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')}`;
}

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);
}