import { toISO, getWeekday } from './utils.js';

function isOnVacation(employee, isoDate) {
  return employee.constraints.vacations.some(v => v.from <= isoDate && isoDate <= v.to);
}

function daysBetween(isoA, isoB) {
  return Math.abs(
    (new Date(isoA + 'T12:00:00Z') - new Date(isoB + 'T12:00:00Z')) / 86400000
  );
}

// Returns sorted array of ISO date strings that are workdays in the given month
export function getWorkdays(year, month, holidays, companyClosures) {
  const days = [];
  const daysInMonth = new Date(year, month, 0).getDate();
  for (let d = 1; d <= daysInMonth; d++) {
    const iso = toISO(year, month, d);
    const wd = getWeekday(iso);
    if (wd >= 6) continue; // weekend
    if (holidays.includes(iso) || companyClosures.includes(iso)) continue;
    days.push(iso);
  }
  return days;
}

function isEligible(employee, isoDate, monthAssignments, allHistory) {
  const c = employee.constraints;
  const wd = getWeekday(isoDate);

  if (c.neverDays.includes(wd)) return false;
  if (c.onlyDays.length > 0 && !c.onlyDays.includes(wd)) return false;
  if (isOnVacation(employee, isoDate)) return false;

  if (c.maxPerMonth !== null) {
    const count = monthAssignments.filter(a => a.employeeId === employee.id).length;
    if (count >= c.maxPerMonth) return false;
  }

  if (c.minGapDays != null && c.minGapDays > 0) {
    // Latest prior assignment date for this employee — single pass, no sort/alloc
    let lastDate = null;
    const isLater = a => a.employeeId === employee.id && a.date < isoDate &&
      (lastDate === null || a.date > lastDate);
    for (const a of allHistory) if (isLater(a)) lastDate = a.date;
    for (const a of monthAssignments) if (isLater(a)) lastDate = a.date;
    if (lastDate && daysBetween(lastDate, isoDate) < c.minGapDays) return false;
  }

  return true;
}

// randomFn injectable so tests can pass () => 0.5 for deterministic output.
// histCount is a precomputed Map<employeeId, number> of past assignments.
function scoreCandidate(employee, isoDate, histCount, randomFn) {
  const wd = getWeekday(isoDate);
  const myTotal = histCount.get(employee.id) || 0;

  // Fewer past assignments → higher (less negative) score
  let score = -myTotal;

  // Low priority: increase negative score (less likely to be picked)
  if (employee.constraints.lowPriorityDays.includes(wd)) score -= 5;

  // Jitter ±0.49
  score += (randomFn() - 0.5) * 0.98;

  return score;
}

// Returns { assignments: [{ date, employeeId, manual: false }], skippedDates: [] }
// slotsPerDay controls how many top-scored candidates are picked per day (default 1).
export function generatePlan(workdays, employees, history, randomFn = Math.random, slotsPerDay = 1, preAssigned = []) {
  // preAssigned entries (e.g. manual picks for this month) are kept fixed: they
  // occupy slots, count toward eligibility, and are never double-booked. Only
  // the remaining slots per day are auto-filled. Returns only the newly
  // generated assignments (not the preAssigned ones).
  const monthAssignments = [...preAssigned];
  const generated = [];
  const skippedDates = [];

  // Precompute each employee's history count once (used for fairness scoring)
  const histCount = new Map();
  for (const h of history) histCount.set(h.employeeId, (histCount.get(h.employeeId) || 0) + 1);

  for (const date of workdays) {
    const taken = monthAssignments.filter(a => a.date === date);
    const needed = slotsPerDay - taken.length;
    if (needed <= 0) continue; // already fully staffed (e.g. by manual entries)

    const takenIds = new Set(taken.map(a => a.employeeId));
    const candidates = employees.filter(e =>
      !takenIds.has(e.id) && isEligible(e, date, monthAssignments, history)
    );

    if (candidates.length < needed) {
      skippedDates.push(date);
      continue;
    }

    const scored = candidates
      .map(e => ({ employee: e, score: scoreCandidate(e, date, histCount, randomFn) }))
      .sort((a, b) => b.score - a.score);

    for (let i = 0; i < needed; i++) {
      const entry = { date, employeeId: scored[i].employee.id, manual: false };
      monthAssignments.push(entry);
      generated.push(entry);
    }
  }

  return { assignments: generated, skippedDates };
}