// OPT / STEM OPT unemployment day counter. MIT License. No dependencies.
// Same rules as opt_counter.py. Dates are "YYYY-MM-DD" strings, turned into
// whole UTC day numbers from their Y-M-D integers, so DST can never shift a day.

const DISCLAIMER =
  "Counts dates only. It does not decide whether a job qualifies or whether you are in status.";

function dayNum(iso) {
  const m = /^(\d{4})-(\d{2})-(\d{2})$/.exec(iso);
  if (!m) throw new Error(`Expected YYYY-MM-DD, got ${iso}`);
  const [y, mo, d] = [Number(m[1]), Number(m[2]), Number(m[3])];
  const ms = Date.UTC(y, mo - 1, d);
  const check = new Date(ms);
  if (check.getUTCMonth() !== mo - 1 || check.getUTCDate() !== d) throw new Error(`Not a real date: ${iso}`);
  return ms / 86400000;
}

function isoOf(n) {
  return new Date(n * 86400000).toISOString().slice(0, 10);
}

// jobs: [{ start: "YYYY-MM-DD", end: "YYYY-MM-DD" | null, qualifying: true | false }]
// end = null means the job is ongoing.
function countUnemployment(eadStart, authEnd, asOf, jobs, stemStart = null) {
  const ead = dayNum(eadStart);
  const countTo = Math.min(dayNum(asOf), dayNum(authEnd));
  const stem = stemStart ? dayNum(stemStart) : null;

  const covered = new Set(); // a Set, so overlapping jobs never double count
  for (const job of jobs) {
    if (!job.qualifying) continue;
    const end = job.end == null ? countTo : Math.min(dayNum(job.end), countTo);
    for (let d = Math.max(dayNum(job.start), ead); d <= end; d++) covered.add(d);
  }

  const unemployed = [];
  let firstOver = null;
  for (let d = ead; d <= countTo; d++) {
    if (covered.has(d)) continue;
    unemployed.push(d);
    const limitThatDay = stem !== null && d >= stem ? 150 : 90;
    if (firstOver === null && unemployed.length > limitThatDay) firstOver = d;
  }

  const limit = stem !== null && countTo >= stem ? 150 : 90;
  return {
    days_used: unemployed.length,
    limit,
    days_remaining: limit - unemployed.length,
    first_day_over_limit: firstOver === null ? null : isoOf(firstOver),
    unemployed_dates: unemployed.map(isoOf),
  };
}

module.exports = { countUnemployment, DISCLAIMER };