using System;

using System.Globalization;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var start = DateTime.ParseExact("1600-02-05 02:55:51", "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);

        var end = DateTime.ParseExact("2016-02-29 22:55:51", "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);

        GetDocumentsInRange(start, end);

    }

    // This is made void to ease testing

    public static void GetDocumentsInRange(DateTime st, DateTime en)

    {

        Console.WriteLine("start:"+st+ "\nend:"+en);

        foreach (var prefix in GetAllPrefix( st, en))

        {

            // ideally return the values of this call

            Console.WriteLine(GetDocumentsPrefix(prefix));

        }

    }

    public static string GetDocumentsPrefix(string prefix)

    {

        return "GetDocuments called for prefix: " + prefix;

    }

    internal static IEnumerable<string> GetAllPrefix(DateTime st, DateTime en)

    {

        if (st <= en)

        {

            var start = new MyDateTime(st);

            var end = new MyDateTime(en);

​

            List<string> prefixes = new List<string>();

            string prefix;

            var nextStart = GetPrefix(start, end, out prefix);

            prefixes.Add(prefix);

            if(IsValidPrefix(prefix)) yield return prefix.TrimEnd('*');

​

            // string comparison of the datetime representation

            while (string.Compare(nextStart.ToString(), end.ToString()) <= 0)

            {

                nextStart = GetPrefix(nextStart, end, out prefix);

                if (IsValidPrefix(prefix)) yield return prefix.TrimEnd('*');

            }

        }

    }

    private static bool IsValidPrefix(string prefix)

    {

        // remove sequence if day or month are '00' as they are invalid

        return !(prefix.EndsWith("00********") || prefix.EndsWith("00******"));

    }

    internal static MyDateTime GetPrefix(MyDateTime start, MyDateTime end, out string prefix)

    {

        var startString = start.ToString();

        var numTrailZeroes = CountTrailingZeroes(startString);

        int pos = numTrailZeroes;

        MyDateTime nextStart = end;

​

        while (pos >= 0)

        {

            nextStart = Increment(start, pos);

​

            // string comparison of the datetime representation

            if (string.Compare(nextStart.ToString(), end.ToString()) <= 0)

            {

                break;

            }

            --pos;

        }

​

        // incase start == end pos will become -1

        if (pos < 0)

        {

            pos = 0;

        }

        prefix = ReplaceSuffix(startString, pos);

        return nextStart;

    }

    private static int CountTrailingZeroes(string input)

    {

        // Console.WriteLine(input);

        return input.Reverse().TakeWhile(c => c == '0').Count();

    }

    // Function that increments the digit pointed by the position

    // digit can be second. 10Seconds, minutes, 10Minture, ....decades, centuries, millennia

     internal static MyDateTime Increment(MyDateTime moment, int pos)

     {

         switch (pos)

         {

             case 0:

                 return moment.AddSeconds(1);

             case 1:

                 return moment.AddSeconds(10);

             case 2:

                 return moment.AddMinutes(1);

             case 3:

                 return moment.AddMinutes(10);

             case 4:

                 return moment.AddHours(1);

             case 5:

                 return moment.AddHours(10);

             case 6:

                 return moment.AddDays(1);

             case 7:

                 return moment.AddDays(10);

             case 8:

                 return moment.AddMonths(1);

             case 9:

                 return moment.AddMonths(10);

             case 10:

                 return moment.AddYears(1);

             case 11:

                 return moment.AddYears(10);

             case 12:

                 return moment.AddYears(100);

             case 13:

                 return moment.AddYears(1000);

            //seat belt

             default:

                 throw new ArgumentException("pos not supported:" + pos);

         }

     }

    // add * at the end to help debuggin

    private static string ReplaceSuffix(string input, int endPos)

    {

        var prefix = input.Substring(0, input.Length - endPos);

        var suffix = string.Concat(Enumerable.Repeat('*', endPos));

        return prefix + suffix;

    }   

​

    internal class MyDateTime

    {

        public int Year;

        public int Month;

        public int Day;

        public int Hour;

        public int Minute;

        public int Second;

​

        public MyDateTime(int y, int mon, int d, int h, int min, int s)

        {

            Year = y;

            Month = mon;

            Day = d;

            Hour = h;

            Minute = min;

            Second = s;

        }

​

        public MyDateTime(DateTime dt)

        {

            Year = dt.Year;

            Month = dt.Month;

            Day = dt.Day;

            Hour = dt.Hour;

            Minute = dt.Minute;

            Second = dt.Second;

        }

​

        public MyDateTime(MyDateTime dt)

        {

            Year = dt.Year;

            Month = dt.Month;

            Day = dt.Day;

            Hour = dt.Hour;

            Minute = dt.Minute;

            Second = dt.Second;

        }

​

        // Custom Add functions that increment the value of certain digit

        // Add check : increment can be add 1 0r 10

        public MyDateTime AddSeconds(int sec)

        {

            var temp = new MyDateTime(this);

            temp.Second = temp.Second + sec;

​

            if (temp.Second >= 60)

            {

                temp.Second = temp.Second % 60;

                // currently we dont add more than 60 seconds

                return temp.AddMinutes(1);

            }

            return temp;

        }

​

        public MyDateTime AddMinutes(int min)

        {

            var temp = new MyDateTime(this);

            temp.Minute += min;

            if (temp.Minute >= 60)

            {

                temp.Minute = 0;

                return temp.AddHours(1);

            }

            return temp;

        }

​

        // Add check : increment can be add 1 0r 10

        public MyDateTime AddHours(int hour)

        {

            var temp = new MyDateTime(this);

            temp.Hour += hour;

            if (temp.Hour > 24)

            {

                temp.Hour = 0;

                return temp.AddDays(1);

            }

            return temp;

        }

​

        // Add check : increment can be add 1 0r 10

        public MyDateTime AddDays(int day)

        {

            var temp = new MyDateTime(this);

            temp.Day += day;

​

            if (temp.Day > DateTime.DaysInMonth(temp.Year, temp.Month))

            {

                // this is invalid date, but thats okay as we need 

                // to represent first nine days of month

                // eg: 2018010* represents first nine days of october2018

                temp.Day = 0;

                return temp.AddMonths(1);

            }

            return temp;

        }

​

        // Add check : increment can be add 1 0r 10

        public MyDateTime AddMonths(int mon)

        {

            var temp = new MyDateTime(this);

            temp.Month += mon;

            if (temp.Month > 12)

            {

                // this is invalid date, but thats okay as we need 

                // to represent Jan-Sep

                // eg: 2018* represent Jan-Sep of 2018

                temp.Month = 0;

                return temp.AddYears(1);

            }

            return temp;

        }

​

       // Add check : increment can be add 1 0r 10

        public MyDateTime AddYears(int y)

        {

            var temp = new MyDateTime(this);

            temp.Year += y;

            return temp;

        }

​

        public override string ToString()

        {

            string value= string.Format("{0:0000}{1:00}{2:00}{3:00}{4:00}{5:00}",Year,Month,Day,Hour,Minute,Second);

            // seat belt

            if (value.Length != 14)

            { throw new Exception("My datestring is invalid:"+ value);}

​

            return value;

        }

    }

}

​

​