// F-1 to H-1B transition date estimates. Same rules as transition_dates.py.
// Dates are "YYYY-MM-DD" strings, handled in UTC so time zones can't shift a day.
// Estimates only. USCIS decides; the grace period is discretionary.

const LABEL = "Estimate from the dates you entered. Not legal advice. " +
  "USCIS decides; the grace period is discretionary.";
const FLAG = "FLAG: your last day paid differs from your last day worked. " +
  "USCIS's archived page says the period starts the day after " +
  "termination of employment, typically determined by the last day " +
  "for which a salary or wage is paid. Ask an immigration attorney " +
  "which date applies to you.";

function parse(s) {
  if (typeof s !== "string" || !/^\d{4}-\d{2}-\d{2}$/.test(s)) {
    throw new Error(`Expected a YYYY-MM-DD date, got ${s}`);
  }
  const d = new Date(s + "T00:00:00Z");
  if (isNaN(d) || d.toISOString().slice(0, 10) !== s) throw new Error(`Invalid date ${s}`);
  return d;
}

function addDays(s, n) {
  const d = parse(s);
  d.setUTCDate(d.getUTCDate() + n);
  return d.toISOString().slice(0, 10);
}

// YYYY-MM-DD strings sort the same as the dates they name.
const minDate = (a, b) => (parse(a), parse(b), a < b ? a : b);
const maxDate = (a, b) => (parse(a), parse(b), a > b ? a : b);

// Day 1 = the day after lastDayOfEmployment; capped at the I-797/I-94 validity end.
function gracePeriodEnd(lastDayOfEmployment, validityEnd) {
  if (validityEnd < lastDayOfEmployment) {
    throw new Error("validityEnd is before lastDayOfEmployment; check your dates");
  }
  return minDate(addDays(lastDayOfEmployment, 60), validityEnd);
}

function gracePeriodReport(lastDayOfEmployment, validityEnd, lastDayPaid = null) {
  const report = { byLastDayWorked: gracePeriodEnd(lastDayOfEmployment, validityEnd), label: LABEL };
  if (lastDayPaid && lastDayPaid !== lastDayOfEmployment) {
    report.byLastDayPaid = gracePeriodEnd(minDate(lastDayPaid, validityEnd), validityEnd);
    report.flag = FLAG;
  }
  return report;
}

// FY N runs Oct 1 (N-1) to Sep 30 N, so FY2027 -> "2027-04-01".
function capGapEnd(fiscalYear, approvedValidityStart = null) {
  if (!Number.isInteger(fiscalYear)) throw new Error("fiscalYear must be an integer");
  const april1 = `${fiscalYear}-04-01`;
  return approvedValidityStart ? minDate(april1, approvedValidityStart) : april1;
}

// optEadEnd: OPT EAD end, or STEM EAD end if on STEM OPT; null if you never had OPT.
function capGapEligibility({ filedOn, optEadEnd = null, programEnd, changeOfStatus, capSubject }) {
  parse(filedOn);
  const result = (statusExtended, workExtended, reason) =>
    ({ statusExtended, workExtended, reason, label: LABEL });

  if (!changeOfStatus) return result(false, false, "consular processing is not cap-gap eligible");
  if (!capSubject) return result(false, false, "cap-exempt petitions are not cap-gap eligible");

  const f1GraceEnd = addDays(optEadEnd ? maxDate(optEadEnd, programEnd) : programEnd, 60);
  if (filedOn > f1GraceEnd) {
    return result(false, false, `filed after the F-1 60-day grace period ended on ${f1GraceEnd}`);
  }
  if (optEadEnd && filedOn <= optEadEnd) {
    return result(true, true, `filed on or before the EAD end date ${optEadEnd}: ` +
      "F-1 status and work authorization may be extended");
  }
  return result(true, false, `filed while F-1 status was in effect (grace period runs to ${f1GraceEnd}) ` +
    "but not during a valid EAD: status may be extended, no work authorization");
}

module.exports = { LABEL, FLAG, gracePeriodEnd, gracePeriodReport, capGapEnd, capGapEligibility };