// Reine Planungslogik ohne Abhängigkeiten. Datumsangaben sind immer ISO-Strings (YYYY-MM-DD).

export function addDays(iso, n) {
  const d = new Date(iso + 'T00:00:00Z');
  d.setUTCDate(d.getUTCDate() + n);
  return d.toISOString().slice(0, 10);
}

export function isWeekend(iso) {
  const day = new Date(iso + 'T00:00:00Z').getUTCDay();
  return day === 0 || day === 6;
}

/** ISO-Wochentag 1 (Mo) … 7 (So). */
export function isoWeekday(iso) {
  const day = new Date(iso + 'T00:00:00Z').getUTCDay();
  return day === 0 ? 7 : day;
}

export function isValidDate(iso) {
  return typeof iso === 'string' && /^\d{4}-\d{2}-\d{2}$/.test(iso) && !Number.isNaN(Date.parse(iso + 'T00:00:00Z'))
    && new Date(iso + 'T00:00:00Z').toISOString().slice(0, 10) === iso;
}

/** Alle Werktage (Mo–Fr) im Bereich, ohne die blockierten Tage (Feiertage, betriebsfrei). */
export function workingDays(from, to, blocked = new Set()) {
  const out = [];
  for (let d = from; d <= to; d = addDays(d, 1)) {
    if (!isWeekend(d) && !blocked.has(d)) out.push(d);
  }
  return out;
}

export function isAbsent(personId, date, absences) {
  return absences.some((a) => a.personId === personId && a.from <= date && a.to >= date);
}

/** Verfügbar = aktiv, nicht abwesend und kein fester freier Wochentag (Teilzeit). */
export function isAvailable(person, date, absences) {
  if (person.active === false) return false;
  if (Array.isArray(person.freeDays) && person.freeDays.includes(isoWeekday(date))) return false;
  return !isAbsent(person.id, date, absences);
}

/**
 * Erzeugt Zuteilungen für die angegebenen Tage.
 *
 * - persons:   [{ id, canMain, canCo, active, freeDays }]
 * - absences:  [{ personId, from, to }]
 * - existing:  { date: { main, co, locked, off, keep, fill } } – locked/off/keep bleiben unverändert,
 *              fill: vorhandene Slots behalten, nur leere auslosen
 * - blocked:   Set blockierter Tage (Feiertage), damit „nicht zwei Werktage in Folge" darüber hinweg gilt
 * - history:   { date: { main, co, off } } – frühere Zuteilungen für die Fairness-Zählung
 * - rng:       Zufallsfunktion [0,1), austauschbar für Tests
 *
 * Fairness: Wer im Betrachtungsfenster am seltensten dran war, wird bevorzugt; unter Gleichen
 * entscheidet die Rollenverteilung, danach der Zufall. Niemand moderiert an zwei Werktagen
 * hintereinander, wenn es sich vermeiden lässt, und dasselbe Paar wiederholt sich ungern.
 */
export function generateSchedule({ dates, persons, absences = [], existing = {}, history = {}, blocked = new Set(), rng = Math.random, historyWindowDays = 60 }) {
  const result = {};
  const counts = new Map();
  const stat = (id) => {
    if (!counts.has(id)) counts.set(id, { total: 0, main: 0, co: 0, lastPartner: null });
    return counts.get(id);
  };
  const bump = (id, role, partner) => {
    const s = stat(id);
    s.total += 1;
    s[role] += 1;
    s.lastPartner = partner || null;
  };

  const first = dates[0];
  if (first) {
    const windowStart = addDays(first, -historyWindowDays);
    const past = Object.entries(history).filter(([date, a]) => date < first && date >= windowStart && a && !a.off).sort();
    for (const [, a] of past) {
      if (a.main) bump(a.main, 'main', a.co);
      if (a.co) bump(a.co, 'co', a.main);
    }
  }

  const assignedOn = (date) => {
    const a = result[date] || history[date];
    return a && !a.off ? [a.main, a.co].filter(Boolean) : [];
  };
  const previousWorkingDay = (date) => {
    let d = addDays(date, -1);
    while (isWeekend(d) || blocked.has(d)) d = addDays(d, -1);
    return d;
  };

  const active = persons.filter((p) => p.active !== false);

  for (const date of dates) {
    const ex = existing[date];
    if (ex && (ex.locked || ex.off || ex.keep)) {
      const { keep, fill, ...rest } = ex;
      result[date] = { ...rest };
      if (!ex.off) {
        if (ex.main) bump(ex.main, 'main', ex.co);
        if (ex.co) bump(ex.co, 'co', ex.main);
      }
      continue;
    }
    const fixedMain = ex && ex.fill ? ex.main || null : null;
    const fixedCo = ex && ex.fill ? ex.co || null : null;
    const yesterday = assignedOn(previousWorkingDay(date));
    const available = active.filter((p) => isAvailable(p, date, absences));
    const main = fixedMain || pick(available.filter((p) => p.canMain && p.id !== fixedCo), 'main', yesterday, null, stat, rng);
    const co = fixedCo || pick(available.filter((p) => p.canCo && p.id !== main), 'co', yesterday, main, stat, rng);
    result[date] = { main, co, locked: false };
    if (main) bump(main, 'main', co);
    if (co) bump(co, 'co', main);
  }
  return result;
}

function pick(candidates, role, yesterday, partner, stat, rng) {
  if (candidates.length === 0) return null;
  let pool = candidates.filter((p) => !yesterday.includes(p.id));
  if (pool.length === 0) pool = candidates;

  const minTotal = Math.min(...pool.map((p) => stat(p.id).total));
  pool = pool.filter((p) => stat(p.id).total === minTotal);

  const minRole = Math.min(...pool.map((p) => stat(p.id)[role]));
  pool = pool.filter((p) => stat(p.id)[role] === minRole);

  if (partner) {
    const fresh = pool.filter((p) => stat(p.id).lastPartner !== partner);
    if (fresh.length > 0) pool = fresh;
  }
  return pool[Math.floor(rng() * pool.length)].id;
}

/** Zählt Einsätze je Person im Bereich – für die Statistik in der Oberfläche. */
export function countAssignments(assignments, from, to) {
  const out = {};
  for (const [date, a] of Object.entries(assignments)) {
    if (date < from || date > to || !a || a.off) continue;
    for (const [role, id] of [['main', a.main], ['co', a.co]]) {
      if (!id) continue;
      out[id] = out[id] || { main: 0, co: 0, total: 0 };
      out[id][role] += 1;
      out[id].total += 1;
    }
  }
  return out;
}