/*
 * Datumsrechnung ohne Zeitzonenfallen: alles laeuft ueber 'YYYY-MM-DD' und
 * UTC-Mitternacht, damit ein Tag nie durch die Sommerzeit verrutscht.
 */
export function parseDate(value) {
  if (typeof value !== 'string' || !/^\d{4}-\d{2}-\d{2}$/.test(value)) return null;
  const date = new Date(`${value}T00:00:00Z`);
  return Number.isNaN(date.getTime()) || iso(date) !== value ? null : date;
}

export function iso(date) {
  return date.toISOString().slice(0, 10);
}

export function today() {
  const now = new Date();
  // Lokales Datum des Servers (Europe/Berlin), nicht UTC.
  const local = new Date(now.getTime() - now.getTimezoneOffset() * 60000);
  return local.toISOString().slice(0, 10);
}

export function addDays(value, days) {
  const date = parseDate(value);
  date.setUTCDate(date.getUTCDate() + days);
  return iso(date);
}

export function daysBetween(from, to) {
  return Math.round((parseDate(to) - parseDate(from)) / 86400000);
}

/** Arbeitstage Mo-Fr im geschlossenen Intervall. */
export function workdays(from, to) {
  let count = 0;
  const end = parseDate(to);
  for (const d = parseDate(from); d <= end; d.setUTCDate(d.getUTCDate() + 1)) {
    const day = d.getUTCDay();
    if (day !== 0 && day !== 6) count++;
  }
  return count;
}

export function monthRange(month) {
  if (!/^\d{4}-\d{2}$/.test(month || '')) return null;
  const [y, m] = month.split('-').map(Number);
  if (m < 1 || m > 12) return null;
  const last = new Date(Date.UTC(y, m, 0)).getUTCDate();
  return { from: `${month}-01`, to: `${month}-${String(last).padStart(2, '0')}` };
}

export function isoWeek(value) {
  const date = parseDate(value);
  const day = date.getUTCDay() || 7;
  date.setUTCDate(date.getUTCDate() + 4 - day);
  const yearStart = new Date(Date.UTC(date.getUTCFullYear(), 0, 1));
  return { year: date.getUTCFullYear(), week: Math.ceil(((date - yearStart) / 86400000 + 1) / 7) };
}

export function mondayOf(value) {
  const date = parseDate(value);
  const day = date.getUTCDay() || 7;
  date.setUTCDate(date.getUTCDate() - day + 1);
  return iso(date);
}

export const MONTHS_DE = ['Januar', 'Februar', 'März', 'April', 'Mai', 'Juni', 'Juli', 'August', 'September', 'Oktober', 'November', 'Dezember'];

export function formatDateDe(value) {
  const [y, m, d] = String(value).slice(0, 10).split('-');
  return `${d}.${m}.${y}`;
}

/** Zeitraum, in dem `value` liegt: day | week | month | quarter | year. */
export function periodRange(granularity, value) {
  const [y, m] = value.split('-').map(Number);
  switch (granularity) {
    case 'day':
      return { from: value, to: value };
    case 'week': {
      const from = mondayOf(value);
      return { from, to: addDays(from, 6) };
    }
    case 'month':
      return monthRange(value.slice(0, 7));
    case 'quarter': {
      const q = Math.floor((m - 1) / 3);
      const startMonth = String(q * 3 + 1).padStart(2, '0');
      const end = monthRange(`${y}-${String(q * 3 + 3).padStart(2, '0')}`);
      return { from: `${y}-${startMonth}-01`, to: end.to };
    }
    case 'year':
      return { from: `${y}-01-01`, to: `${y}-12-31` };
    default:
      return null;
  }
}

/** Beginn des vorherigen Zeitraums (fuer Reihen und Vergleich). */
export function previousStart(granularity, from) {
  const d = parseDate(from);
  if (granularity === 'day') d.setUTCDate(d.getUTCDate() - 1);
  if (granularity === 'week') d.setUTCDate(d.getUTCDate() - 7);
  if (granularity === 'month') d.setUTCMonth(d.getUTCMonth() - 1);
  if (granularity === 'quarter') d.setUTCMonth(d.getUTCMonth() - 3);
  if (granularity === 'year') d.setUTCFullYear(d.getUTCFullYear() - 1);
  return iso(d);
}

export function periodLabel(granularity, from) {
  const [y, m, d] = from.split('-');
  if (granularity === 'day') return `${d}.${m}.`;
  if (granularity === 'week') return `KW ${isoWeek(from).week}`;
  if (granularity === 'month') return `${MONTHS_DE[Number(m) - 1].slice(0, 3)} ${y.slice(2)}`;
  if (granularity === 'quarter') return `Q${Math.floor((Number(m) - 1) / 3) + 1} ${y.slice(2)}`;
  return y;
}

/** Ostersonntag (Gauss/Meeus, gregorianisch) als 'YYYY-MM-DD'. */
export function easterSunday(year) {
  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);
  const day = ((h + l - 7 * m + 114) % 31) + 1;
  return `${year}-${String(month).padStart(2, '0')}-${String(day).padStart(2, '0')}`;
}

/**
 * Name eines deutschen Feiertags zum Datum. HRworks sagt nur, DASS ein Tag
 * Feiertag ist (welcher, haengt am Standort der Person); der Name kommt von
 * hier. Unbekannt: null.
 */
export function germanHolidayName(value) {
  const year = Number(value.slice(0, 4));
  const fixed = {
    '01-01': 'Neujahr',
    '01-06': 'Heilige Drei Könige',
    '03-08': 'Frauentag',
    '05-01': 'Tag der Arbeit',
    '08-08': 'Friedensfest',
    '08-15': 'Mariä Himmelfahrt',
    '09-20': 'Weltkindertag',
    '10-03': 'Tag der Deutschen Einheit',
    '10-31': 'Reformationstag',
    '11-01': 'Allerheiligen',
    '12-24': 'Heiligabend',
    '12-25': '1. Weihnachtstag',
    '12-26': '2. Weihnachtstag',
    '12-31': 'Silvester',
  };
  if (fixed[value.slice(5)]) return fixed[value.slice(5)];
  const easter = easterSunday(year);
  const offset = daysBetween(easter, value);
  const moving = {
    '-2': 'Karfreitag',
    0: 'Ostersonntag',
    1: 'Ostermontag',
    39: 'Christi Himmelfahrt',
    49: 'Pfingstsonntag',
    50: 'Pfingstmontag',
    60: 'Fronleichnam',
  };
  if (moving[offset]) return moving[offset];
  // Buss- und Bettag: Mittwoch vor dem 23. November.
  const nov23 = parseDate(`${year}-11-23`);
  const back = ((nov23.getUTCDay() - 3 + 7) % 7) || 7;
  if (value === addDays(`${year}-11-23`, -back)) return 'Buß- und Bettag';
  return null;
}