/**
 * Shared timezone-aware date helpers for statistics aggregations.
 */

/** Milliseconds in a single day. */
export const DAY_MS = 24 * 60 * 60 * 1000;

/** Maps a short weekday name (as produced by Intl) to its index (Sun = 0). */
export const WEEKDAY: Record<string, number> = {
  Sun: 0,
  Mon: 1,
  Tue: 2,
  Wed: 3,
  Thu: 4,
  Fri: 5,
  Sat: 6,
};

/** Parses a `YYYY-MM-DD` string into `[year, month, day]` numbers. */
export const parseYmd = (dateStr: string): [number, number, number] => {
  const [y, m, d] = dateStr.split("-").map(Number);
  return [y, m, d];
};

/** Builds a zero-padded `YYYY-MM-DD` string from numeric parts. */
export const ymdToStr = (y: number, m: number, d: number): string =>
  `${y}-${String(m).padStart(2, "0")}-${String(d).padStart(2, "0")}`;

/** Parses a `YYYY-MM-DD` string as midnight UTC. */
export const toUtcDate = (dateStr: string): Date =>
  new Date(`${dateStr}T00:00:00.000Z`);

/** Formats a date as a `YYYY-MM-DD` string in UTC. */
export const formatDate = (date: Date): string =>
  date.toISOString().slice(0, 10);

/** Adds `days` to a `YYYY-MM-DD` string, returning the resulting `YYYY-MM-DD`. */
export const addDaysToDateStr = (dateStr: string, days: number): string =>
  formatDate(new Date(toUtcDate(dateStr).getTime() + days * DAY_MS));

/** Next civil calendar day (proleptic Gregorian). Avoids DST 24h steps that can stall iteration. */
export const incrementYmd = (
  y: number,
  m: number,
  d: number,
): [number, number, number] => {
  const next = new Date(Date.UTC(y, m - 1, d + 1));
  return [next.getUTCFullYear(), next.getUTCMonth() + 1, next.getUTCDate()];
};

/**
 * Formats a date as the calendar day in the given timezone.
 * `en-CA` yields ISO-8601 `YYYY-MM-DD`, which is lexicographically sortable
 * (string compare == chronological compare) and matches MongoDB's `%Y-%m-%d`.
 */
export const formatYmdInTz = (date: Date, timezone: string): string =>
  new Intl.DateTimeFormat("en-CA", { timeZone: timezone }).format(date);

/**
 * Returns the calendar day in the given timezone as numeric parts.
 * Reads parts by `type` (not string order), so `en-US` is fine here.
 */
export const getYmdInTz = (date: Date, timezone: string) => {
  const parts = Object.fromEntries(
    new Intl.DateTimeFormat("en-US", {
      timeZone: timezone,
      year: "numeric",
      month: "numeric",
      day: "numeric",
    })
      .formatToParts(date)
      .map((p) => [p.type, Number(p.value)]),
  );
  return { y: parts.year, m: parts.month, d: parts.day };
};

/** Weekday index (Sun = 0) of a date in the given timezone. */
export const getWeekdayInTz = (date: Date, timezone: string): number =>
  WEEKDAY[
    new Intl.DateTimeFormat("en-US", {
      timeZone: timezone,
      weekday: "short",
    }).format(date)
  ] ?? 0;

/**
 * Returns the given `YYYY-MM-DD` day at noon UTC — a stable anchor inside the
 * calendar day, far from midnight so timezone/DST shifts can't cross a date boundary.
 */
export const dateAtNoonUtc = (dateStr: string): Date => {
  const [y, m, d] = parseYmd(dateStr);
  return new Date(Date.UTC(y, m - 1, d, 12, 0, 0));
};

/**
 * UTC instant at the start of the given `YYYY-MM-DD` calendar day in `timezone`.
 * Binary-searches the boundary so it stays correct across DST transitions.
 */
export const startOfCalendarDayInTz = (
  dateStr: string,
  timezone: string,
): Date => {
  const anchor = dateAtNoonUtc(dateStr).getTime();
  let low = anchor - 2 * DAY_MS;
  let high = anchor + DAY_MS;

  while (high - low > 1 /* ms */) {
    const mid = Math.floor((low + high) / 2);
    if (formatYmdInTz(new Date(mid), timezone) >= dateStr) high = mid;
    else low = mid;
  }

  return new Date(high);
};

/** UTC instant at the last millisecond of the given calendar day in `timezone`. */
export const endOfCalendarDayInTz = (
  dateStr: string,
  timezone: string,
): Date => {
  const [y, m, d] = parseYmd(dateStr);
  const nextDay = new Date(Date.UTC(y, m - 1, d + 1))
    .toISOString()
    .slice(0, 10);
  return new Date(startOfCalendarDayInTz(nextDay, timezone).getTime() - 1);
};