export const TZ = 'America/Toronto'

/** YYYY-MM-DD for right now in Toronto */
export function todayToronto(): string {
  return toDateToronto(new Date())
}

/** YYYY-MM-DD in Toronto for any Date */
export function toDateToronto(date: Date): string {
  return new Intl.DateTimeFormat('en-CA', { timeZone: TZ }).format(date)
}

/**
 * Add n calendar days to a YYYY-MM-DD string.
 * Uses noon UTC so the shift is always exactly 24 h and never crosses
 * a Toronto DST boundary ambiguously.
 */
export function addDays(isoDate: string, n: number): string {
  const d = new Date(isoDate + 'T12:00:00Z')
  d.setUTCDate(d.getUTCDate() + n)
  return d.toISOString().slice(0, 10)
}

/** HH:MM in Toronto for a UTC ISO string */
export function fmtTime(utcIso: string): string {
  return new Date(utcIso).toLocaleTimeString('en-CA', {
    timeZone: TZ,
    hour: '2-digit',
    minute: '2-digit',
    hour12: false,
  })
}

/** "Wednesday, April 20" style label for a YYYY-MM-DD string */
export function fmtDateLong(isoDate: string): string {
  return new Date(isoDate + 'T12:00:00Z').toLocaleDateString('en-CA', {
    timeZone: TZ,
    weekday: 'long',
    month: 'long',
    day: 'numeric',
  })
}

/** { weekday: "Wed", short: "Apr 20" } for a YYYY-MM-DD string */
export function fmtDayHeader(isoDate: string): { weekday: string; short: string } {
  const d = new Date(isoDate + 'T12:00:00Z')
  return {
    weekday: d.toLocaleDateString('en-CA', { timeZone: TZ, weekday: 'short' }),
    short: d.toLocaleDateString('en-CA', { timeZone: TZ, month: 'short', day: 'numeric' }),
  }
}

/**
 * YYYY-MM-DD of the Monday that starts the ISO week containing isoDate.
 * Uses the en-CA weekday name to determine day-of-week in Toronto.
 */
export function weekMonday(isoDate: string): string {
  const d = new Date(isoDate + 'T12:00:00Z')
  const wd = new Intl.DateTimeFormat('en-CA', { timeZone: TZ, weekday: 'short' }).format(d)
  const offsets: Record<string, number> = { Mon: 0, Tue: 1, Wed: 2, Thu: 3, Fri: 4, Sat: 5, Sun: 6 }
  const offset = offsets[wd] ?? 0
  const monday = new Date(d)
  monday.setUTCDate(d.getUTCDate() - offset)
  return monday.toISOString().slice(0, 10)
}

/** All 7 YYYY-MM-DD strings for the ISO week containing isoDate (Mon → Sun) */
export function weekDates(isoDate: string): string[] {
  const mon = weekMonday(isoDate)
  return Array.from({ length: 7 }, (_, i) => addDays(mon, i))
}

/**
 * UTC range string pair that safely covers all Toronto-local timestamps
 * falling within the isoFrom…isoTo date range.
 * Adds a 1-day buffer on each side so client-side date filtering is authoritative.
 */
export function utcRange(isoFrom: string, isoTo: string): { from: string; to: string } {
  return {
    from: addDays(isoFrom, -1) + 'T00:00:00Z',
    to:   addDays(isoTo,   +1) + 'T23:59:59Z',
  }
}