"""
cron_schedule.py
================

Minimal cron-expression parser — no external dependency.

Supports the classic 5-field syntax::

    ┌──────── minute        (0-59)
    │ ┌────── hour          (0-23)
    │ │ ┌──── day of month  (1-31)
    │ │ │ ┌── month         (1-12 or jan-dec)
    │ │ │ │ ┌ day of week   (0-7 or sun-sat; 0 and 7 = Sunday)
    │ │ │ │ │
    0 19 * * 1,4    ->  19:00 every Monday and Thursday

Field syntax: ``*``, single values, ranges (``a-b``), steps (``*/n``,
``a-b/n``) and comma lists.  Month / weekday names (``jan``, ``mon``, …)
are accepted case-insensitively.

As in Vixie cron, when *both* day-of-month and day-of-week are restricted
the job runs when **either** matches.

Times are evaluated against the local system clock (``datetime.now()``) —
in Docker set the ``TZ`` environment variable so "19:00" means local time.
"""

from __future__ import annotations

from datetime import datetime, timedelta


_MONTH_NAMES = {"jan": 1, "feb": 2, "mar": 3, "apr": 4, "may": 5, "jun": 6,
                "jul": 7, "aug": 8, "sep": 9, "oct": 10, "nov": 11, "dec": 12}
_DAY_NAMES   = {"sun": 0, "mon": 1, "tue": 2, "wed": 3, "thu": 4,
                "fri": 5, "sat": 6}


def _parse_value(token: str, lo: int, hi: int,
                 names: "dict[str, int] | None") -> int:
    token = token.strip().lower()
    if names and token in names:
        return names[token]
    try:
        value = int(token)
    except ValueError:
        raise ValueError(f"invalid cron value {token!r}") from None
    if not (lo <= value <= hi):
        raise ValueError(f"cron value {value} out of range {lo}-{hi}")
    return value


def _parse_field(field: str, lo: int, hi: int,
                 names: "dict[str, int] | None" = None) -> "set[int]":
    """Parses one cron field into the set of matching integer values."""
    result: set[int] = set()
    for part in field.split(","):
        part = part.strip()
        if not part:
            raise ValueError(f"empty element in cron field {field!r}")

        step = 1
        if "/" in part:
            part, step_text = part.split("/", 1)
            try:
                step = int(step_text)
            except ValueError:
                raise ValueError(f"invalid cron step {step_text!r}") from None
            if step < 1:
                raise ValueError(f"cron step must be >= 1, got {step}")

        if part == "*":
            start, end = lo, hi
        elif "-" in part:
            a, b = part.split("-", 1)
            start = _parse_value(a, lo, hi, names)
            end   = _parse_value(b, lo, hi, names)
            if end < start:
                raise ValueError(f"inverted cron range {part!r}")
        else:
            start = end = _parse_value(part, lo, hi, names)

        result.update(range(start, end + 1, step))
    return result


class CronSchedule:
    """
    Parsed 5-field cron expression with ``next_after()`` evaluation.

    Usage::

        cron = CronSchedule("0 19 * * mon,thu")
        run_at = cron.next_after(datetime.now())
    """

    def __init__(self, expression: str):
        self.expression = expression.strip()
        fields = self.expression.split()
        if len(fields) != 5:
            raise ValueError(
                f"cron expression needs 5 fields "
                f"(minute hour dom month dow), got {len(fields)}: "
                f"{expression!r}")

        minute, hour, dom, month, dow = fields
        self._minutes = _parse_field(minute, 0, 59)
        self._hours   = _parse_field(hour,   0, 23)
        self._dom     = _parse_field(dom,    1, 31)
        self._months  = _parse_field(month,  1, 12, _MONTH_NAMES)
        dow_values    = _parse_field(dow,    0, 7,  _DAY_NAMES)
        # 7 is an alias for Sunday (= 0)
        self._dow = {0 if v == 7 else v for v in dow_values}

        # Vixie-cron rule: dom/dow are OR-combined when both are restricted.
        self._dom_restricted = dom != "*"
        self._dow_restricted = dow != "*"

    def __repr__(self) -> str:
        return f"CronSchedule({self.expression!r})"

    # ------------------------------------------------------------------
    def _day_matches(self, day: "datetime.date") -> bool:
        if day.month not in self._months:
            return False
        dom_ok = day.day in self._dom
        # Python: Monday=0 … Sunday=6  ->  cron: Sunday=0 … Saturday=6
        dow_ok = ((day.weekday() + 1) % 7) in self._dow
        if self._dom_restricted and self._dow_restricted:
            return dom_ok or dow_ok
        if self._dom_restricted:
            return dom_ok
        if self._dow_restricted:
            return dow_ok
        return True

    def next_after(self, dt: datetime) -> datetime:
        """
        Returns the first matching time strictly after ``dt``
        (second/microsecond precision is dropped).
        """
        cand = (dt + timedelta(minutes=1)).replace(second=0, microsecond=0)
        hours   = sorted(self._hours)
        minutes = sorted(self._minutes)

        # Walk day by day (covers rare dom/month combos like Feb 29).
        for _ in range(366 * 5):
            if self._day_matches(cand.date()):
                for h in hours:
                    if h < cand.hour:
                        continue
                    for m in minutes:
                        if h == cand.hour and m < cand.minute:
                            continue
                        return cand.replace(hour=h, minute=m)
            cand = (cand + timedelta(days=1)).replace(hour=0, minute=0)

        raise ValueError(
            f"cron {self.expression!r}: no occurrence within 5 years")