Calendar_Frontend/lib/dates.ts

import { TZDate } from "@date-fns/tz";

/**
 * Every wall-clock time in this app means Europe/Berlin, whatever time zone the
 * editor's browser happens to be in.
 *
 * The Angular app used the browser's local time, which is only correct while
 * the editor is in Germany: an editor abroad would enter "19:00" and save an
 * event hours off. The API has no notion of time zones at all - it stores what
 * it is sent and writes the iCal feed with the *server's* local `getHours()`,
 * labelled `TZID=Europe/Berlin` (icalgenerator.service.ts) - so the frontend is
 * the one place that can get this right, by always sending the UTC instant of a
 * Berlin wall time.
 *
 * Dates travel through the form as "YYYY-MM-DD" and times as "HH:mm", exactly
 * what <input type="date|time"> produce, so there is no Date object to mutate
 * in place (the other Angular bug).
 *
 * DST, as @date-fns/tz resolves it and the tests pin:
 * - a time that does not exist (02:30 on the last Sunday of March) moves
 *   forward to 03:30 CEST;
 * - a time that exists twice (02:30 on the last Sunday of October) is the
 *   second one, CET.
 */

export const TIME_ZONE = "Europe/Berlin";
const LOCALE = "de-DE";

export interface WallTime {
  /** "YYYY-MM-DD" */
  date: string;
  /** "HH:mm" */
  time: string;
}

const pad = (n: number): string => n.toString().padStart(2, "0");

const splitDate = (date: string): [number, number, number] => {
  const [y, m, d] = date.split("-").map(Number);
  return [y, m, d];
};

/** Berlin wall time → instant. */
export const fromWallTime = (date: string, time: string): Date => {
  const [y, m, d] = splitDate(date);
  const [hh, mm] = time.split(":").map(Number);
  return new Date(new TZDate(y, m - 1, d, hh, mm, TIME_ZONE).getTime());
};

/** Instant → Berlin wall time. */
export const toWallTime = (instant: Date): WallTime => {
  const t = new TZDate(instant.getTime(), TIME_ZONE);
  return {
    date: `${t.getFullYear()}-${pad(t.getMonth() + 1)}-${pad(t.getDate())}`,
    time: `${pad(t.getHours())}:${pad(t.getMinutes())}`,
  };
};

/** The Berlin calendar day an instant falls on, as "YYYY-MM-DD" (sorts as a string). */
export const berlinDay = (instant: Date): string => toWallTime(instant).date;

/** Calendar arithmetic on "YYYY-MM-DD", independent of any time zone. */
export const addDays = (date: string, days: number): string => {
  const [y, m, d] = splitDate(date);
  const u = new Date(Date.UTC(y, m - 1, d + days));
  return `${u.getUTCFullYear()}-${pad(u.getUTCMonth() + 1)}-${pad(u.getUTCDate())}`;
};

/** Whole days from `a` to `b` ("YYYY-MM-DD"); negative when b is earlier. */
export const daysBetween = (a: string, b: string): number => {
  const [ay, am, ad] = splitDate(a);
  const [by, bm, bd] = splitDate(b);
  return Math.round((Date.UTC(by, bm - 1, bd) - Date.UTC(ay, am - 1, ad)) / 86_400_000);
};

/**
 * A whole-day event's stored range: 00:00 on the first day to 23:59 on the
 * last, both Berlin.
 *
 * The end is the *inclusive* last day because that is what the iCal generator
 * assumes - it adds one day to a whole-day end (`formatDate(…, true, true)`).
 * 23:59 rather than 00:00 keeps the event "upcoming" for the whole of its last
 * day in the list filter.
 */
export const wholeDayRange = (startDate: string, endDate: string): { start: Date; end: Date } => ({
  start: fromWallTime(startDate, "00:00"),
  end: fromWallTime(endDate, "23:59"),
});

/** The next full hour after `now`, as Berlin wall time. */
export const nextFullHour = (now: Date): WallTime => {
  const { date, time } = toWallTime(now);
  const hour = Number(time.slice(0, 2)) + 1;
  return hour === 24 ? { date: addDays(date, 1), time: "00:00" } : { date, time: `${pad(hour)}:00` };
};

/**
 * The first occurrence of a repeating event that starts today (Berlin) or
 * later, keeping its wall-clock time. Non-repeating events, and frequencies
 * the API may hold but this app does not know, return `start` unchanged.
 *
 * Only YEARLY is ever written by this app (birthdays), but the API accepts any
 * `FREQ` string, so older data may hold others.
 *
 * A yearly event on 29 February falls on 28 February in other years - the
 * choice a birthday list wants, though RFC 5545 would skip those years.
 */
export const nextOccurrence = (start: Date, repeatFrequency: string, now: Date): Date => {
  const today = berlinDay(now);
  const { date, time } = toWallTime(start);
  if (date >= today) {
    return start;
  }
  const [, m, d] = splitDate(date);

  switch (repeatFrequency) {
    case "YEARLY": {
      const year = Number(today.slice(0, 4));
      const candidate = (yy: number) => {
        const lastDay = new Date(Date.UTC(yy, m, 0)).getUTCDate();
        return `${yy}-${pad(m)}-${pad(Math.min(d, lastDay))}`;
      };
      const thisYear = candidate(year);
      return fromWallTime(thisYear >= today ? thisYear : candidate(year + 1), time);
    }
    case "MONTHLY": {
      const [ty, tm] = splitDate(today);
      const candidate = (yy: number, mm: number) => {
        const lastDay = new Date(Date.UTC(yy, mm, 0)).getUTCDate();
        return `${yy}-${pad(mm)}-${pad(Math.min(d, lastDay))}`;
      };
      const thisMonth = candidate(ty, tm);
      if (thisMonth >= today) {
        return fromWallTime(thisMonth, time);
      }
      return fromWallTime(tm === 12 ? candidate(ty + 1, 1) : candidate(ty, tm + 1), time);
    }
    case "WEEKLY":
    case "DAILY": {
      const step = repeatFrequency === "WEEKLY" ? 7 : 1;
      const steps = Math.ceil(daysBetween(date, today) / step);
      return fromWallTime(addDays(date, steps * step), time);
    }
    default:
      return start;
  }
};

const dayFormat = new Intl.DateTimeFormat(LOCALE, {
  timeZone: TIME_ZONE,
  weekday: "short",
  day: "numeric",
  month: "short",
  year: "numeric",
});
const timeFormat = new Intl.DateTimeFormat(LOCALE, { timeZone: TIME_ZONE, hour: "2-digit", minute: "2-digit" });
const dateTimeFormat = new Intl.DateTimeFormat(LOCALE, { timeZone: TIME_ZONE, dateStyle: "medium", timeStyle: "short" });
const monthFormat = new Intl.DateTimeFormat(LOCALE, { timeZone: TIME_ZONE, month: "long", year: "numeric" });

/** "Sa., 3. Okt. 2026" */
export const formatDay = (instant: Date): string => dayFormat.format(instant);
/** "19:00" */
export const formatTime = (instant: Date): string => timeFormat.format(instant);
/** "03.10.2026, 19:00" */
export const formatDateTime = (instant: Date): string => dateTimeFormat.format(instant);
/** "Oktober 2026" */
export const formatMonth = (instant: Date): string => monthFormat.format(instant);
/** "2026-10" - a sortable month key. */
export const monthKey = (instant: Date): string => berlinDay(instant).slice(0, 7);