import dayjs from 'dayjs';

/**
 * [Gitea #139] One place that turns a timestamp into words, honouring the
 * user's 12/24 h clock and date-format preferences everywhere.
 *
 * Rules for the `auto` styles, relative to `now`:
 *   today            → "21:14"
 *   yesterday        → "Yesterday 21:14"
 *   tomorrow         → "Tomorrow 21:14"        (scheduled sends, reminders)
 *   last 6 days      → "Mon 21:14"
 *   older / further  → "5 Sep 2026 21:14"      (date per `dateFormatString`)
 */
export type TimestampPrefs = {
  hour24Clock: boolean;
  dateFormatString: string;
};

export type TimestampStyle =
  /** Time for today, day word + time for nearby days, date + time otherwise. */
  | 'auto'
  /** Like `auto` but never the time: "Today", "Yesterday", "Mon", "5 Sep 2026". */
  | 'autoDate'
  /** Always the clock time. */
  | 'time'
  /** Always the date per `dateFormatString`. */
  | 'date'
  /** Always date + time. */
  | 'dateTime';

const DEFAULT_DATE_FORMAT = 'D MMM YYYY';

const dateFormat = (prefs: TimestampPrefs) => prefs.dateFormatString || DEFAULT_DATE_FORMAT;

export const formatTime = (ts: number, prefs: Pick<TimestampPrefs, 'hour24Clock'>): string =>
  dayjs(ts).format(prefs.hour24Clock ? 'HH:mm' : 'hh:mm A');

export const formatDate = (ts: number, prefs: TimestampPrefs): string =>
  dayjs(ts).format(dateFormat(prefs));

/** Whole days between the start of `ts`'s day and the start of `now`'s day (negative = future). */
export const dayDistance = (ts: number, now: number): number =>
  dayjs(now).startOf('day').diff(dayjs(ts).startOf('day'), 'day');

/** "Today" / "Yesterday" / "Tomorrow" / "Mon" for nearby days, else undefined. */
export const dayWord = (ts: number, now: number): string | undefined => {
  const d = dayDistance(ts, now);
  if (d === 0) return 'Today';
  if (d === 1) return 'Yesterday';
  if (d === -1) return 'Tomorrow';
  if (d > 1 && d < 7) return dayjs(ts).format('ddd');
  return undefined;
};

export function formatTimestamp(
  ts: number,
  prefs: TimestampPrefs,
  style: TimestampStyle = 'auto',
  now: number = Date.now(),
): string {
  const time = formatTime(ts, prefs);
  if (style === 'time') return time;
  const date = formatDate(ts, prefs);
  if (style === 'date') return date;
  if (style === 'dateTime') return `${date} ${time}`;

  const word = dayWord(ts, now);
  if (style === 'autoDate') return word ?? date;
  if (word === 'Today') return time;
  return `${word ?? date} ${time}`;
}

/** Short relative age for dense lists: "now", "5m", "3h", "Yesterday", "Mon", "5 Sep". */
export function formatShortAge(
  ts: number,
  prefs: TimestampPrefs,
  now: number = Date.now(),
): string {
  const diffMin = Math.floor((now - ts) / 60_000);
  if (diffMin < 60) return diffMin < 1 ? 'now' : `${diffMin}m`;
  const diffHours = Math.floor(diffMin / 60);
  if (diffHours < 24 && dayDistance(ts, now) === 0) return `${diffHours}h`;
  const word = dayWord(ts, now);
  if (word && word !== 'Today') return word;
  // Same year: drop the year from the user's format for brevity.
  const sameYear = dayjs(ts).year() === dayjs(now).year();
  const fmt = dateFormat(prefs);
  return dayjs(ts).format(sameYear ? fmt.replace(/[-/.\s]*Y{2,4}[-/.\s]*/, ' ').trim() : fmt);
}

/** Conversational age for list rows: "just now", "5m ago", "3h ago", "yesterday", "3d ago", then the date. */
export function formatRelativeAge(
  ts: number,
  prefs: TimestampPrefs,
  now: number = Date.now(),
): string {
  const diffMin = Math.floor((now - ts) / 60_000);
  if (diffMin < 1) return 'just now';
  if (diffMin < 60) return `${diffMin}m ago`;
  const diffHours = Math.floor(diffMin / 60);
  if (diffHours < 24) return `${diffHours}h ago`;
  const days = dayDistance(ts, now);
  if (days === 1) return 'yesterday';
  if (days < 7) return `${days}d ago`;
  return formatDate(ts, prefs);
}

/** Timeline day divider: "Today", "Yesterday", "Monday" (last 6 days), else the date. */
export function formatDayDivider(
  ts: number,
  prefs: TimestampPrefs,
  now: number = Date.now(),
): string {
  const d = dayDistance(ts, now);
  if (d > 1 && d < 7) return dayjs(ts).format('dddd');
  return formatTimestamp(ts, prefs, 'autoDate', now);
}