/**
 * Date-math helpers shared by the E2E specs. The whole point of this module
 * is that no spec ever hardcodes a year: every date used in an assertion is
 * derived, at run time, from "today" or from the seeded child's birth date.
 *
 * All dates here are UTC-normalized `Date`s at midnight and ISO date
 * strings (`YYYY-MM-DD`) — the same representation the app's calendar
 * grid (`CalendarDay.tsx`) uses (`new Date(date + 'T00:00:00Z')`).
 */

export function toIsoDate(date: Date): string {
  return date.toISOString().slice(0, 10)
}

export function addDays(date: Date, days: number): Date {
  const result = new Date(date.getTime())
  result.setUTCDate(result.getUTCDate() + days)
  return result
}

/**
 * Easter Sunday for `year`, via the Anonymous Gregorian (Meeus/Jones/
 * Butcher) algorithm. Pure date math, no external calendar data — this is
 * intentionally independent of the backend's `swedish_calendar.json` cache,
 * so the test computes its own expectation rather than asking the system
 * under test what the system under test should show.
 */
export function easterSunday(year: number): Date {
  const a = year % 19
  const b = Math.floor(year / 100)
  const c = year % 100
  const d = Math.floor(b / 4)
  const e = b % 4
  const f = Math.floor((b + 8) / 25)
  const g = Math.floor((b - f + 1) / 3)
  const h = (19 * a + b - d - g + 15) % 30
  const i = Math.floor(c / 4)
  const k = c % 4
  const l = (32 + 2 * e + 2 * i - h - k) % 7
  const m = Math.floor((a + 11 * h + 22 * l) / 451)
  const monthPlusDayEncoded = h + l - 7 * m + 114
  const month = Math.floor(monthPlusDayEncoded / 31) // 3 = March, 4 = April
  const day = (monthPlusDayEncoded % 31) + 1
  return new Date(Date.UTC(year, month - 1, day))
}

/** The first Easter Sunday on or after `from` (whole-day comparison). */
export function nextEasterOnOrAfter(from: Date): Date {
  let year = from.getUTCFullYear()
  let candidate = easterSunday(year)
  while (candidate.getTime() < from.getTime()) {
    year += 1
    candidate = easterSunday(year)
  }
  return candidate
}

/** Whit Sunday (Pingstdagen) is always exactly 49 days after Easter Sunday. */
export function whitSunday(easter: Date): Date {
  return addDays(easter, 49)
}

/** Midsummer's Day (Midsommardagen) — the Saturday falling between June 20
 * and June 26 inclusive, per Swedish law. */
export function midsummerDay(year: number): Date {
  for (let day = 20; day <= 26; day++) {
    const candidate = new Date(Date.UTC(year, 5, day)) // June = month 5
    if (candidate.getUTCDay() === 6) return candidate
  }
  /* istanbul ignore next -- a Saturday always falls in a 7-day window */
  throw new Error(`no Saturday found in June 20-26 ${year} — impossible`)
}

/**
 * True for a Saturday, a Sunday, or a Swedish public/"de facto" free day —
 * i.e. exactly the set of dates the planner treats as free (see
 * `is_free_day`/`_FREE_CATEGORIES` in the backend's `app/utils/holidays.py`
 * and `app/services/allocation_service.py`; `PlanningService.
 * _reject_illegal_free_day` rejects a `vacation`-typed pin on any of them).
 * Computed purely from date math, like `easterSunday` above — deliberately
 * independent of the backend's `swedish_calendar.json` cache, so a test
 * that uses this to pick "a range with no free days in it" isn't
 * validating itself against the very data the system under test reads.
 * Not meant to be exhaustive for every calendar nuance (klämdagar are
 * intentionally excluded — they're workable, not free) — only for the
 * free days a test needs to steer clear of.
 */
export function isSwedishFreeDay(date: Date): boolean {
  const day = date.getUTCDay()
  if (day === 0 || day === 6) return true // Sunday or Saturday

  const year = date.getUTCFullYear()
  const iso = toIsoDate(date)

  const fixed = new Set([
    `${year}-01-01`, // Nyårsdagen
    `${year}-01-06`, // Trettondedag jul
    `${year}-05-01`, // Första maj
    `${year}-06-06`, // Sveriges nationaldag
    `${year}-12-24`, // Julafton (de facto)
    `${year}-12-25`, // Juldagen
    `${year}-12-26`, // Annandag jul
    `${year}-12-31`, // Nyårsafton (de facto)
  ])
  if (fixed.has(iso)) return true

  const easter = easterSunday(year)
  const midsummer = midsummerDay(year)
  const moveable = [
    addDays(easter, -2), // Långfredagen
    addDays(easter, -1), // Påskafton (de facto)
    easter, // Påskdagen
    addDays(easter, 1), // Annandag påsk
    addDays(easter, 39), // Kristi himmelsfärdsdag
    addDays(easter, 48), // Pingstafton (de facto)
    addDays(easter, 49), // Pingstdagen
    addDays(midsummer, -1), // Midsommarafton (de facto) — midsummer day itself is always a Saturday, already covered above
  ].map(toIsoDate)

  return moveable.includes(iso)
}

/** Monday of the ISO week containing `date` (Mon=0 .. Sun=6 offset). */
export function mondayOfWeek(date: Date): Date {
  const isoWeekday = (date.getUTCDay() + 6) % 7 // 0 = Monday
  return addDays(date, -isoWeekday)
}

/**
 * Number of Mon–Sun calendar-week rows that the inclusive range
 * [start, end] touches — i.e. how many `CalendarWeek` rows a band spanning
 * that range should render across in the app's Monday-start grid
 * (see `Calendar.tsx`'s `startOffset = (firstDay.getDay() + 6) % 7`).
 */
export function countWeekRows(start: Date, end: Date): number {
  const startWeek = mondayOfWeek(start)
  const endWeek = mondayOfWeek(end)
  const diffDays = Math.round((endWeek.getTime() - startWeek.getTime()) / 86_400_000)
  return diffDays / 7 + 1
}