];
            soldier.ActivatedPrograms.Push(program);
        }

        public static void Relearn(int number)
        {
            CaseOfNull(SoldiersList, number);
            var soldier = SoldiersList[number];
            if (soldier.CanceledPrograms.Count > 0)
                soldier.ActivatedPrograms.Push(soldier.CanceledPrograms.Pop());
        }

        public static void Rollback(int number)
        {
            CaseOfNull(SoldiersList, number);
            var soldier = SoldiersList[number];
            if (soldier.ActivatedPrograms.Count > 0)
                soldier.CanceledPrograms.Push(soldier.ActivatedPrograms.Pop());
        }

        public static void CloneIt(int number)
        {
            CaseOfNull(SoldiersList, number);
            var soldier = SoldiersList[number];
            SoldiersList.Add(new Trooper()
            {
                ActivatedPrograms = soldier.ActivatedPrograms,
                CanceledPrograms = soldier.CanceledPrograms
            });
        }

        public static string CheckIt(int number)
        {
            CaseOfNull(SoldiersList, number);
            var soldier = SoldiersList[number];
            if (soldier.ActivatedPrograms.Count > 0)
             {
                 var lastProgram = soldier.ActivatedPrograms.Peek();
                 return lastProgram.ToString();
             }
            return "basic";
        }
        private static void CaseOfNull(List<Trooper> soldiersList, int number)
        {
            if (soldiersList[number] == null)
                soldiersList.Add(new Trooper());
        }

    }
}