using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main(string[] args)

    {

        LetsGetItDone();

    }

    public static void LetsGetItDone()

    {

        char[] array = "01234567".ToCharArray();

        var list = GetPer(array);

        int[] date;

        int[] days31 = {1, 3, 5, 7, 8, 10, 12};

        int[] days30 = { 4, 6, 9, 11};

        int counter = 0;

        DateTime lastDateOf2015 = new DateTime(2015, 12, 31);

        DateTime illion = DateTime.Today;

        SortedSet<DateTime> set = new SortedSet<DateTime>();

        foreach(int i in list)

        {

            if (i % 10000 > 2000 && i % 10000 < 2999)

            {

                date = GetDayMonthYear(i);

                if(date[0] <= 30 && (days30.Contains(date[1])))

                {

                    counter = ProcessDate(ref illion, date, counter);

                    set.Add(new DateTime(illion.Ticks));

                }

                else if (date[0] <= 31 && days31.Contains(date[1]))

                {

                    counter = ProcessDate(ref illion, date, counter);

                    set.Add(new DateTime(illion.Ticks));

                }

                else if(date[0] < 29 && date[1] == 2)

                {

                    counter = ProcessDate(ref illion, date, counter);

                    set.Add(new DateTime(illion.Ticks));

                }

                else if(date[0] == 29 && date[1] == 2 && DateTime.IsLeapYear(date[2]))

                {

                    counter = ProcessDate(ref illion, date, counter);

                    set.Add(new DateTime(illion.Ticks));

                }

            }

        }

        Console.WriteLine("Number of valid illion dates: {0}\nFirst Illion after 2015: {1}\n\nFirst 5 illions are: ", counter, set.ElementAt(0));

        counter = 0;

        foreach(DateTime t in set)

        {

            ++counter;

            Console.WriteLine(t);

            if(counter == 5)

            {

                break;

            }

        }

    }

    public static int ProcessDate(ref DateTime illion, int[] date, int counter)

    {

        DateTime tempDate = new DateTime(date[2], date[1], date[0]);

        if (illion == DateTime.Today)

        {

            illion = tempDate;

        }

        else if (tempDate.Year > 2015)

        {

            illion = tempDate;

        }

        return ++counter;

    }

    static int[] GetDayMonthYear(int num) 

    {

        int[] date = new int[3];

        date[0] = num / 1000000;

        date[1] = (num % 1000000) / 10000;

        date[2] = num % 10000;

        return date;

    }

    public static List<int> GetPer(char[] list)

    {

        int x = list.Length - 1;

        List<int> permutationList = new List<int>();

        return GetPer(list, 0, x, permutationList);

    }

    private static List<int> GetPer(char[] list, int k, int m, List<int> permutationList)

    {

        if (k == m)

        {

            permutationList.Add(Convert.ToInt32(new string(list)));

        }

        else

        {

            for (int i = k; i <= m; i++)

            {

                Swap(ref list[k], ref list[i]);

                GetPer(list, k + 1, m, permutationList);

                Swap(ref list[k], ref list[i]);

            }

        }

        return permutationList;

    }

    private static void Swap(ref char a, ref char b)

    {

        if (a == b) return;

        a ^= b;

        b ^= a;

        a ^= b;

    }

}