using System;

namespace Factorial
{
    class Program
    {
        static void Main(string[] args)
        {
            Factorial<intFact> f = new Factorial<intFact>(new intFact(5));
            Console.WriteLine(f.Value);
        }

    }
    
    struct intFact : IFactoriable<intFact>
    {
        public int n;
        public intFact(int num)
        {
            n = num;
        }
        public bool isZero()
        {
            return n == 0;
        }
        public bool isBelowZero()
        {
            return n < 0;
        }
        public intFact Step()
        {
            return new intFact(n - 1);
        }
        public intFact Multiply(intFact iF)
        {
            return new intFact(n * iF.n);
        }
        public intFact getNeutralElement()
        {
            return new intFact(1);
        }
        public override string ToString()
        {
            return n.ToString();
        }
    }

    [Serializable]
    class Factorial<T> where T : IFactoriable<T>
    {
        T p;

        public T Value
        {
            get;
            private set;
        }
        public T Param
        {
            get 
            { 
                return p; 
            }
            set 
            {
                if (value.isBelowZero()) throw new ArgumentOutOfRangeException("n or Param");
                Value = fact(value); 
                p = value; 
            }
        }

        public Factorial(T n)
        {
            Param = n;
        }

        T fact(T n)
        {
            if (n.isZero()) return n.getNeutralElement();
            return n.Multiply(fact(n.Step()));
        }
    }

    interface IFactoriable<T>
    {
        bool isZero();
        bool isBelowZero();
        T Step();
        T getNeutralElement();
        T Multiply(T t);
    }
}