/**
 * Time helpers. Everything here works in *local* time — the prototype derived day
 * keys from `toISOString()`, which is UTC and files late/early work under the wrong
 * day. Timestamps are epoch milliseconds; rounding is applied at render only.
 */

export const MINUTE = 60_000;
export const HOUR = 3_600_000;
export const DAY = 86_400_000;

const pad = (n) => String(n).padStart(2, '0');

/** Local 'YYYY-MM-DD' for a timestamp. */
export function localDateKey(ts = Date.now()) {
  const d = new Date(ts);
  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}`;
}

/** Timestamp of local midnight starting `dateKey`. */
export function startOfDay(dateKey) {
  const [y, m, d] = dateKey.split('-').map(Number);
  return new Date(y, m - 1, d).getTime();
}

/** Timestamp of 'HH:MM' local time on `dateKey`. */
export function atLocalTime(dateKey, hhmm) {
  const [y, m, d] = dateKey.split('-').map(Number);
  const [h, min] = hhmm.split(':').map(Number);
  return new Date(y, m - 1, d, h, min, 0, 0).getTime();
}

/** `dateKey` shifted by `n` days, DST-safe. */
export function addDays(dateKey, n) {
  const [y, m, d] = dateKey.split('-').map(Number);
  return localDateKey(new Date(y, m - 1, d + n).getTime());
}

/** JS weekday for a date key: 0 = Sunday … 6 = Saturday. */
export function weekday(dateKey) {
  return new Date(startOfDay(dateKey)).getDay();
}

/**
 * Round to the nearest `minutes`, half up, relative to *local midnight* rather
 * than the epoch — so zones with a :30 or :45 offset still land on whole
 * wall-clock steps. Always drops seconds.
 */
export function roundTs(ts, minutes) {
  const step = Math.max(1, minutes || 1) * MINUTE;
  const midnight = startOfDay(localDateKey(ts));
  return midnight + Math.round((ts - midnight) / step) * step;
}

/** 'HH:MM' in local time, rounded to `roundMinutes`. */
export function fmtTime(ts, roundMinutes = 1) {
  const d = new Date(roundTs(ts, roundMinutes));
  return `${pad(d.getHours())}:${pad(d.getMinutes())}`;
}

/** A duration as '8h15' / '0h45'. */
export function fmtDuration(ms) {
  const total = Math.max(0, Math.round(ms / MINUTE));
  return `${Math.floor(total / 60)}h${pad(total % 60)}`;
}

/** 'vendredi 5 septembre' — the resume heading. */
export function fmtDayLabel(dateKey) {
  const s = new Intl.DateTimeFormat('fr-FR', {
    weekday: 'long',
    day: 'numeric',
    month: 'long',
  }).format(new Date(startOfDay(dateKey)));
  return s.charAt(0).toUpperCase() + s.slice(1);
}

/** 'HH:MM' → minutes since midnight. */
export function hhmmToMinutes(hhmm) {
  const [h, m] = hhmm.split(':').map(Number);
  return h * 60 + m;
}