using Elternbeirat.Contracts;
using Elternbeirat.Web.Shared;

namespace Elternbeirat.Web.Features.Events;

/// <summary>
///     Formats the date range of an <see cref="Event"/> for display in German.
/// </summary>
/// <remarks>
///     Shared by the full event list and the home-page events embed, so both render
///     dates the same way.
///     <para>
///         This is the event counterpart to
///         <see cref="Posts.PostDateFormat"/>, which formats a plain
///         <see cref="DateOnly"/> day instead of a point-in-time range.
///     </para>
/// </remarks>
public static class EventDateFormat
{
    /// <summary>
    ///     Date format for the start, and for a single-day event.
    /// </summary>
    private const string LongDate = "dddd, d. MMMM yyyy";

    /// <summary>
    ///     Date format for the end of a multi-day event; the weekday is omitted.
    /// </summary>
    private const string ShortDate = "d. MMMM yyyy";

    /// <summary>
    ///     Formats the date range of an event.
    /// </summary>
    /// <param name="event">
    ///     The event to format.
    /// </param>
    /// <returns>
    ///     The formatted range, depending on <see cref="Event.End"/> and <see cref="Event.HasTime"/>:
    ///     <list type="bullet">
    ///         <item>
    ///             <description>No end: <c>Donnerstag, 8. Oktober 2026, 19:30 Uhr</c></description>
    ///         </item>
    ///         <item>
    ///             <description>Same day: <c>Donnerstag, 8. Oktober 2026, 19:30–21:00 Uhr</c></description>
    ///         </item>
    ///         <item>
    ///             <description>Several days: <c>Freitag, 9. Oktober 2026 – 11. Oktober 2026</c></description>
    ///         </item>
    ///     </list>
    ///     Without a time, the time parts are omitted.
    /// </returns>
    public static string Range(Event @event)
        => (@event.Start, @event.End, @event.HasTime) switch
        {
            (var start, null, var hasTime)
                => Stamp(start, LongDate, hasTime),
            (var start, { } end, true) when end.Date == start.Date
                => $"{start.ToString(LongDate, Cultures.German)}, {start.ToString("HH:mm", Cultures.German)}–{end.ToString("HH:mm", Cultures.German)} Uhr",
            (var start, { } end, false) when end.Date == start.Date
                => Stamp(start, LongDate, withTime: false),
            (var start, { } end, var hasTime)
                => $"{Stamp(start, LongDate, hasTime)} – {Stamp(end, ShortDate, hasTime)}",
        };

    /// <summary>
    ///     Formats a single point in time as a date with an optional time.
    /// </summary>
    /// <param name="value">
    ///     The date and time to format.
    /// </param>
    /// <param name="dateFormat">
    ///     The date format, e.g. <see cref="LongDate"/>.
    /// </param>
    /// <param name="withTime">
    ///     <see langword="true"/> to append the time, e.g. <c>, 19:30 Uhr</c>.
    /// </param>
    /// <returns>
    ///     The formatted date, with the time if requested.
    /// </returns>
    private static string Stamp(DateTime value, string dateFormat, bool withTime)
        => withTime
            ? value.ToString(dateFormat + ", HH:mm", Cultures.German) + " Uhr"
            : value.ToString(dateFormat, Cultures.German);
}