circularPrimes 2/24

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

namespace circularPrimes
{
    class Program
    {
        /*static void Main(string[] args)
        {
            int primesCount = 0;

            Console.WriteLine("Input p: ");
            int p = Int32.Parse(Console.ReadLine());

            //Console.WriteLine((!fermatIsPrime(p)).ToString());

            for (int i = 2; i < p; i += 2)
            {
                if (!fermatIsPrime(i))
                {
                    primesCount++;
                }
            }

            Console.WriteLine(primesCount);

            Console.ReadKey();
        }*/

        static void Main(string[] args)
        {
            /*Console.WriteLine("Number to check: ");
            int p = Int32.Parse(Console.ReadLine());

            Console.WriteLine("Number is prime: " + isPrime(p));

            Console.ReadKey();*/

            int count = 2;

            Console.WriteLine("number to count primes to: ");
            int n = Int32.Parse(Console.ReadLine());

            for (int i = 5; i < n; i++)
            {
                if (isPrime(i))
                {
                    Console.WriteLine(i);
                    count++;
                }
            }

            Console.WriteLine("count: " + count);

            Console.ReadKey();
        }

        public static bool isPrime(int p)
        {
            if (p == 1 || p == 2)
            {
                return true;
            }
            else if (p % 2 == 0 || p % 5 == 0)
            {
                return false;
            }
            else
            {
                if (fermatIsPrime(p))
                {
                    // Number is possibly prime
                    // Verify
                    for (int i = 3; i <= Math.Truncate(Math.Sqrt(p)); i++)
                    {
                        // Loop as far as is necessary
                        if (p % i == 0)
                        {
                            // Number is divisible by another number
                            // Number is not prime
                            return false;
                        }
                    }

                    // All tests have failed
                    // Number is prime
                    return true;
                }
                else
                {
                    // Number is not possibly prime according to Fermat's theorem
                    return false;
                }
            }
        }

        public static int powerMod(int b, int e, int m)
        {
            int x = 1;
            while (e > 0)
            {
                if (e % 2 == 1)
                {
                    x = (x * b) % m;
                    e -= 1;
                }
                else
                {
                    b = (b * b) % m;
                    e = e / 2;
                }
            }

            return x;
        }

        public static bool fermatIsPrime(int p)
        {
            BigInteger intermediateValue = new BigInteger();
            BigInteger pBigInt = new BigInteger(p);
            BigInteger a = new BigInteger();
            Random randomGen = new Random();
            //int a = 0;
            for (int i = 0; i < p * .2; i++)
            {
                // Loop for amount of times equal to 20% of the number
                //a = randomGen.Next(3, p - 1);

                /* if (Math.Pow(a, p - 1) % p != 1)
                {
                    // Number is not prime
                    return false;
                } */



                intermediateValue = intermediateValue.ModPow(a, pBigInt - 1, pBigInt);

                // Number is probably prime
            }

            // Number is PROBABLY prime
            // Must be checked to be sure
            return true;
        }
    }
}