export interface Occasion {
  key:               string
  label:             string
  emoji:             string
  blurb:             string
  /** The slug that Square items must have as their category to appear in this tab.
   *  Derived from the Square category name: lowercased, non-alphanumeric chars → '-'.
   *  If omitted the tab shows all products (fallback). */
  squareCategorySlug?: string
  ranges:            (year: number) => Array<{ startMonth: number; startDay: number; endMonth: number; endDay: number }>
}

// ── Easter calculation (Anonymous Gregorian algorithm) ────────────────────────
function easterDate(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 month = Math.floor((h + l - 7 * m + 114) / 31)   // 1-12
  const day   = ((h + l - 7 * m + 114) % 31) + 1
  return new Date(year, month - 1, day)
}

/** Returns the nth occurrence of a weekday (0=Sun…6=Sat) in a given month. */
function nthWeekday(year: number, month: number, weekday: number, n: number): Date {
  const first = new Date(year, month, 1)
  const offset = (weekday - first.getDay() + 7) % 7
  return new Date(year, month, 1 + offset + (n - 1) * 7)
}

function addDays(d: Date, n: number): Date {
  return new Date(d.getTime() + n * 86_400_000)
}

function toRange(start: Date, end: Date) {
  return {
    startMonth: start.getMonth() + 1,
    startDay:   start.getDate(),
    endMonth:   end.getMonth()   + 1,
    endDay:     end.getDate(),
  }
}

// ── Occasions ─────────────────────────────────────────────────────────────────
export const OCCASIONS: Occasion[] = [
  {
    key:               'new-year',
    label:             "New Year's",
    emoji:             '🥂',
    blurb:             "Ring in the new year in style — balloons for every party and celebration.",
    squareCategorySlug: 'new-years',
    ranges: (y) => [
      toRange(new Date(y - 1, 11, 26), new Date(y - 1, 11, 31)),  // Dec 26-31 prior year
      toRange(new Date(y, 0, 1), new Date(y, 0, 3)),               // Jan 1-3
    ],
  },
  {
    key:               'valentine',
    label:             "Valentine's Day",
    emoji:             '❤️',
    blurb:             "Say it with balloons — romantic bouquets, arches, and custom arrangements.",
    squareCategorySlug: 'valentines-day',
    ranges: (y) => [toRange(new Date(y, 1, 1), new Date(y, 1, 14))],
  },
  {
    key:               'st-patricks',
    label:             "St. Patrick's Day",
    emoji:             '🍀',
    blurb:             "Go green! Festive balloon décor for your St. Patrick's Day celebration.",
    squareCategorySlug: 'st-patricks-day',
    ranges: (y) => [toRange(new Date(y, 2, 10), new Date(y, 2, 17))],
  },
  {
    key:   'easter',
    label: 'Easter',
    emoji: '🐣',
    blurb: "Pastel bouquets, organic arrangements, and cheerful décor for Easter.",
    squareCategorySlug: 'easter',
    // Palm Sunday (−7) through Easter Monday (+1)
    ranges: (y) => {
      const e = easterDate(y)
      return [toRange(addDays(e, -7), addDays(e, 1))]
    },
  },
  {
    key:               'mothers-day',
    label:             "Mother's Day",
    emoji:             '💐',
    blurb:             "Make mom feel extra special with a beautiful custom balloon arrangement.",
    squareCategorySlug: 'mothers-day',
    ranges: (y) => [toRange(new Date(y, 3, 28), new Date(y, 4, 12))],
  },
  {
    key:               'graduation',
    label:             'Graduation',
    emoji:             '🎓',
    blurb:             "Celebrate the big milestone — arches, columns, and bouquets for grads.",
    squareCategorySlug: 'graduation',
    ranges: (y) => [toRange(new Date(y, 4, 1), new Date(y, 5, 30))],
  },
  {
    key:               'fathers-day',
    label:             "Father's Day",
    emoji:             '👔',
    blurb:             "Show dad some love — balloons and arrangements for the best dad around.",
    squareCategorySlug: 'fathers-day',
    // Third Sunday in June; window opens 2 weeks prior
    ranges: (y) => {
      const fd = nthWeekday(y, 5, 0, 3)   // 3rd Sunday of June (month index 5)
      return [toRange(addDays(fd, -14), fd)]
    },
  },
  {
    key:               'fourth-july',
    label:             '4th of July',
    emoji:             '🇺🇸',
    blurb:             "Red, white & blue — festive balloon décor for your Independence Day party.",
    squareCategorySlug: '4th-of-july',
    ranges: (y) => [toRange(new Date(y, 5, 25), new Date(y, 6, 4))],
  },
  {
    key:               'back-to-school',
    label:             'Back to School',
    emoji:             '🎒',
    blurb:             "Kick off the school year with fun balloon bouquets and decorations.",
    squareCategorySlug: 'back-to-school',
    ranges: (y) => [toRange(new Date(y, 7, 15), new Date(y, 8, 10))],
  },
  {
    key:               'halloween',
    label:             'Halloween',
    emoji:             '🎃',
    blurb:             "Spooky, fun, and festive — balloon décor for your Halloween bash.",
    squareCategorySlug: 'halloween',
    ranges: (y) => [toRange(new Date(y, 9, 1), new Date(y, 9, 31))],
  },
  {
    key:               'thanksgiving',
    label:             'Thanksgiving',
    emoji:             '🦃',
    blurb:             "Warm autumn arrangements and centerpieces for your Thanksgiving gathering.",
    squareCategorySlug: 'thanksgiving',
    ranges: (y) => [toRange(new Date(y, 10, 15), new Date(y, 10, 28))],
  },
  {
    key:               'christmas',
    label:             'Christmas',
    emoji:             '🎄',
    blurb:             "Deck the halls — festive balloon arches, columns, and centerpieces.",
    squareCategorySlug: 'christmas',
    ranges: (y) => [toRange(new Date(y, 11, 1), new Date(y, 11, 25))],
  },
]

// ── Override config ───────────────────────────────────────────────────────────

export interface OccasionOverride {
  enabled?:            boolean  // omit = true
  squareCategorySlug?: string   // omit = use occasion default
  windowStart?:        string   // "MM-DD" — override computed start date
  windowEnd?:          string   // "MM-DD" — override computed end date
}

/** A fully user-defined holiday (not in the built-in OCCASIONS list). */
export interface CustomOccasionDef {
  custom:              true
  enabled?:            boolean
  label:               string
  emoji:               string
  blurb:               string
  squareCategorySlug?: string
  windowStart:         string   // "MM-DD" required
  windowEnd:           string   // "MM-DD" required
}

export type OccasionsConfig = Record<string, OccasionOverride | CustomOccasionDef>

// ── Helpers ───────────────────────────────────────────────────────────────────

function mmddNum(mmdd: string): number {
  const [mm, dd] = mmdd.split('-').map(Number)
  return mm * 100 + dd
}

function mmddToDate(year: number, mmdd: string): Date {
  const [mm, dd] = mmdd.split('-').map(Number)
  return new Date(year, mm - 1, dd)
}

/**
 * Returns the primary display window for an occasion in a given year,
 * applying any windowStart / windowEnd overrides.
 */
export function getOccasionWindow(
  occ: Occasion,
  year: number,
  ov?: Pick<OccasionOverride, 'windowStart' | 'windowEnd'>,
): { start: Date; end: Date } | null {
  const ranges = occ.ranges(year)
  if (!ranges.length) return null
  // Use the last range as primary (handles New Year's which has two ranges)
  const r     = ranges[ranges.length - 1]
  const start = ov?.windowStart
    ? mmddToDate(year, ov.windowStart)
    : new Date(year, r.startMonth - 1, r.startDay)
  const end   = ov?.windowEnd
    ? mmddToDate(year, ov.windowEnd)
    : new Date(year, r.endMonth - 1, r.endDay)
  return { start, end }
}

/** Returns all occasions currently active for the given date, with optional config overrides. */
export function getActiveOccasions(date: Date = new Date(), config: OccasionsConfig = {}): Occasion[] {
  const y    = date.getFullYear()
  const mmdd = (date.getMonth() + 1) * 100 + date.getDate()

  const results: Occasion[] = []

  // Built-in occasions with optional overrides
  for (const occ of OCCASIONS) {
    const ov = config[occ.key]
    if (ov?.enabled === false) continue

    let active = false
    if (ov && !('custom' in ov) && (ov.windowStart || ov.windowEnd)) {
      const win = getOccasionWindow(occ, y, ov)
      if (win) {
        const s = (win.start.getMonth() + 1) * 100 + win.start.getDate()
        const e = (win.end.getMonth()   + 1) * 100 + win.end.getDate()
        active = mmdd >= s && mmdd <= e
      }
    } else {
      active = occ.ranges(y).some(({ startMonth, startDay, endMonth, endDay }) =>
        mmdd >= startMonth * 100 + startDay && mmdd <= endMonth * 100 + endDay
      )
    }

    if (active) {
      const slug = ov && !('custom' in ov) ? ov.squareCategorySlug : undefined
      results.push(slug ? { ...occ, squareCategorySlug: slug } : occ)
    }
  }

  // User-defined custom occasions
  for (const [key, ov] of Object.entries(config)) {
    if (!('custom' in ov)) continue
    const def = ov as CustomOccasionDef
    if (def.enabled === false) continue
    const s = mmddNum(def.windowStart)
    const e = mmddNum(def.windowEnd)
    if (mmdd >= s && mmdd <= e) {
      results.push({
        key,
        label:              def.label,
        emoji:              def.emoji,
        blurb:              def.blurb,
        squareCategorySlug: def.squareCategorySlug,
        ranges:             () => [],
      })
    }
  }

  return results
}