namespace Aeshnidae.Trials;

/// <summary>
/// The trophy week. An ISO week key ("2026-W38") whose boundary is the award moment
/// (Settings.AwardDay / AwardHour, server-local) rather than Monday midnight: every
/// server-local time is shifted forward by the distance from the award moment to the
/// following Monday 00:00 before the ISO week is read, so a clear at Sunday 18:00
/// belongs to the week that has just begun and the award at Sunday 17:00 closes a
/// week that ends exactly then. With Sunday 17:00 the shift is seven hours.
/// </summary>
public static class Weeks
{
    /// <summary>From the award moment to the next Monday 00:00.</summary>
    public static TimeSpan Shift(Settings s)
    {
        var day = (int)s.AwardDayOfWeek;             // Sunday 0 .. Saturday 6
        var hour = Math.Clamp(s.AwardHour, 0, 23);

        // Days from the award day to Monday, going forward: Sunday -> 1, Monday -> 7, Tuesday -> 6 ...
        var daysToMonday = ((int)DayOfWeek.Monday - day + 7) % 7;
        if (daysToMonday == 0)
            daysToMonday = 7;

        return TimeSpan.FromDays(daysToMonday) - TimeSpan.FromHours(hour);
    }

    /// <summary>The week a server-local time falls in.</summary>
    public static string KeyOf(DateTime localTime, Settings s)
    {
        var shifted = localTime + Shift(s);
        return $"{ISOWeek.GetYear(shifted)}-W{ISOWeek.GetWeekOfYear(shifted):00}";
    }

    /// <summary>The week that ended at the most recent award moment, from a server-local time.</summary>
    public static string PreviousKey(DateTime localTime, Settings s) => KeyOf(localTime.AddDays(-7), s);

    /// <summary>The next award moment after a server-local time.</summary>
    public static DateTime NextAward(DateTime localTime, Settings s)
    {
        var hour = Math.Clamp(s.AwardHour, 0, 23);
        var candidate = localTime.Date.AddHours(hour);

        while (candidate.DayOfWeek != s.AwardDayOfWeek || candidate <= localTime)
            candidate = candidate.AddDays(1);

        return candidate;
    }

    /// <summary>The award moment that began the current week (the last one that passed).</summary>
    public static DateTime CurrentWeekStart(DateTime localTime, Settings s) => NextAward(localTime, s).AddDays(-7);

    /// <summary>The Monday the shifted ISO week of a key starts on, for the human "week of" line.</summary>
    public static string Describe(string key, Settings s)
    {
        try
        {
            var parts = key.Split("-W");
            var monday = ISOWeek.ToDateTime(int.Parse(parts[0]), int.Parse(parts[1]), DayOfWeek.Monday);
            var start = monday - Shift(s);
            return start.ToString("d MMM yyyy", CultureInfo.InvariantCulture);
        }
        catch
        {
            return key;
        }
    }
}