export type Recurrence = {
    type: 'daily' | 'weekly' | 'monthly';
    days?: number[];
    dayOfMonth?: number;
};

const startOfDay = (date: Date) => new Date(date.getFullYear(), date.getMonth(), date.getDate());

const addDays = (date: Date, days: number) => {
    const d = new Date(date);
    d.setDate(d.getDate() + days);
    return d;
};

const getNextDaily = (completedAt: Date) => addDays(startOfDay(completedAt), 1);

const getNextWeekly = (completedAt: Date, days: number[]) => {
    if (!days || days.length === 0) return null;
    const base = startOfDay(completedAt);
    for (let offset = 1; offset <= 7; offset++) {
        const candidate = addDays(base, offset);
        if (days.includes(candidate.getDay())) return candidate;
    }
    return null;
};

const getNextMonthly = (completedAt: Date, dayOfMonth?: number) => {
    if (!dayOfMonth || dayOfMonth < 1 || dayOfMonth > 31) return null;
    const base = startOfDay(completedAt);
    const baseDay = base.getDate();
    const startOffset = dayOfMonth > baseDay ? 0 : 1;

    for (let offset = startOffset; offset <= 12; offset++) {
        const year = base.getFullYear();
        const monthIndex = base.getMonth() + offset;
        const candidate = new Date(year, monthIndex, dayOfMonth);
        if (candidate.getMonth() === (monthIndex % 12 + 12) % 12) {
            if (candidate.getTime() > completedAt.getTime()) {
                return startOfDay(candidate);
            }
        }
    }
    return null;
};

export const getNextRecurrenceDate = (recurrence: Recurrence, completedAt: Date) => {
    if (recurrence.type === 'daily') return getNextDaily(completedAt);
    if (recurrence.type === 'weekly') return getNextWeekly(completedAt, recurrence.days || []);
    if (recurrence.type === 'monthly') return getNextMonthly(completedAt, recurrence.dayOfMonth);
    return null;
};

export const getRecurrenceProgress = (
    recurrence: Recurrence | null | undefined,
    completedAtIso: string | null | undefined,
    nowMs: number
) => {
    if (!recurrence || !completedAtIso) return null;
    const completedAt = new Date(completedAtIso);
    if (isNaN(completedAt.getTime())) return null;
    const next = getNextRecurrenceDate(recurrence, completedAt);
    if (!next) return null;
    const total = next.getTime() - completedAt.getTime();
    if (total <= 0) return null;
    const progress = (nowMs - completedAt.getTime()) / total;
    return Math.max(0, Math.min(1, progress));
};