using System;

using System.Diagnostics;

public class Program

{

​

    public static DateTime A(DateTime d)

    {

        var precedingDay = d.AddDays(-1);

        var monday = precedingDay.AddDays(-(int)precedingDay.DayOfWeek).AddDays(1);

​

        return monday;

    }

    private static DateTime B(DateTime date)

    {

      if (date.DayOfWeek == DayOfWeek.Monday)

      {

        return date;      

      }

​

      int distanceToPrevMonday = date.DayOfWeek - DayOfWeek.Monday;

​

      if (distanceToPrevMonday < 0)

      {

          const int BackDistanceFromSundayToMonday = 6; 

        distanceToPrevMonday = BackDistanceFromSundayToMonday;

      }

​

      return date.AddDays(-distanceToPrevMonday);

    }

    private static DateTime C(DateTime date)

    {

        var weekStartDay = DayOfWeek.Monday;

        int difference = (7 + (date.DayOfWeek - weekStartDay)) % 7;

        return date.AddDays(-1 * difference).Date;

    }

    public static TimeSpan Bench(int iters, Action bench)

    {

        Stopwatch sw = Stopwatch.StartNew();

        for (int i = 0; i < iters; i++)

        {

            bench();

        }

        sw.Stop();

        return sw.Elapsed;

    }

    public static void Main()

    {

        // some warmup

        for (int i = 0; i < 100; i++)

        {

            _ = A(DateTime.Now);

            _ = B(DateTime.Now);

            _ = C(DateTime.Now);    

        }

        TimeSpan timeA = TimeSpan.Zero;

        TimeSpan timeB = TimeSpan.Zero;

        TimeSpan timeC = TimeSpan.Zero;

        for (int i = 0; i < 365; i++)

        {

            DateTime stamp = new DateTime(2021, 1, 1).AddDays(i);

            DateTime resultA = DateTime.MaxValue;

            DateTime resultB = DateTime.MinValue;

            DateTime resultC = DateTime.MinValue;

            timeC += Bench(10000, () => resultC = C(stamp));

            timeB += Bench(10000, () => resultB = B(stamp));

            timeA += Bench(10000, () => resultA = A(stamp));

            if (resultA != resultB)

            {

                throw new InvalidOperationException("Does not work");

            }

        }

        Console.WriteLine("A: " + timeA.ToString());

        Console.WriteLine("B: " + timeB.ToString());

        Console.WriteLine("C: " + timeC.ToString());

    }

}