using HSchool.Content;

namespace HSchool.Schedule;

/// <summary>
/// Which lessons are happening at a game instant. Breaks, nights, weekends and holidays are empty
/// — occupancy is derived, never stored.
/// </summary>
public static class TimetableClock
{
    public static IReadOnlyList<LessonPlacement> OccurringAt(
        Timetable table,
        DefCatalog catalog,
        DateTime time,
        int weekDays)
    {
        ArgumentNullException.ThrowIfNull(table);
        var slot = SchoolDay.At(catalog, time, weekDays);
        if (slot.Kind != DaySlotKind.Lesson)
        {
            return [];
        }

        var day = SchoolDay.WeekdayIndex(time);
        return table.Lessons
            .Where(lesson => lesson.Day == day && lesson.Period == slot.Index)
            .ToArray();
    }

    public static OccupancyKey Key(DefCatalog catalog, DateTime time, int weekDays)
    {
        var utc = DateTime.SpecifyKind(time, DateTimeKind.Utc);
        var slot = SchoolDay.At(catalog, utc, weekDays);
        return new OccupancyKey(DateOnly.FromDateTime(utc), slot.Kind, slot.Index);
    }
}

public readonly record struct OccupancyKey(DateOnly Day, DaySlotKind Kind, int Index);