const MONTH_NAMES = [
  'فروردین',
  'اردیبهشت',
  'خرداد',
  'تیر',
  'مرداد',
  'شهریور',
  'مهر',
  'آبان',
  'آذر',
  'دی',
  'بهمن',
  'اسفند',
] as const;

function div(a: number, b: number) {
  return Math.trunc(a / b);
}

function mod(a: number, b: number) {
  return a - Math.trunc(a / b) * b;
}

export function toJalali(gy: number, gm: number, gd: number) {
  const gdm = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334];
  const gy2 = gm > 2 ? gy + 1 : gy;
  let days =
    355666 +
    365 * gy +
    div(gy2 + 3, 4) -
    div(gy2 + 99, 100) +
    div(gy2 + 399, 400) +
    gd +
    gdm[gm - 1];
  let jy = -1595 + 33 * div(days, 12053);
  days %= 12053;
  jy += 4 * div(days, 1461);
  days %= 1461;
  if (days > 365) {
    jy += div(days - 1, 365);
    days = (days - 1) % 365;
  }
  const jm = days < 186 ? 1 + div(days, 31) : 7 + div(days - 186, 30);
  const jd = 1 + (days < 186 ? days % 31 : (days - 186) % 30);
  return { jy, jm, jd };
}

export function toGregorian(jy: number, jm: number, jd: number) {
  const jy2 = jy + 1595;
  let days =
    -355668 +
    365 * jy2 +
    div(jy2, 33) * 8 +
    div(mod(jy2, 33) + 3, 4) +
    jd +
    (jm < 7 ? (jm - 1) * 31 : (jm - 7) * 30 + 186);
  let gy = 400 * div(days, 146097);
  days %= 146097;
  if (days > 36524) {
    gy += 100 * div(--days, 36524);
    days %= 36524;
    if (days >= 365) days += 1;
  }
  gy += 4 * div(days, 1461);
  days %= 1461;
  if (days > 365) {
    gy += div(days - 1, 365);
    days = (days - 1) % 365;
  }
  const gd = days + 1;
  const salA = [
    0, 31, (gy % 4 === 0 && gy % 100 !== 0) || gy % 400 === 0 ? 29 : 28,
    31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
  ];
  let gm = 0;
  let dayAcc = gd;
  for (gm = 1; gm <= 12 && dayAcc > salA[gm]; gm += 1) {
    dayAcc -= salA[gm];
  }
  return { gy, gm, gd: dayAcc };
}

export function dateToJalali(d: Date) {
  return toJalali(d.getFullYear(), d.getMonth() + 1, d.getDate());
}

export function jalaliToDate(jy: number, jm: number, jd: number): Date {
  const { gy, gm, gd } = toGregorian(jy, jm, jd);
  return new Date(gy, gm - 1, gd);
}

export function addJalaliMonths(jy: number, jm: number, delta: number) {
  const idx = jy * 12 + (jm - 1) + delta;
  return { jy: Math.floor(idx / 12), jm: (idx % 12) + 1 };
}

export function jalaliMonthName(jm: number): string {
  return MONTH_NAMES[jm - 1] ?? '';
}

export function jalaliMonthKey(d: Date): string {
  const { jy, jm } = dateToJalali(d);
  return `${jy}-${String(jm).padStart(2, '0')}`;
}

const FA_DIGITS = '۰۱۲۳۴۵۶۷۸۹';

function toAsciiDigits(value: string) {
  return value
    .replace(/[۰-۹]/g, (d) => String(FA_DIGITS.indexOf(d)))
    .replace(/[٠-٩]/g, (d) => String('٠١٢٣٤٥٦٧٨٩'.indexOf(d)));
}

function toFaDigits(value: string | number) {
  return String(value).replace(/\d/g, (d) => FA_DIGITS[Number(d)] ?? d);
}

function tehranYmd(d: Date) {
  const [gy, gm, gd] = new Intl.DateTimeFormat('en-CA', {
    timeZone: 'Asia/Tehran',
    year: 'numeric',
    month: '2-digit',
    day: '2-digit',
  })
    .format(d)
    .split('-')
    .map(Number);
  return { gy, gm, gd };
}

/** تاریخ شمسی تهران، مثل ۱۴۰۲/۰۶/۱۲ */
export function formatTehranJalali(d: Date): string {
  const { gy, gm, gd } = tehranYmd(d);
  const j = toJalali(gy, gm, gd);
  return `${toFaDigits(j.jy)}/${toFaDigits(String(j.jm).padStart(2, '0'))}/${toFaDigits(String(j.jd).padStart(2, '0'))}`;
}

const GREG_MONTHS: Array<[string, number]> = [
  ['ژانویه', 1],
  ['january', 1],
  ['فوریه', 2],
  ['february', 2],
  ['مارس', 3],
  ['march', 3],
  ['آوریل', 4],
  ['april', 4],
  ['ژوئن', 6],
  ['june', 6],
  ['ژوئیه', 7],
  ['جولای', 7],
  ['july', 7],
  ['اوت', 8],
  ['آگوست', 8],
  ['آگست', 8],
  ['august', 8],
  ['سپتامبر', 9],
  ['september', 9],
  ['اکتبر', 10],
  ['october', 10],
  ['نوامبر', 11],
  ['november', 11],
  ['دسامبر', 12],
  ['december', 12],
  ['مه', 5],
  ['می', 5],
  ['may', 5],
];

function gregMonthNumber(name: string): number | null {
  const key = name.trim().toLowerCase();
  return GREG_MONTHS.find(([label]) => label === key)?.[1] ?? null;
}

function inferGregorianYear(month: number, day: number, now = new Date()): number {
  const { gy, gm, gd } = tehranYmd(now);
  if (month > gm || (month === gm && day > gd)) return gy - 1;
  return gy;
}

function jalaliFromGregorian(year: number, month: number, day: number): string | null {
  if (month < 1 || month > 12 || day < 1 || day > 31 || year < 1700 || year > 2500) return null;
  const j = toJalali(year, month, day);
  return `${toFaDigits(j.jy)}/${toFaDigits(String(j.jm).padStart(2, '0'))}/${toFaDigits(String(j.jd).padStart(2, '0'))}`;
}

/** «21 سپتامبر» و تاریخ میلادی عددی را در متن کاربر به شمسی تبدیل می‌کند. */
export function rewriteGregorianDates(text: string, now = new Date()): string {
  const monthAlt = GREG_MONTHS.map(([name]) => name).join('|');
  const dayMonth = new RegExp(
    `(^|[^\\p{L}\\d])([0-9۰-۹]{1,2})\\s*(${monthAlt})(?![\\p{L}\\u200c])(?:\\s+([0-9۰-۹]{4}))?`,
    'giu',
  );
  const monthDay = new RegExp(
    `(^|[^\\p{L}\\d])(${monthAlt})(?![\\p{L}\\u200c])\\s+([0-9۰-۹]{1,2})(?:\\s*,?\\s*([0-9۰-۹]{4}))?`,
    'giu',
  );
  const iso = /\b((?:19|20)\d{2})-(\d{2})-(\d{2})\b/g;

  const fromParts = (dayRaw: string, monthName: string, yearRaw?: string) => {
    const day = Number(toAsciiDigits(dayRaw));
    const month = gregMonthNumber(monthName);
    if (!month) return null;
    const year = yearRaw ? Number(toAsciiDigits(yearRaw)) : inferGregorianYear(month, day, now);
    return jalaliFromGregorian(year, month, day);
  };

  let out = text.replace(dayMonth, (full, lead: string, day: string, month: string, year?: string) => {
    const next = fromParts(day, month, year);
    return next ? `${lead}${next}` : full;
  });
  out = out.replace(monthDay, (full, lead: string, month: string, day: string, year?: string) => {
    const next = fromParts(day, month, year);
    return next ? `${lead}${next}` : full;
  });
  out = out.replace(iso, (full, year: string, month: string, day: string) => {
    return jalaliFromGregorian(Number(year), Number(month), Number(day)) ?? full;
  });
  return out;
}

export function startOfJalaliMonth(jy: number, jm: number): Date {
  return jalaliToDate(jy, jm, 1);
}

export function endOfJalaliMonth(jy: number, jm: number): Date {
  const next = addJalaliMonths(jy, jm, 1);
  const startNext = jalaliToDate(next.jy, next.jm, 1);
  return new Date(startNext.getTime() - 1);
}