using System;
using System.Numerics;
using System.Text;

namespace ConsoleApp3a
{
    class Program
    {
        static void Main(string[] args)
        {
            //BigInteger bInt
            Console.OutputEncoding = Encoding.UTF8;
            Console.WriteLine("Hello World!");

            Console.WriteLine(SayTheDigit(3));
            Console.WriteLine(IsPointIn(1.2, 2.3, 10));

            // 0!= 1;
            // n! = (n-1)! *n;
            // 10
            int factorial = 0; 
            for (int i = 0; i < 10; i++)
            {
                if (i == 0)
                {
                    factorial = 1;
                }
                else
                {
                    factorial *= i;
                }              
            }

            Console.WriteLine("10! = {0}", factorial);
            Console.WriteLine(Factorial(10));

            // 1,1,2,3,5,8,13,21,34
            Console.WriteLine(Fibonacci(40));

            //for (int i = 0; i < 5; i++)
            //{
            //    Console.Write("{0}, ", Fibonacci(i));
            //}

            Console.Write("Please enter a = ");
            double a = double.Parse(Console.ReadLine());
            Console.WriteLine();

            Console.Write("Please enter b = ");
            double b = double.Parse(Console.ReadLine());
            Console.WriteLine();

            Console.Write("Please enter c  = ");
            double c = double.Parse(Console.ReadLine());
            Console.WriteLine();


            Console.WriteLine("Is existing? - {0}.", CheckTriangleExists(a,b,c));
            if (CheckTriangleExists(a, b, c)) {
                Console.WriteLine("The surface is: {0}", CalculateSurface(a, b, c));
            }
        }

        static double InputSite(string nameVariable) {
            Console.Write($"Please enter {nameVariable} = ");
            return  double.Parse(Console.ReadLine());
            // Console.WriteLine();
        }

        static bool CheckTriangleExists(double a, double b, double c)
        {
            bool isExists = false;
            if (a < b + c && b < c + a && c < b + a) isExists = true;
            // return (a < b + c && b < c + a && c < b + a);
            return isExists;
        }

        static double CalculateSurface(double a, double b, double c) {
            double perimeter = a + b + c;
            return Math.Sqrt(perimeter * (perimeter - a) * (perimeter - b) * (perimeter - c));
        }

        static int Factorial(int n)
        {
            if (n == 0) return 1; // 0!= 1;
            return Factorial(n - 1) * n; // n! = (n-1)! *n;
        }

        static int Fibonacci(int number)
        {
            if (number == 1 || number == 2) return 1;
            return Fibonacci(number - 1) + Fibonacci(number - 2);
        }

        static bool IsPointIn(double x, double y, double r)
        {
            bool isIn = false;
            double d = Math.Sqrt(Math.Pow((x - 1), 2) + Math.Pow((y - 0), 2));

            isIn = d > r;

            return isIn;
        }

        static string SayTheDigit(int digit) {
            string digitName = "";
            switch (digit)
            {
                case 1:
                    digitName = "едно";
                    break;
                case 2:
                    digitName = "две";
                    break;
                case 3:
                    digitName = "три";
                    break;
                case 4:
                    digitName = "четири";
                    break;
                case 5:
                    digitName = "пет";
                    break;
                case 6:
                    digitName = "шест";
                    break;

                default:
                    break;
            }
            return digitName;
        }
    }
}