using System;

using System.Collections.Generic;

using System.Linq;

​

public class Subscription {

  public Subscription() {}

  public Subscription(int id, int customerId, int monthlyPriceInDollars) {

    this.Id = id;

    this.CustomerId = customerId;

    this.MonthlyPriceInDollars = monthlyPriceInDollars;

  }

​

  public int Id;

  public int CustomerId;

  public int MonthlyPriceInDollars;

}

​

public class User {

  public User() {}

  public User(int id, string name, DateTime activatedOn, DateTime deactivatedOn, int customerId) {

    this.Id = id;

    this.Name = name;

    this.ActivatedOn = activatedOn;

    this.DeactivatedOn = deactivatedOn;

    this.CustomerId = customerId;

  }

​

  public int Id;

  public string Name;

  public DateTime ActivatedOn;

  public DateTime DeactivatedOn;

  public int CustomerId;

}

​

public class Challenge

{

  public static Decimal BillFor(string month, Subscription activeSubscription, User[] users) {

    if(string.IsNullOrEmpty(month)){

        return 0;

    }

    if(activeSubscription?.MonthlyPriceInDollars == null){

        return 0;

    }

    var currentMonthDate = DateTime.Parse(month);

    var numberOfDays = DateTime.DaysInMonth(currentMonthDate.Year, currentMonthDate.Month);

    decimal dailyRate = activeSubscription.MonthlyPriceInDollars / numberOfDays;

    var roundedDailyRate = Math.Round(dailyRate, 9);

     Console.WriteLine($"Daily rate is {activeSubscription.MonthlyPriceInDollars} / {numberOfDays} days => {dailyRate} / day");

     decimal subTotal = 0;

     for(int i=0; i < numberOfDays; i++){

         currentMonthDate = currentMonthDate.AddDays(i);

         var activeUsers = users.Where(x =>  x.ActivatedOn <= currentMonthDate && x.DeactivatedOn > currentMonthDate && x.CustomerId == activeSubscription.CustomerId).ToList();

         var dailyTotal = Math.Round(dailyRate * activeUsers.Count, 9);

         subTotal += dailyTotal;

         Console.WriteLine($"{currentMonthDate}  {activeUsers.Count} active users * ${dailyRate} = ${dailyTotal}  (subtotal: ${subTotal})");         

     }

     Console.WriteLine($"Total = ${subTotal} (round subtotal to nearest cent)");

     return Math.Round(subTotal, 2);

  }

​

  /*******************

  * Helper functions *

  *******************/

​

  /**

  Takes a DateTime object and returns a DateTime which is the first day

  of that month. For example:

​

  FirstDayOfMonth(new DateTime(2019, 2, 7)) // => new DateTime(2019, 2, 1)

  **/

  private static DateTime FirstDayOfMonth(DateTime date) {

    return new DateTime(date.Year, date.Month, 1);

  }

​

  /**

  Takes a DateTime object and returns a DateTime which is the last day

  of that month. For example:

​

  LastDayOfMonth(new DateTime(2019, 2, 7)) // => new DateTime(2019, 2, 28)

  **/

  private static DateTime LastDayOfMonth(DateTime date) {

    return new DateTime(date.Year, date.Month, DateTime.DaysInMonth(date.Year, date.Month));

  }

​

  /**

  Takes a DateTime object and returns a DateTime which is the next day.

  For example:

​

  NextDay(new DateTime(2019, 2, 7))  // => new DateTime(2019, 2, 8)

  NextDay(new DateTime(2019, 2, 28)) // => new DateTime(2019, 3, 1)

  **/

  private static DateTime NextDay(DateTime date) {

    return date.AddDays(1);

  }

}