from __future__ import annotations

from dataclasses import dataclass
from datetime import UTC, datetime
from zoneinfo import ZoneInfo, ZoneInfoNotFoundError

from croniter import croniter

from deerflow.domain.schedule.model.enums import ScheduleType
from deerflow.domain.schedule.model.errors import InvalidScheduleError

CRON_FIELD_COUNT = 5


@dataclass(frozen=True)
class SchedulePolicy:
    """Operator-tunable thresholds the domain needs but must not read itself."""

    min_once_delay_seconds: int = 0


@dataclass(frozen=True)
class ScheduleSpec:
    """Parsed, validated view of (schedule_type, schedule_spec, timezone).

    The stored JSON spec is mapped in and out by the adapter layer, never here:
    a `Mapping[str, Any]` in a domain signature would mean the domain is
    handling a persistence/transport format. The two halves of that parsing
    split cleanly — structural checks (is the key present? is it a str?) belong
    to the boundary, value rules (5-field cron, resolvable timezone, run_at
    present) belong to __post_init__ below. Storage keeps the same raw JSON, so
    this needs no migration.

    Normalization happens in __post_init__ rather than in the factories below,
    so direct construction cannot bypass it: a frozen dataclass is still
    constructible field-by-field, and "valid on construction" has to hold for
    that path too.
    """

    schedule_type: ScheduleType
    timezone: str
    cron: str | None = None
    run_at: datetime | None = None

    def __post_init__(self) -> None:
        # The timezone is checked first because normalizing a naive run_at
        # below needs it to already be known-good.
        try:
            zone = ZoneInfo(self.timezone)
        except ZoneInfoNotFoundError as exc:
            raise InvalidScheduleError(f"Unknown timezone: {self.timezone}") from exc

        if self.schedule_type is ScheduleType.CRON:
            if not self.cron:
                raise InvalidScheduleError("cron schedule requires schedule_spec.cron")
            fields = [part for part in self.cron.split() if part]
            if len(fields) != CRON_FIELD_COUNT:
                raise InvalidScheduleError(f"Cron expression must contain exactly {CRON_FIELD_COUNT} fields")
            object.__setattr__(self, "cron", " ".join(fields))

        if self.schedule_type is ScheduleType.ONCE:
            if self.run_at is None:
                raise InvalidScheduleError("once schedule requires run_at")
            if self.run_at.tzinfo is None:
                # A naive run_at means wall-clock time in this schedule's own
                # timezone (schedules.py:40-43). Localizing here rather than at
                # every read site keeps the rest of this class tz-aware only.
                object.__setattr__(self, "run_at", self.run_at.replace(tzinfo=zone))

    @classmethod
    def cron_schedule(cls, expr: str, timezone: str) -> ScheduleSpec:
        """Readability sugar — all validation lives in __post_init__."""
        return cls(ScheduleType.CRON, timezone, cron=expr)

    @classmethod
    def once_at(cls, run_at: datetime, timezone: str) -> ScheduleSpec:
        """Readability sugar — all validation lives in __post_init__."""
        return cls(ScheduleType.ONCE, timezone, run_at=run_at)

    def next_after(self, now: datetime) -> datetime | None:
        """Next fire time in UTC, or None when there is no future occurrence.

        The dispatch-path calculation (was `next_run_at` in schedules.py:24-55).
        It applies no submission-time policy — see ensure_launchable for that,
        and do not swap the two: re-arming a cron task through the stricter one
        would reject it right after a perfectly normal launch.

        ONCE returns run_at while it is still ahead of `now`, else None (the
        single occurrence is in the past). CRON is evaluated in this schedule's
        timezone and returned as UTC. A naive `now` is read as UTC
        (schedules.py:32-33).
        """
        if now.tzinfo is None:
            now = now.replace(tzinfo=UTC)

        if self.schedule_type is ScheduleType.ONCE:
            return self.run_at if self.run_at > now else None

        zone = ZoneInfo(self.timezone)
        next_local = croniter(self.cron, now.astimezone(zone)).get_next(datetime)
        if next_local.tzinfo is None:
            next_local = next_local.replace(tzinfo=zone)
        return next_local.astimezone(UTC)

    def ensure_launchable(self, now: datetime, policy: SchedulePolicy) -> datetime | None:
        """Next fire time, with the constraints that only apply at submission.

        Used by create/update; the dispatch path must use next_after instead.

        Raises:
            InvalidScheduleError: a ONCE schedule with no future occurrence, or
                one closer than policy.min_once_delay_seconds. CRON is never
                subject to the delay floor (router:105, router:196).
        """
        if now.tzinfo is None:
            now = now.replace(tzinfo=UTC)

        next_at = self.next_after(now)
        if self.schedule_type is not ScheduleType.ONCE:
            return next_at
        if next_at is None:
            raise InvalidScheduleError("once schedule must be in the future")
        if (next_at - now).total_seconds() < policy.min_once_delay_seconds:
            raise InvalidScheduleError(f"once schedule must be at least {policy.min_once_delay_seconds} seconds in the future")
        return next_at