using System.Text;

[assembly: System.Runtime.CompilerServices.InternalsVisibleTo("UnitTests")]

namespace System
{
	/// <summary>
	/// Provides extension methods for date and time manipulation.
	/// </summary>
	public static class DateTimeExtensions
	{
		#region Kind

		/// <summary>
		/// Specifies the <see cref="DateTime.Kind"/> as UTC.
		/// </summary>
		/// <param name="dt">The <see cref="DateTime"/> instance.</param>
		/// <returns>A <see cref="DateTime"/> with correct <see cref="DateTime.Kind"/>.</returns>
		public static DateTime AsUtc(this DateTime dt)
		{
			return dt.Kind switch
			{
				DateTimeKind.Local => dt.ToUniversalTime(),
				DateTimeKind.Utc => dt,
				_ => DateTime.SpecifyKind(dt, DateTimeKind.Utc),
			};
		}

		/// <summary>
		/// Specifies the <see cref="DateTime.Kind"/> as local time.
		/// </summary>
		/// <param name="dt">The <see cref="DateTime"/> instance.</param>
		/// <returns>A <see cref="DateTime"/> with correct <see cref="DateTime.Kind"/>.</returns>
		public static DateTime AsLocal(this DateTime dt)
		{
			return dt.Kind switch
			{
				DateTimeKind.Local => dt,
				DateTimeKind.Utc => dt.ToLocalTime(),
				_ => DateTime.SpecifyKind(dt, DateTimeKind.Local),
			};
		}

		#endregion Kind

		#region Aligned Interval

		/// <summary>
		/// Aligns the <see cref="TimeSpan"/> to the UTC clock.
		/// </summary>
		/// <param name="timeSpan">The timespan to align.</param>
		/// <param name="offset">A specific offset to the timespan.</param>
		/// <returns>The timespan until the aligned time.</returns>
		public static TimeSpan GetAlignedIntervalUtc(this TimeSpan timeSpan, TimeSpan offset = default)
			=> timeSpan.GetAlignedInterval(DateTime.UtcNow, offset);

		/// <summary>
		/// Aligns the <see cref="TimeSpan"/> to the local clock and respects daylight saving time.
		/// </summary>
		/// <param name="timeSpan">The timespan to align.</param>
		/// <param name="offset">A specific offset to the timespan.</param>
		/// <returns>The timespan until the aligned time.</returns>
		public static TimeSpan GetAlignedIntervalLocal(this TimeSpan timeSpan, TimeSpan offset = default)
			=> timeSpan.GetAlignedInterval(DateTime.Now, offset);

		internal static TimeSpan GetAlignedInterval(this TimeSpan timeSpan, DateTime now, TimeSpan offset = default)
		{
			var nowWithOffset = new DateTimeOffset(now);

			var nextTime = new DateTime(nowWithOffset.Ticks / timeSpan.Ticks * timeSpan.Ticks, now.Kind).Add(offset);
			var nextTimeWithOffset = new DateTimeOffset(nextTime);

			if (nextTimeWithOffset <= nowWithOffset)
				nextTimeWithOffset = nextTimeWithOffset.Add(timeSpan);

			if (now.Kind == DateTimeKind.Local)
				return nextTimeWithOffset.LocalDateTime - nowWithOffset.LocalDateTime;

			return nextTimeWithOffset - nowWithOffset;
		}

		#endregion Aligned Interval

		/// <summary>
		/// Prints the timespan as shortended string.
		/// </summary>
		/// <param name="timeSpan">The timespan</param>
		/// <param name="withMilliseconds">A value indicating whether to show milliseconds.</param>
		/// <returns>The timespan as string.</returns>
		public static string ToShortString(this TimeSpan timeSpan, bool withMilliseconds = false)
		{
			var sb = new StringBuilder();

			if (timeSpan < TimeSpan.Zero)
				sb.Append("-");

			if (timeSpan.TotalDays != 0)
				sb.Append(Math.Abs(timeSpan.Days)).Append("d ");

			if (timeSpan.TotalHours != 0)
				sb.Append(Math.Abs(timeSpan.Hours)).Append("h ");

			if (timeSpan.TotalMinutes != 0)
				sb.Append(Math.Abs(timeSpan.Minutes)).Append("m ");

			sb.Append(Math.Abs(timeSpan.Seconds)).Append("s ");

			if (withMilliseconds)
				sb.Append(Math.Abs(timeSpan.Milliseconds)).Append("ms");

			return sb.ToString().Trim();
		}

		#region Round DateTime

		/// <summary>
		/// Rounds the <see cref="DateTime"/> to full seconds.
		/// </summary>
		/// <param name="dt">The time value to round.</param>
		/// <returns></returns>

		public static DateTime RoundToSecond(this DateTime dt)
			=> new(RoundTicks(dt.Ticks, TimeSpan.TicksPerSecond), dt.Kind);

		/// <summary>
		/// Rounds the <see cref="DateTime"/> to full minutes.
		/// </summary>
		/// <param name="dt">The time value to round.</param>
		/// <returns></returns>
		public static DateTime RoundToMinute(this DateTime dt)
			=> new(RoundTicks(dt.Ticks, TimeSpan.TicksPerMinute), dt.Kind);

		/// <summary>
		/// Rounds the <see cref="DateTime"/> to full hours.
		/// </summary>
		/// <param name="dt">The time value to round.</param>
		/// <returns></returns>
		public static DateTime RoundToHour(this DateTime dt)
			=> new(RoundTicks(dt.Ticks, TimeSpan.TicksPerHour), dt.Kind);

		/// <summary>
		/// Rounds the <see cref="DateTime"/> to full days.
		/// </summary>
		/// <param name="dt">The time value to round.</param>
		/// <returns></returns>
		public static DateTime RoundToDay(this DateTime dt)
			=> new(RoundTicks(dt.Ticks, TimeSpan.TicksPerDay), dt.Kind);

		#endregion Round DateTime

		#region Round TimeSpan

		/// <summary>
		/// Rounds the <see cref="TimeSpan"/> to full seconds.
		/// </summary>
		/// <param name="timeSpan">The time value to round.</param>
		/// <returns></returns>
		public static TimeSpan RoundToSecond(this TimeSpan timeSpan)
			=> new(RoundTicks(timeSpan.Ticks, TimeSpan.TicksPerSecond));

		/// <summary>
		/// Rounds the <see cref="TimeSpan"/> to full minutes.
		/// </summary>
		/// <param name="timeSpan">The time value to round.</param>
		/// <returns></returns>
		public static TimeSpan RoundToMinute(this TimeSpan timeSpan)
			=> new(RoundTicks(timeSpan.Ticks, TimeSpan.TicksPerMinute));

		/// <summary>
		/// Rounds the <see cref="TimeSpan"/> to full hours.
		/// </summary>
		/// <param name="timeSpan">The time value to round.</param>
		/// <returns></returns>
		public static TimeSpan RoundToHour(this TimeSpan timeSpan)
			=> new(RoundTicks(timeSpan.Ticks, TimeSpan.TicksPerHour));

		/// <summary>
		/// Rounds the <see cref="TimeSpan"/> to full days.
		/// </summary>
		/// <param name="timeSpan">The time value to round.</param>
		/// <returns></returns>
		public static TimeSpan RoundToDay(this TimeSpan timeSpan)
			=> new(RoundTicks(timeSpan.Ticks, TimeSpan.TicksPerDay));

		#endregion Round TimeSpan

		private static long RoundTicks(long ticks, long value)
			=> (ticks + value / 2) / value * value;
	}
}