using System;

public class Program

{

    private static int _staleCount = 4;

    private static int _staleCountNext = 4;

    private static int _activatedThisMonth = 0;

    private static int _includedListing = 5;

    private static int _excessListing = 0;

    private static int _excessListingNext = 0;

    public static void Main()

    {

        // Create stale listings

        var listing1 = new Listing(new DateTime(2022, 03, 26));

        var listing2 = new Listing(new DateTime(2022, 03, 26));

        var listing3 = new Listing(new DateTime(2022, 03, 26));

        var listing4 = new Listing(new DateTime(2022, 03, 26));

        // Init

        PrintStats("Init");

        // Plus #5

        var listing5 = new Listing(new DateTime(2022, 04, 26));     

        ActivateListing(listing5);

        PrintStats("+ #5");

        // Plus #6

        var listing6 = new Listing(new DateTime(2022, 04, 26));

        ActivateListing(listing6);

        PrintStats("+ #6");

        // Minus #6

        RemoveListing(listing6);

        PrintStats("- #6");

        // Plus #7

        var listing7 = new Listing(new DateTime(2022, 04, 26)); 

        ActivateListing(listing7);

        PrintStats("+ #7");

        // Plus #6

        ActivateListing(listing6);

        PrintStats("+ #6");

        // Minus #1

        RemoveListing(listing1);

        PrintStats("- #1");

        // Plus #1

        ActivateListing(listing1);

        PrintStats("+ #1");

        // Minus #1

        RemoveListing(listing1);

        PrintStats("- #1");

        // Plus #1

        ActivateListing(listing1);

        PrintStats("+ #1");

        // Minus #1

        RemoveListing(listing1);

        PrintStats("- #1");

        // Plus #8

        var listing8 = new Listing(new DateTime(2022, 04, 26));     

        ActivateListing(listing8);

        PrintStats("+ #8");

        // Minus #2

        RemoveListing(listing2);

        PrintStats("- #2");

        // Minus #3

        RemoveListing(listing3);

        PrintStats("- #3");

        // Minus #4

        RemoveListing(listing4);

        PrintStats("- #4");

        // Plus #9

        var listing9 = new Listing(new DateTime(2022, 04, 26));

        ActivateListing(listing9);

        PrintStats("+ #9");

        // Plus #10

        var listing10 = new Listing(new DateTime(2022, 04, 26));

        ActivateListing(listing10);

        PrintStats("+ #10");

        // Plus #11

        var listing11 = new Listing(new DateTime(2022, 04, 26));

        ActivateListing(listing11);

        PrintStats("+ #11");

        // Plus #12

        var listing12 = new Listing(new DateTime(2022, 04, 26));

        ActivateListing(listing12);

        PrintStats("+ #12");

    }

    public static void PrintStats(string notes)

    {

        Console.WriteLine($"{notes}\t\t Listing included: {_includedListing}\t Used this month: {_staleCount + _activatedThisMonth}\t Excess: {_excessListing}\t Stale: {_staleCount}\t Activated this Month: {_activatedThisMonth}\t Stale count next month: {_staleCountNext}\t Excess listing next month: {_excessListingNext}");

        Console.WriteLine();

    }

    // This will be called at the START of the add listing flow

    public static string CheckExcessCharge()

    {

        if(_staleCount + _activatedThisMonth + 1 > _includedListing + _excessListing)

            return "Excess charge";

        else

            return "No excess charge";

    }

    // This will be called at the END when saving the listing status

    public static string RemoveListing(Listing listing)

    {

        _staleCountNext = _staleCountNext > 0? _staleCountNext -1 : 0;

        _excessListingNext = _excessListingNext > 0? _excessListingNext - 1 : 0;

        if(listing.Status == "DELETED")

            return string.Empty; // No further processing

        // TODO: Check if stale has been deactivated this month, this can potentially be given by combination of status & date

        // We can make an educated assumption that the existence of ReActivationDate within the month means that the stale listing

        // has been reactivated before, and has no more need to reduce the stale count

        if(listing.IsStale)

            _staleCount = _staleCount > 0? _staleCount -1 : 0;

        listing.Status = "DELETED";

        return string.Empty;

    }

    // This will be called at the END when saving the listing status

    public static string ActivateListing(Listing listing)

    {       

        if (++_staleCountNext > _includedListing)

            _excessListingNext++;

        // Check if listing is activated, this means that it already has a valid slots

        if(listing.IsActive && listing.Status == "DELETED")

            return "No excess charge"; // No further processing needed

        // TODO: Check if stale listing is reactivated (which means it should have previous slot)

        // There is an exception though, if it is above threshold then it needs to apply charge

        // Update the listing status

        if(listing.Status == "DELETED")

        {

            listing.ReActivationDate = DateTime.Now;

            //listing.Status = string.Empty;

        }

        // If listing has not been reactivated, it will consume a slot

        // Doesn't matter whether the listing was stale or not      

        _activatedThisMonth++;

        if(IsAboveThreshold()) 

        {

            _excessListing++;

            return "Excess charge applied";

        }

        else

            return "No excess charge";

    }

    public static bool IsAboveThreshold()

    {

        return _staleCount + _activatedThisMonth > _includedListing + _excessListing;

    }

}

​

public class Listing

{

    public Listing(DateTime activeDate)

    {

        ActiveDate = activeDate;

    }

    public DateTime? ActiveDate {get; set;}

    public DateTime? ReActivationDate {get;set;}

    public string Status {get;set;}

    // Derived members

    // These are missing null checks, however use the null checks sparingly because by default ReActivationDate is supposed to be null

    // Stale listing is when the listing was created (ActiveDate) last month OR reactivated (ReActivationDate) last month

    public bool IsStale => ActiveDate < new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1) || ReActivationDate < new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1); 

    // Activated listing is when the listing was created (ActiveDate) this month OR reactivated (ReActivationDate) this month

    public bool IsActive => ActiveDate >= new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1) || ReActivationDate >= new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1);

}