using System;
using System.Collections.Generic;
using System.Text;

namespace VanWassenhove.Util
{
    /// <summary>
    /// This class contains helper methods for the manipulation of DateTime instances.
    /// </summary>
    public class DateTimeHelpers
    {
        /// <summary>
        /// The base DateTime.
        /// </summary>
        public static DateTime UnixTimeBase = new DateTime(1970, 1, 1);

        /// <summary>
        /// Converts a unixtime into a DateTime.
        /// </summary>
        /// <param name="timestamp">The unixtime to convert.</param>
        /// <returns>An equivalent datetime for the given unixtime.</returns>
        public static DateTime FromUnixTime(double timestamp)
        {
            return UnixTimeBase.AddSeconds(timestamp);
        }

        /// <summary>
        /// Converts a DateTime to a unixtime.
        /// </summary>
        /// <param name="dateTime">The datetime to convert.</param>
        /// <returns>An equivalent unixtime for the given datetime.</returns>
        public static double ToUnixTime(DateTime dateTime)
        {
            TimeSpan timeSpan = dateTime - UnixTimeBase;
            return timeSpan.TotalSeconds;
        }

        /// <summary>
        /// Returns the first day of the week for a given datetime.
        /// </summary>
        /// <param name="dateTime">The given datetime.</param>
        /// <returns>The first day of the week.</returns>
        public static DateTime FirstDayOfWeek(DateTime dateTime)
        {
            if (dateTime.DayOfWeek == DayOfWeek.Sunday)
            {
                dateTime = dateTime.AddDays(-7);
            }
            return dateTime.AddDays(-Convert.ToInt32(dateTime.DayOfWeek) + 1);
        }

        /// <summary>
        /// Returns the last day of the week for a given datetime.
        /// </summary>
        /// <param name="dateTime">The given datetime.</param>
        /// <returns>The last day of the week.</returns>
        public static DateTime LastDayOfWeek(DateTime dateTime)
        {
            if (dateTime.DayOfWeek == DayOfWeek.Sunday)
            {
                dateTime = dateTime.AddDays(-7);
            }
            return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day).AddDays(7 - Convert.ToInt32(dateTime.DayOfWeek));
        }

        /// <summary>
        /// Returns the first day of the month of a given datetime.
        /// </summary>
        /// <param name="dateTime">The given datetime.</param>
        /// <returns>The first day of the month.</returns>
        public static DateTime FirstDayOfMonth(DateTime dateTime)
        {
            return new DateTime(dateTime.Year, dateTime.Month, 1);
        }

        /// <summary>
        /// Returns the last day of the month for a given datetme.
        /// </summary>
        /// <param name="dateTime">The given datetime.</param>
        /// <returns>The last day of the month.</returns>
        public static DateTime LastDayOfMonth(DateTime dateTime)
        {
            return new DateTime(dateTime.Year, dateTime.Month, DateTime.DaysInMonth(dateTime.Year, dateTime.Month));
        }

        /// <summary>
        /// Returns the interval that exist between end and begin expressed in units of the given spanUnit.
        /// </summary>
        /// <param name="begin">The begin of the interval.</param>
        /// <param name="end">The end of the interval>.</param>
        /// <param name="spanUnit">The unit in which the interval should be expressed.</param>
        /// <returns>The number of units that exist between end and begin.</returns>
        public static long TimeSpan(DateTime begin, DateTime end, SpanUnit spanUnit)
        {
            TimeSpan ts = ts = end - begin;

            switch (spanUnit)
            {
                case SpanUnit.Year:
                    return end.Year - begin.Year;
                case SpanUnit.Month:
                    return (end.Month - begin.Month) + (12 * (end.Year - begin.Year));
                case SpanUnit.Weekday:
                    return Fix(ts.TotalDays) / 7;
                case SpanUnit.Day:
                    return Fix(ts.TotalDays);
                case SpanUnit.Hour:
                    return Fix(ts.TotalHours);
                case SpanUnit.Minute:
                    return Fix(ts.TotalMinutes);
                default:
                    return Fix(ts.TotalSeconds);
            }
        }

        private static long Fix(double number)
        {
            if (number >= 0)
            {
                return (long)Math.Floor(number);
            }
            return (long)Math.Ceiling(number);
        }
    }

    /// <summary>
    /// This enum represents the Unit in which a TimeSpan (interval) number is expressed.
    /// </summary>
    public enum SpanUnit
    {
        Year,
        Month,
        Weekday,
        Day,
        Hour,
        Minute,
        Second
    }
}