namespace Elternbeirat.Web.Shared;

/// <summary>
///     Tells the current calendar day as people at the school see it.
/// </summary>
/// <remarks>
///     The container's clock runs in UTC, so <see cref="DateTime.Today"/> would flip to
///     the next day at 01:00 or 02:00 Berlin time, and an event shortly after midnight
///     would count as past for an hour or two. Event times are Berlin wall-clock
///     values (see <c>LocalDateTimeConverter</c>), so "today" has to be a Berlin day
///     too.
///     <para>
///         The time comes from an injected <see cref="TimeProvider"/>, not the system
///         clock, so tests can pin "today" and check what a page shows before and after
///         an event.
///     </para>
/// </remarks>
public static class BerlinClock
{
    /// <summary>
    ///     IANA id; resolves on every platform .NET supports, Windows included.
    /// </summary>
    private static readonly TimeZoneInfo Berlin =
        TimeZoneInfo.FindSystemTimeZoneById("Europe/Berlin");

    /// <summary>
    ///     Returns today's date in Berlin.
    /// </summary>
    /// <param name="time">The clock to read, usually <see cref="TimeProvider.System"/>.</param>
    /// <returns>The current Berlin calendar day.</returns>
    public static DateOnly Today(TimeProvider time) =>
        DateOnly.FromDateTime(TimeZoneInfo.ConvertTime(time.GetUtcNow(), Berlin).DateTime);
}