如何提高IsItAHoliday函数的效率？

Question

I have a method that is supposed to return 1 if it is a weekend or one of these holidays: New Years Day, Memorial Day, Independence Day, Labor Day, Thanksgiving Day & Christmas Day, and 0 otherwise. I have chosen to implement the code using a combination of DateTime.DayOfYear and DateTime.IsLeapYear, but I feel like this is really inefficient, and also, makes it more difficult to do holidays like Memorial Day (final Monday of May!). How can I achieve my goal efficiently, and without hardcoding the holidays that change dates every year?

Here is my current implementation:

    private int IsItAHoliday(DateTime time)
    {
        // Assume it is not a weekend
        int isWeekendOrHoliday = 0;

        // TODO: Memorial Day, Labor Day, Thanksgiving
        // Declare holidays
        int christmasDayNonLeapYearIndex = 359;
        int christmasDayLeapYearIndex = 360;
        int newYearsDayIndex = 1;
        int independenceDayNonLeapYearIndex = 185;
        int independenceDayLeapYearIndex = 186;

        // Find out if time falls on a weekend
        if (time.DayOfWeek == DayOfWeek.Saturday || time.DayOfWeek == DayOfWeek.Sunday)
        {
            isWeekendOrHoliday = 1;
        }
        // Find out if time falls on a holiday
        // Leap year
        if (DateTime.IsLeapYear(time.Year))
        {
            if ((time.DayOfYear == christmasDayLeapYearIndex) || (time.DayOfYear == independenceDayLeapYearIndex) || (time.DayOfYear == newYearsDayIndex))
            {
                isWeekendOrHoliday = 1;
            }
        }
        // Non-leap year
        else if (!DateTime.IsLeapYear(time.Year))
        {
            if ((time.DayOfYear == christmasDayNonLeapYearIndex) || (time.DayOfYear == independenceDayNonLeapYearIndex) || (time.DayOfYear == newYearsDayIndex))
            {
                isWeekendOrHoliday = 1;
            }
        }

        return isWeekendOrHoliday;
    }

Answer 1

Common approach is to define set of rules that describes a holiday date:

public interface IHoliday
{
  bool isHoliday(DateTime date);
}

Then implement the interfaces for different holiday schemas. For example, exact date holiday:

public class ExactDayHoliday : IHoliday
{
  public int Day {get; set;}
  public int Month {get; set;}

  public bool isHoliday(DateTime date)
  {
    return date.Month == month && date.Day == day;
  }
}

Another example of rule: a day of n-th weeks of the month.

public class DayOfWeekHoliday : IHoliday
{
    public int Nth { get; set; }
    public DayOfWeek DayOfWeek { get; set; }
    public int Month { get; set; }

    public bool isHoliday(DateTime date)
    {
        return date.Month == Month && date.DayOfWeek == DayOfWeek && (date.Day - 1) / 7 == (Nth - 1);
    }
} 

Create list of rules for all holidays. For example:

List<IHoliday> holidays = new List<IHoliday> {new ExactDayHoliday {Month = 12, Day = 25}, new DayOfWeekHoliday {Nth = 1, DayOfWeek = DayOfWeek.Monday, Month = 9}};

And the last step (check for given date):

bool isHoliday = holidays.Any(rule => rule.IsHoliday(date));

Answer 2

I Suggest you use Enumerations if you will be hard coding every event you need your application to support for example :

    public enum Holidays{
    newYearsDayIndex = 1,
    independenceDayNonLeapYearIndex = 185,
        independenceDayLeapYearIndex = 186,
    christmasDayNonLeapYearIndex = 359,
        christmasDayLeapYearIndex = 360
    }

Then use the value of each as the day index of a holiday and the name to explain why this day is a holiday

    Enum.GetValues(typeof(Holidays)); --> For values which you will use as days Index
    Enum.GetNames(typeof(Holidays)); --> For Names which you will use to describe the picked date

You may also consider separating Leap holidays and non leap holidays each in separate Enumeration with your naming conventions

Answer 3

You dont need to check for YEAR in holidays , because they appear on the same dates every year, like say Dec 25, 1 Jan etc.

I quickly compiled some code:

public class Holiday
{
    public int Day {get; private set;}
    public int Month {get; private set;}
    public Holiday(int day, int month)
    {
        this.Day = day;
        this.Month = month;
    }

    public bool IsEqual(DateTime dateTime)
    {
        return ((this.Day == dateTime.Day) && (this.Month == dateTime.Month));
    }
}

public static class DateTimeExtensions
{
    private static List<Holiday> holidays = new List<Holiday>();

    public static void RegisterHoliday(Holiday holiday)
    {
        holidays.Add(holiday);
    }

    public static bool IsHoliday(this DateTime dateTime)
    {
        foreach (Holiday holiday in holidays)
        {
            if (holiday.IsEqual(dateTime)) return true;
        }
        return false;
    }

    public static bool IsWeekend(this DateTime dateTime)
    {
        return (dateTime.DayOfWeek == DayOfWeek.Sunday) || (dateTime.DayOfWeek == DayOfWeek.Saturday);
    }
}

Now Add Holidays using the Register method and use the extensions. DateTimeExtensions.RegisterHoliday(new Holiday(25, 12));

DateTime dateTimeToCheck = new DateTime(2013, 12, 25); return dateTimeToCheck.IsHoliday() || dateTimeToCheck.IsWeekend();