// date-utils.js — Restored & clean utilities (date-fns + timezone aware)
import * as dateFns from 'date-fns'
import { fromZonedTime, toZonedTime } from 'date-fns-tz'

const DEFAULT_TZ = Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC'

// Re-exported iso helpers (keep the same exported names used elsewhere)
const getISOWeek = dateFns.getISOWeek
const getISOWeekYear = dateFns.getISOWeekYear

// Constants
const monthAbbr = [
  'jan',
  'feb',
  'mar',
  'apr',
  'may',
  'jun',
  'jul',
  'aug',
  'sep',
  'oct',
  'nov',
  'dec',
]
const MIN_YEAR = 100 // less than 100 is interpreted as 19xx
const MAX_YEAR = 9999

// Core helpers ------------------------------------------------------------
/**
 * Construct a date at local midnight in the specified IANA timezone.
 * Returns a native Date whose wall-clock components in that zone are (Y, M, D 00:00:00).
 */
function makeTZDate(year, monthIndex, day, timeZone = DEFAULT_TZ) {
  const iso = `${String(year).padStart(4, '0')}-${String(monthIndex + 1).padStart(2, '0')}-${String(
    day,
  ).padStart(2, '0')}`
  const utcDate = fromZonedTime(`${iso}T00:00:00`, timeZone)
  return toZonedTime(utcDate, timeZone)
}

/**
 * Alias constructor for timezone-specific calendar date (semantic sugar over makeTZDate).
 */
const TZDate = (year, monthIndex, day, timeZone = DEFAULT_TZ) =>
  makeTZDate(year, monthIndex, day, timeZone)

/**
 * Construct a UTC-based date/time (wrapper for Date.UTC for consistency).
 */
const UTCDate = (year, monthIndex, day, hour = 0, minute = 0, second = 0, ms = 0) =>
  new Date(Date.UTC(year, monthIndex, day, hour, minute, second, ms))

function toLocalString(date = new Date(), timeZone = DEFAULT_TZ) {
  return dateFns.format(toZonedTime(date, timeZone), 'yyyy-MM-dd')
}

function fromLocalString(dateString, timeZone = DEFAULT_TZ) {
  if (!dateString) return makeTZDate(1970, 0, 1, timeZone)
  const parsed = dateFns.parseISO(dateString)
  const utcDate = fromZonedTime(`${dateString}T00:00:00`, timeZone)
  return toZonedTime(utcDate, timeZone) || parsed
}

function getMondayOfISOWeek(date, timeZone = DEFAULT_TZ) {
  const d = toZonedTime(date, timeZone)
  const dow = (dateFns.getDay(d) + 6) % 7 // Monday=0
  return dateFns.addDays(dateFns.startOfDay(d), -dow)
}

const mondayIndex = (d) => (dateFns.getDay(d) + 6) % 7

// Count how many days in [startDate..endDate] match the boolean `pattern` array
function countPatternDaysInInterval(startDate, endDate, patternArr) {
  const days = dateFns.eachDayOfInterval({
    start: dateFns.startOfDay(startDate),
    end: dateFns.startOfDay(endDate),
  })
  return days.reduce((c, d) => c + (patternArr[dateFns.getDay(d)] ? 1 : 0), 0)
}

// Recurrence: Weekly ------------------------------------------------------
function _getRecur(event) {
  return event?.recur ?? null
}

function getWeeklyOccurrenceIndex(event, dateStr, timeZone = DEFAULT_TZ) {
  const recur = _getRecur(event)
  if (!recur || recur.freq !== 'weeks') return null
  const pattern = recur.weekdays || []
  if (!pattern.some(Boolean)) return null

  const target = fromLocalString(dateStr, timeZone)
  const baseStart = fromLocalString(event.startDate, timeZone)
  if (target < baseStart) return null

  const dow = dateFns.getDay(target)
  if (!pattern[dow]) return null // target not active

  const interval = recur.interval || 1
  const baseBlockStart = getMondayOfISOWeek(baseStart, timeZone)
  const currentBlockStart = getMondayOfISOWeek(target, timeZone)
  // Number of weeks between block starts (each block start is a Monday)
  const weekDiff = dateFns.differenceInCalendarWeeks(currentBlockStart, baseBlockStart)
  if (weekDiff < 0 || weekDiff % interval !== 0) return null

  const baseDow = dateFns.getDay(baseStart)
  const baseCountsAsPattern = !!pattern[baseDow]

  // Same ISO week as base: count pattern days from baseStart up to target (inclusive)
  if (weekDiff === 0) {
    let n = countPatternDaysInInterval(baseStart, target, pattern) - 1
    if (!baseCountsAsPattern) n += 1
    const maxCount = recur.count === 'unlimited' ? Infinity : parseInt(recur.count, 10)
    return n < 0 || n >= maxCount ? null : n
  }

  const baseWeekEnd = dateFns.addDays(baseBlockStart, 6)
  // Count pattern days in the first (possibly partial) week from baseStart..baseWeekEnd
  const firstWeekCount = countPatternDaysInInterval(baseStart, baseWeekEnd, pattern)
  const alignedWeeksBetween = weekDiff / interval - 1
  const fullPatternWeekCount = pattern.filter(Boolean).length
  const middleWeeksCount = alignedWeeksBetween > 0 ? alignedWeeksBetween * fullPatternWeekCount : 0
  // Count pattern days in the current (possibly partial) week from currentBlockStart..target
  const currentWeekCount = countPatternDaysInInterval(currentBlockStart, target, pattern)
  let n = firstWeekCount + middleWeeksCount + currentWeekCount - 1
  if (!baseCountsAsPattern) n += 1
  const maxCount = recur.count === 'unlimited' ? Infinity : parseInt(recur.count, 10)
  return n >= maxCount ? null : n
}

// Recurrence: Monthly -----------------------------------------------------
function getMonthlyOccurrenceIndex(event, dateStr, timeZone = DEFAULT_TZ) {
  const recur = _getRecur(event)
  if (!recur || recur.freq !== 'months') return null
  const baseStart = fromLocalString(event.startDate, timeZone)
  const d = fromLocalString(dateStr, timeZone)
  const diffMonths = dateFns.differenceInCalendarMonths(d, baseStart)
  if (diffMonths < 0) return null
  const interval = recur.interval || 1
  if (diffMonths % interval !== 0) return null
  const baseDay = dateFns.getDate(baseStart)
  const effectiveDay = Math.min(baseDay, dateFns.getDaysInMonth(d))
  if (dateFns.getDate(d) !== effectiveDay) return null
  const n = diffMonths / interval
  const maxCount = recur.count === 'unlimited' ? Infinity : parseInt(recur.count, 10)
  return n >= maxCount ? null : n
}

function getOccurrenceIndex(event, dateStr, timeZone = DEFAULT_TZ) {
  const recur = _getRecur(event)
  if (!recur) return null
  if (dateStr < event.startDate) return null
  if (recur.freq === 'weeks') return getWeeklyOccurrenceIndex(event, dateStr, timeZone)
  if (recur.freq === 'months') return getMonthlyOccurrenceIndex(event, dateStr, timeZone)
  return null
}

// Reverse lookup: given a recurrence index (0-based) return the occurrence start date string.
// Returns null if the index is out of range or the event is not repeating.
function getWeeklyOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ) {
  const recur = _getRecur(event)
  if (!recur || recur.freq !== 'weeks') return null
  if (occurrenceIndex < 0 || !Number.isInteger(occurrenceIndex)) return null
  const maxCount = recur.count === 'unlimited' ? Infinity : parseInt(recur.count, 10)
  if (occurrenceIndex >= maxCount) return null
  const pattern = recur.weekdays || []
  if (!pattern.some(Boolean)) return null
  const interval = recur.interval || 1
  const baseStart = fromLocalString(event.startDate, timeZone)
  if (occurrenceIndex === 0) return toLocalString(baseStart, timeZone)
  const baseWeekMonday = getMondayOfISOWeek(baseStart, timeZone)
  const baseDow = dateFns.getDay(baseStart)
  const baseCountsAsPattern = !!pattern[baseDow]
  // Adjust index if base weekday is not part of the pattern (pattern occurrences shift by +1)
  let occ = occurrenceIndex
  if (!baseCountsAsPattern) occ -= 1
  if (occ < 0) return null
  // Sorted list of active weekday indices
  const patternDays = []
  for (let d = 0; d < 7; d++) if (pattern[d]) patternDays.push(d)
  // First (possibly partial) week: only pattern days >= baseDow and >= baseStart date
  const firstWeekDates = []
  for (const d of patternDays) {
    if (d < baseDow) continue
    const date = dateFns.addDays(baseWeekMonday, d)
    if (date < baseStart) continue
    firstWeekDates.push(date)
  }
  const F = firstWeekDates.length
  if (occ < F) {
    return toLocalString(firstWeekDates[occ], timeZone)
  }
  const remaining = occ - F
  const P = patternDays.length
  if (P === 0) return null
  // Determine aligned week group (k >= 1) in which the remaining-th occurrence lies
  const k = Math.floor(remaining / P) + 1 // 1-based aligned week count after base week
  const indexInWeek = remaining % P
  const dow = patternDays[indexInWeek]
  const occurrenceDate = dateFns.addDays(baseWeekMonday, k * interval * 7 + dow)
  return toLocalString(occurrenceDate, timeZone)
}

function getMonthlyOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ) {
  const recur = _getRecur(event)
  if (!recur || recur.freq !== 'months') return null
  if (occurrenceIndex < 0 || !Number.isInteger(occurrenceIndex)) return null
  const maxCount = recur.count === 'unlimited' ? Infinity : parseInt(recur.count, 10)
  if (occurrenceIndex >= maxCount) return null
  const interval = recur.interval || 1
  const baseStart = fromLocalString(event.startDate, timeZone)
  const targetMonthOffset = occurrenceIndex * interval
  const monthDate = dateFns.addMonths(baseStart, targetMonthOffset)
  // Adjust day for shorter months (clamp like forward logic)
  const baseDay = dateFns.getDate(baseStart)
  const daysInTargetMonth = dateFns.getDaysInMonth(monthDate)
  const day = Math.min(baseDay, daysInTargetMonth)
  const actual = makeTZDate(dateFns.getYear(monthDate), dateFns.getMonth(monthDate), day, timeZone)
  return toLocalString(actual, timeZone)
}

function getOccurrenceDate(event, occurrenceIndex, timeZone = DEFAULT_TZ) {
  const recur = _getRecur(event)
  if (!recur) return null
  if (recur.freq === 'weeks') return getWeeklyOccurrenceDate(event, occurrenceIndex, timeZone)
  if (recur.freq === 'months') return getMonthlyOccurrenceDate(event, occurrenceIndex, timeZone)
  return null
}

function getVirtualOccurrenceEndDate(event, occurrenceStartDate, timeZone = DEFAULT_TZ) {
  const spanDays = Math.max(0, (event.days || 1) - 1)
  const occurrenceStart = fromLocalString(occurrenceStartDate, timeZone)
  return toLocalString(dateFns.addDays(occurrenceStart, spanDays), timeZone)
}

// Utility formatting & localization ---------------------------------------
const pad = (n) => String(n).padStart(2, '0')

function daysInclusive(aStr, bStr, timeZone = DEFAULT_TZ) {
  const a = fromLocalString(aStr, timeZone)
  const b = fromLocalString(bStr, timeZone)
  return (
    Math.abs(dateFns.differenceInCalendarDays(dateFns.startOfDay(a), dateFns.startOfDay(b))) + 1
  )
}

function addDaysStr(str, n, timeZone = DEFAULT_TZ) {
  return toLocalString(dateFns.addDays(fromLocalString(str, timeZone), n), timeZone)
}

// Weekday name helpers now return Sunday-first ordering (index 0 = Sunday ... 6 = Saturday)
function getLocalizedWeekdayNames(timeZone = DEFAULT_TZ) {
  const sunday = makeTZDate(2025, 0, 5, timeZone) // a Sunday
  return Array.from({ length: 7 }, (_, i) =>
    new Intl.DateTimeFormat(undefined, { weekday: 'short', timeZone }).format(
      dateFns.addDays(sunday, i),
    ),
  )
}

// Long (wide) localized weekday names, Sunday-first ordering
function getLocalizedWeekdayNamesLong(timeZone = DEFAULT_TZ) {
  const sunday = makeTZDate(2025, 0, 5, timeZone)
  return Array.from({ length: 7 }, (_, i) =>
    new Intl.DateTimeFormat(undefined, { weekday: 'long', timeZone }).format(
      dateFns.addDays(sunday, i),
    ),
  )
}

function getLocaleFirstDay() {
  const day = new Intl.Locale(navigator.language).weekInfo?.firstDay ?? 1
  return day % 7
}

function getLocaleWeekendDays() {
  const wk = new Set(new Intl.Locale(navigator.language).weekInfo?.weekend ?? [6, 7])
  return Array.from({ length: 7 }, (_, i) => wk.has(1 + ((i + 6) % 7)))
}

function reorderByFirstDay(days, firstDay) {
  return Array.from({ length: 7 }, (_, i) => days[(i + firstDay) % 7])
}

function getLocalizedMonthName(idx, short = false, timeZone = DEFAULT_TZ) {
  const d = makeTZDate(2025, idx, 1, timeZone)
  return new Intl.DateTimeFormat(undefined, { month: short ? 'short' : 'long', timeZone }).format(d)
}

function formatDateRange(startDate, endDate, timeZone = DEFAULT_TZ) {
  const a = toLocalString(startDate, timeZone)
  const b = toLocalString(endDate, timeZone)
  if (a === b) return a
  const [ay, am] = a.split('-')
  const [by, bm, bd] = b.split('-')
  if (ay === by && am === bm) return `${a}/${bd}`
  if (ay === by) return `${a}/${bm}-${bd}`
  return `${a}/${b}`
}

function lunarPhaseSymbol(date) {
  // Reference new moon (J2000 era) used for approximate phase calculations
  const ref = UTCDate(2000, 0, 6, 18, 14, 0)
  const obs = new Date(date)
  obs.setHours(12, 0, 0, 0)
  const synodic = 29.530588853 // mean synodic month length in days
  const daysSince = dateFns.differenceInMinutes(obs, ref) / 60 / 24
  const phase = (((daysSince / synodic) % 1) + 1) % 1 // normalize to [0,1)
  const phases = [
    { t: 0.0, s: '🌑' }, // New
    { t: 0.25, s: '🌓' }, // First Quarter
    { t: 0.5, s: '🌕' }, // Full
    { t: 0.75, s: '🌗' }, // Last Quarter
  ]
  const thresholdDays = 0.5 // within ~12h of exact phase
  for (const p of phases) {
    let delta = Math.abs(phase - p.t)
    if (delta > 0.5) delta = 1 - delta // wrap shortest arc
    if (delta * synodic <= thresholdDays) return p.s
  }
  return ''
}

// Exports -----------------------------------------------------------------
/**
 * Format date as short localized string (e.g., "Jan 15")
 */
function formatDateShort(date) {
  return date.toLocaleDateString(undefined, { month: 'short', day: 'numeric' }).replace(/, /, ' ')
}

/**
 * Format date as long localized string with optional year (e.g., "Mon Jan 15" or "Mon Jan 15, 2025")
 */
function formatDateLong(date, includeYear = false) {
  const opts = {
    weekday: 'short',
    month: 'short',
    day: 'numeric',
    ...(includeYear ? { year: 'numeric' } : {}),
  }
  return date.toLocaleDateString(undefined, opts)
}

/**
 * Format date as today string (e.g., "Monday\nJanuary 15")
 */
function formatTodayString(date) {
  const formatted = date
    .toLocaleDateString(undefined, { weekday: 'long', month: 'long', day: 'numeric' })
    .replace(/,? /, '\n')
  return formatted.charAt(0).toUpperCase() + formatted.slice(1)
}

export {
  // constants
  monthAbbr,
  MIN_YEAR,
  MAX_YEAR,
  DEFAULT_TZ,
  // core tz helpers
  makeTZDate,
  toLocalString,
  fromLocalString,
  // recurrence
  getMondayOfISOWeek,
  mondayIndex,
  getOccurrenceIndex,
  getOccurrenceDate,
  getVirtualOccurrenceEndDate,
  // formatting & localization
  pad,
  daysInclusive,
  addDaysStr,
  getLocalizedWeekdayNames,
  getLocalizedWeekdayNamesLong,
  getLocaleFirstDay,
  getLocaleWeekendDays,
  reorderByFirstDay,
  getLocalizedMonthName,
  formatDateRange,
  formatDateShort,
  formatDateLong,
  formatTodayString,
  lunarPhaseSymbol,
  // iso helpers re-export
  getISOWeek,
  getISOWeekYear,
  // constructors
  TZDate,
  UTCDate,
}