"""转正套件：入职日 + 试用期月数 → 建议转正日（YYYY-MM-DD）。"""
from __future__ import annotations

import calendar
import re
from datetime import date
from typing import Optional

_YMD = re.compile(r"^(\d{4})-(\d{2})-(\d{2})$")


def add_months_ymd(hire_ymd: str, months: int) -> Optional[str]:
    s = (hire_ymd or "").strip()
    m = _YMD.match(s)
    if not m:
        return None
    if not isinstance(months, int) or isinstance(months, bool) or months < 0:
        return None
    y, mo, d = int(m.group(1)), int(m.group(2)), int(m.group(3))
    try:
        date(y, mo, d)
    except ValueError:
        return None
    total = y * 12 + (mo - 1) + months
    ny, nm = divmod(total, 12)
    nm += 1
    last = calendar.monthrange(ny, nm)[1]
    nd = min(d, last)
    return f"{ny:04d}-{nm:02d}-{nd:02d}"