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, the home-page events embed and the next-event
///     card, so all render dates the same way: <see cref="Range"/> as the full date,
///     the <c>Sheet…</c> methods and <see cref="Span"/> for the calendar sheet and the
///     time line beside it, <see cref="PastDays"/> and <see cref="PastTime"/> for the list of past events.
///     <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>
    ///     Date format for the list of past events, which is grouped by year.
    /// </summary>
    private const string DayMonth = "d. MMMM";

    /// <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)}, {Clock(start)}–{Clock(end)} 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>
    ///     The short month for the top band of a calendar sheet.
    /// </summary>
    /// <param name="value">The event's start.</param>
    /// <returns>The German short month without its period, e.g. <c>Okt</c> or <c>März</c>.</returns>
    /// <remarks>
    ///     Returned in normal case; the sheet's CSS sets it in capitals. The period of the
    ///     abbreviation (<c>Okt.</c>) is dropped because on a sheet it reads as a stray dot.
    /// </remarks>
    /// <seealso cref="CalendarSheet"/>
    public static string SheetMonth(DateTime value)
        => value.ToString("MMM", Cultures.German).TrimEnd('.');

    /// <summary>
    ///     The day of the month for the large number on a calendar sheet.
    /// </summary>
    /// <param name="value">The event's start.</param>
    /// <returns>The day without a leading zero, e.g. <c>8</c>.</returns>
    public static string SheetDay(DateTime value)
        => value.Day.ToString(Cultures.German);

    /// <summary>
    ///     The short weekday for the bottom of a calendar sheet.
    /// </summary>
    /// <param name="value">The event's start.</param>
    /// <returns>The German short weekday without its period, e.g. <c>Do</c>.</returns>
    public static string SheetWeekday(DateTime value)
        => value.ToString("ddd", Cultures.German).TrimEnd('.');

    /// <summary>
    ///     The time line beside a calendar sheet: what the sheet itself cannot show.
    /// </summary>
    /// <param name="event">The event to format.</param>
    /// <returns>
    ///     Depending on <see cref="Event.End"/> and <see cref="Event.HasTime"/>:
    ///     <list type="bullet">
    ///         <item><description>No end: <c>19:30 Uhr</c></description></item>
    ///         <item><description>Same day: <c>19:30–21:00 Uhr</c></description></item>
    ///         <item><description>Several days: <c>09:00 Uhr bis Sonntag, 11. Oktober 2026, 13:00 Uhr</c></description></item>
    ///         <item><description>Several days, no time: <c>bis Sonntag, 11. Oktober 2026</c></description></item>
    ///         <item><description>One day, no time: an empty string; the sheet says it all.</description></item>
    ///     </list>
    /// </returns>
    /// <remarks>
    ///     The sheet shows the start day only, so a multi-day event must name its last
    ///     day here, with the weekday, as the sheet does for the first one.
    /// </remarks>
    public static string Span(Event @event)
        => (@event.Start, @event.End, @event.HasTime) switch
        {
            (var start, { } end, var hasTime) when end.Date != start.Date
                => (hasTime ? Clock(start) + " Uhr " : "") + "bis " + Stamp(end, LongDate, hasTime),
            (var start, null, true)
                => $"{Clock(start)} Uhr",
            (var start, { } end, true)
                => $"{Clock(start)}–{Clock(end)} Uhr",
            _ => "",
        };

    /// <summary>
    ///     The day column of a past event in the folded-up list on the events page.
    /// </summary>
    /// <param name="event">The event to format.</param>
    /// <returns>
    ///     Day and full month, depending on <see cref="Event.End"/>:
    ///     <list type="bullet">
    ///         <item><description>One day: <c>13. November</c></description></item>
    ///         <item><description>Several days in one month: <c>8.–9. Februar</c></description></item>
    ///         <item><description>Across a month: <c>30. Januar – 2. Februar</c></description></item>
    ///         <item><description>Across a year: <c>30. Dezember – 2. Januar 2027</c></description></item>
    ///     </list>
    /// </returns>
    /// <remarks>
    ///     The list is grouped by the start year under its own small heading, so the
    ///     year is only named where the end leaves that year. The weekday is left out
    ///     to keep the column short; the time follows the title (<see cref="PastTime"/>).
    /// </remarks>
    public static string PastDays(Event @event)
        => (@event.Start, @event.End) switch
        {
            (var start, { } end) when end.Year != start.Year
                => $"{start.ToString(DayMonth, Cultures.German)} – {end.ToString(ShortDate, Cultures.German)}",
            (var start, { } end) when end.Month != start.Month
                => $"{start.ToString(DayMonth, Cultures.German)} – {end.ToString(DayMonth, Cultures.German)}",
            (var start, { } end) when end.Day != start.Day
                => $"{start.Day.ToString(Cultures.German)}.–{end.ToString(DayMonth, Cultures.German)}",
            (var start, _)
                => start.ToString(DayMonth, Cultures.German),
        };

    /// <summary>
    ///     The time after the title of a past event in the folded-up list.
    /// </summary>
    /// <param name="event">The event to format.</param>
    /// <returns>
    ///     Depending on <see cref="Event.HasTime"/> and <see cref="Event.End"/>:
    ///     <list type="bullet">
    ///         <item><description>No time: an empty string; nothing is shown.</description></item>
    ///         <item><description>No end: <c>19:00 Uhr</c></description></item>
    ///         <item><description>Same day: <c>19:00–21:00 Uhr</c></description></item>
    ///         <item><description>Several days: the start time only, <c>09:00 Uhr</c></description></item>
    ///     </list>
    /// </returns>
    /// <remarks>
    ///     For several days the day column already shows the span; a time range beside
    ///     it would read as the daily hours, so only the start time is given.
    /// </remarks>
    public static string PastTime(Event @event)
        => (@event.HasTime, @event.End) switch
        {
            (false, _) => "",
            (true, { } end) when end.Date != @event.Start.Date => $"{Clock(@event.Start)} Uhr",
            _ => Span(@event),
        };

    /// <summary>
    ///     Formats the clock time of a point in time.
    /// </summary>
    /// <param name="value">The date and time.</param>
    /// <returns>The time as e.g. <c>19:30</c>.</returns>
    private static string Clock(DateTime value)
        => value.ToString("HH:mm", Cultures.German);

    /// <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);
}