A Flexible Plugin System

using System;
using Xunit;

namespace Fadd
{
    /// <summary>
    /// Calculates the number of months and days between two dates (and add the overlapping days).
    /// </summary>
    public struct MonthSpan
    {
        private int _days;
        private int _months;

        /// <summary>
        /// Number of months
        /// </summary>
        public int Months
        {
            get { return _months; }
            set { _months = value; }
        }

        /// <summary>
        /// Number of days
        /// </summary>
        public int Days
        {
            get { return _days; }
            set { _days = value; }
        }

        /// <summary>
        /// Creates a monthspan between two dates
        /// </summary>
        /// <param name="from">from which date to calculate.</param>
        /// <param name="to">end date.</param>
        /// <returns>Number of months and days between the two dates.</returns>
        public static MonthSpan Create(DateTime from, DateTime to)
        {
            int month = from.Month, year = from.Year;
            MonthSpan span = new MonthSpan();
            if (from.Day != 1)
            {
                // same month
                if (to.Month == from.Month && to.Year == from.Year)
                {
                    TimeSpan tspan = to - from;
                    span.Days = tspan.Days;
                    return span;
                }

                span.Days = DateTime.DaysInMonth(from.Year, from.Month) - from.Day;
                ++month;
                if (month == 13)
                {
                    month = 1;
                    ++year;
                }
            }

            while (year < to.Year || (year == to.Year && month < to.Month))
            {
                ++month;
                if (month == 12)
                {
                    ++year;
                    month = 1;
                }

                ++span.Months;
            }

            if (to.Day != DateTime.DaysInMonth(to.Year, to.Month))
                span.Days += to.Day;
            else if (year == to.Year && month == to.Month)
                span.Months += 1;

            return span;
        }

#if DEBUG


        [Fact]
        private static void TestOneMonth()
        {
            DateTime from = new DateTime(2008, 1, 1);
            DateTime to = new DateTime(2008, 1, 31);

            MonthSpan span = Create(from, to);
            Assert.Equal(1, span.Months);
        }

        [Fact]
        private static void Test1Month5Days()
        {
            DateTime from = new DateTime(2008, 1, 1);
            DateTime to = new DateTime(2008, 2, 5);

            MonthSpan span = Create(from, to);
            Assert.Equal(1, span.Months);
            Assert.Equal(5, span.Days);
        }

        [Fact]
        private static void Test2Month20Days()
        {
            DateTime from = new DateTime(2008, 1, 1);
            DateTime to = new DateTime(2008, 3, 20);

            MonthSpan span = Create(from, to);
            Assert.Equal(2, span.Months);
            Assert.Equal(20, span.Days);
        }
        [Fact]
        private static void TestYearLap()
        {
            DateTime from = new DateTime(2008, 10, 1);
            DateTime to = new DateTime(2009, 1, 5);

            MonthSpan span = Create(from, to);
            Assert.Equal(2, span.Months);
            Assert.Equal(5, span.Days);
        }


        [Fact]
        private static void TestDaysOnly()
        {
            DateTime from = new DateTime(2008, 10, 20);
            DateTime to = new DateTime(2008, 10, 25);

            MonthSpan span = Create(from, to);
            Assert.Equal(0, span.Months);
            Assert.Equal(5, span.Days);
        }

        [Fact]
        private static void TestDaysSpannedOnly()
        {
            DateTime from = new DateTime(2008, 10, 20);
            DateTime to = new DateTime(2008, 11, 12);

            MonthSpan span = Create(from, to);
            Assert.Equal(0, span.Months);
            Assert.Equal(23, span.Days);
        }
#endif
    }
}