"""F-1 to H-1B transition date estimates: cap-gap end, cap-gap eligibility,
and the H-1B up-to-60-day grace period end.

Estimates only. USCIS decides; the grace period is discretionary.
Sources: 8 CFR 214.1(l)(2), 8 CFR 214.2(f)(5)(vi), USCIS cap-gap page.
"""
import argparse
from datetime import date, timedelta

LABEL = ("Estimate from the dates you entered. Not legal advice. "
         "USCIS decides; the grace period is discretionary.")
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.")


def grace_period_end(last_day_of_employment, validity_end):
    """Last day of the up-to-60-day grace period.

    Day 1 is the day after last_day_of_employment, so day 60 is
    last_day_of_employment + 60 days. Capped at the I-797/I-94 validity end.
    """
    if validity_end < last_day_of_employment:
        raise ValueError("validity_end is before last_day_of_employment; check your dates")
    return min(last_day_of_employment + timedelta(days=60), validity_end)


def grace_period_report(last_day_of_employment, validity_end, last_day_paid=None):
    """grace_period_end plus the label. If last_day_paid differs from the last
    day worked, both dates are shown side by side with the FLAG. Neither is picked."""
    report = {
        "by_last_day_worked": grace_period_end(last_day_of_employment, validity_end),
        "label": LABEL,
    }
    if last_day_paid and last_day_paid != last_day_of_employment:
        # min(): pay running past the validity end still stops at the validity end.
        report["by_last_day_paid"] = grace_period_end(min(last_day_paid, validity_end), validity_end)
        report["flag"] = FLAG
    return report


def cap_gap_end(fiscal_year, approved_validity_start=None):
    """Outer cap-gap date: April 1 of the fiscal year, or the approved
    petition's validity start date if that is earlier.
    FY N runs Oct 1 (N-1) to Sep 30 N, so FY2027 -> 2027-04-01."""
    april_1 = date(fiscal_year, 4, 1)
    return min(april_1, approved_validity_start) if approved_validity_start else april_1


def cap_gap_eligibility(filed_on, opt_ead_end, program_end, change_of_status, cap_subject):
    """Rough cap-gap check from dates.

    opt_ead_end: the OPT EAD end date, or the STEM EAD end date if on STEM OPT.
    Pass None if you never had OPT. program_end: program end date on the I-20.
    Does not check that the petition was nonfrivolous, based on a selected
    registration, filed in the filing period, or that you maintained status.
    """
    def result(status, work, reason):
        return {"status_extended": status, "work_extended": work, "reason": reason, "label": LABEL}

    if not change_of_status:
        return result(False, False, "consular processing is not cap-gap eligible")
    if not cap_subject:
        return result(False, False, "cap-exempt petitions are not cap-gap eligible")

    f1_grace_end = max(d for d in (opt_ead_end, program_end) if d) + timedelta(days=60)
    if filed_on > f1_grace_end:
        return result(False, False,
                      f"filed after the F-1 60-day grace period ended on {f1_grace_end}")
    if opt_ead_end and filed_on <= opt_ead_end:
        return result(True, True,
                      f"filed on or before the EAD end date {opt_ead_end}: "
                      "F-1 status and work authorization may be extended")
    return result(True, False,
                  f"filed while F-1 status was in effect (grace period runs to {f1_grace_end}) "
                  "but not during a valid EAD: status may be extended, no work authorization")


def iso_or_none(text):
    """CLI helper: 'none' (any case) -> None, otherwise a YYYY-MM-DD date."""
    return None if text.strip().lower() == "none" else date.fromisoformat(text)


def _main():
    iso = date.fromisoformat
    p = argparse.ArgumentParser(description=LABEL)
    sub = p.add_subparsers(dest="cmd", required=True)

    g = sub.add_parser("grace", help="H-1B grace period end")
    g.add_argument("last_day_worked", type=iso)
    g.add_argument("validity_end", type=iso, help="I-797 / I-94 end date")
    g.add_argument("--paid", type=iso, help="last day paid, if different")

    c = sub.add_parser("capgap-end", help="cap-gap outer date")
    c.add_argument("fiscal_year", type=int)
    c.add_argument("validity_start", type=iso, nargs="?", help="approved petition start date")

    e = sub.add_parser("eligibility", help="rough cap-gap eligibility check")
    e.add_argument("filed_on", type=iso)
    e.add_argument("opt_ead_end", type=iso_or_none,
                   help="OPT or STEM EAD end date, or 'none' if you never had OPT")
    e.add_argument("program_end", type=iso)
    e.add_argument("--consular", action="store_true", help="petition requests consular processing")
    e.add_argument("--cap-exempt", action="store_true", help="employer is cap-exempt")

    a = p.parse_args()
    if a.cmd == "grace":
        for k, v in grace_period_report(a.last_day_worked, a.validity_end, a.paid).items():
            print(f"{k}: {v}")
    elif a.cmd == "capgap-end":
        print(f"cap_gap_end: {cap_gap_end(a.fiscal_year, a.validity_start)}")
        print(f"label: {LABEL}")
    else:
        r = cap_gap_eligibility(a.filed_on, a.opt_ead_end, a.program_end,
                                change_of_status=not a.consular, cap_subject=not a.cap_exempt)
        for k, v in r.items():
            print(f"{k}: {v}")


if __name__ == "__main__":
    _main()