namespace Elternbeirat.Web.Features.Layout;

/// <summary>
///     Works out the year shown in the footer's copyright line.
/// </summary>
/// <remarks>
///     The container runs on UTC, but visitors live in Germany. Taking the year
///     straight from the server clock would show the old year for the first hour
///     (two in summer time) of every new year, so the instant is converted to
///     Europe/Berlin first.
/// </remarks>
public static class FooterYear
{
    /// <summary>
    ///     IANA id; resolves on every platform .NET supports, Windows included.
    /// </summary>
    private static readonly TimeZoneInfo Berlin =
        TimeZoneInfo.FindSystemTimeZoneById("Europe/Berlin");

    /// <summary>
    ///     Returns the calendar year in Berlin at the given instant.
    /// </summary>
    /// <param name="now">
    ///     The current instant, in any offset; usually
    ///     <see cref="TimeProvider.GetUtcNow"/>.
    /// </param>
    /// <returns>The year a visitor in Germany would see on their calendar.</returns>
    /// <example>
    ///     <code>
    ///         FooterYear.For(new DateTimeOffset(2026, 12, 31, 23, 30, 0, TimeSpan.Zero)); // 2027
    ///     </code>
    /// </example>
    public static int For(DateTimeOffset now) =>
        TimeZoneInfo.ConvertTime(now, Berlin).Year;
}