namespace QuestionsApi.Services.Flashcards;

// Small pure helpers pulled out of FlashcardStudyService/FlashcardScheduleStore so the
// daily-limit and sibling-bury rules can be unit tested without a live Mongo instance.
public static class FlashcardScheduling
{
    // A card reviewed on a given local study day buries its siblings until the next local day.
    public static DateTime NextStudyDayStart(DateTime studyDayStartUtc) => studyDayStartUtc.Date.AddDays(1);

    // Effective per-day budget: deck override wins over the global default, minus what's
    // already been consumed on the client's current local study day.
    public static int ResolveDailyBudget(int globalLimit, int? deckOverride, int consumedToday) =>
        Math.Max(0, (deckOverride ?? globalLimit) - consumedToday);

    // A streak counts consecutive calendar days with at least one review, ending either today or
    // yesterday (so the streak doesn't visibly reset to 0 the moment a new day starts before
    // today's first review) - the same semantics as most habit-streak trackers.
    public static int ComputeStreakDays(IReadOnlyCollection<DateTime> reviewTimestamps, DateTime todayUtc)
    {
        var reviewDays = reviewTimestamps.Select(v => v.Date).ToHashSet();
        var cursor = todayUtc.Date;
        if (!reviewDays.Contains(cursor))
        {
            cursor = cursor.AddDays(-1);
            if (!reviewDays.Contains(cursor)) return 0;
        }
        var streak = 0;
        while (reviewDays.Contains(cursor))
        {
            streak++;
            cursor = cursor.AddDays(-1);
        }
        return streak;
    }
}