using System;

using System.Threading;

​

// A factor is a number that divides into another number with no remainder.

// A prime number is a number that has only two factors, 1 and itself.

// Prime factors satisfy both of these conditions.

// For example, the prime factors of 13195 are 5, 7, 13 and 29.

// This application aims to calculate the largest prime factor of an input number.

//

// ### Challenge 1 - The current implementation gives the correct answer for 13195.

//                   However, it gives incorrect results for some numbers, eg 48.

//                   Find the bug and fix it.

// ### Challenge 2 - This function doesn't scale very well.

//                   If we input a large number like 600851475143, it takes many minutes to run.

//                   (Note - if you are running this at dotnetfiddle.net, it will timeout / kill the process before

//                          it finishes so it will just say "Processing...", "Processing..." and never get to "Finished.")

//                   Change the function so can find the largest prime factor for 600851475143 before timing out.

​

namespace code_review_1

{

    class Program

    {

        // eg: dotnet run 27182

        static void Main(string[] args)

        {

            var number = 13195; // works

            // var number = 48; // does not work

​

            RegisterHandlers();

            Log("Starting...");

            Log("Input: " + number);

            StartProgressDots();

            var largestPrimeFactor = LargestPrimeFactor(number);

            StopProgressDots();

            Log("Finished.");

            Log("Largest Prime: " + largestPrimeFactor);

        }

​

        private static long? LargestPrimeFactor(long number)

        {

            long? lpf = 0;

​

            for (var i = 2; i < number; i++)

            {

                if (IsPrime(i) && IsFactor(i, number) && i > lpf)

                {

                    Log("Found " + i);

                    lpf = i;

                }

            }

​

            return lpf;

        }

​

        private static bool IsPrime(long number)

        {

            for (var i = 2; i < Math.Sqrt(number); i++)

            {

                if (IsFactor(i,number))

                {

                    return false;

                }

            }

​

            return true;

        }

​

        private static bool IsFactor(long denominator, long numerator)

        {

            return numerator % denominator == 0;

        }

​

        // progress

​

        private static Timer _progressDotsTimer;

​

        private static void RegisterHandlers()

        {

            Console.CancelKeyPress += delegate

            {

                StopProgressDots();

                Log("Did not finish.... Aborting...");

            };

        }

​

        private static void Log(string str)

        {

            Console.WriteLine(DateTime.Now.ToLongTimeString() + ": " + str);

        }

​

        private static void StartProgressDots()

        {

            TimerCallback printDots = _ =>

            {

                Thread.Sleep(TimeSpan.FromMilliseconds(1500));

                Log("Processing...");

            };

            _progressDotsTimer = new Timer(printDots, null, TimeSpan.Zero, TimeSpan.FromSeconds(1));

        }

​

        private static void StopProgressDots()

        {

            _progressDotsTimer.Dispose();

        }

    }

}

​