from __future__ import annotations

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

from croniter import CroniterError, croniter

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

CRON_FIELD_COUNT = 5


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

    Built by the composition root from the scheduler configuration and passed
    in. Deliberately not held by any aggregate: a task whose meaning changes
    with deployment config is not a domain object.

    The defaults are the permissive ones on purpose -- "nobody configured a
    policy" must not invent a business constraint. Real values only ever
    arrive from the outer ring.
    """

    min_once_delay_seconds: int = 0
    """How far ahead a one-shot schedule must be at submission time. Read by
    `ScheduleSpec.ensure_launchable`; a cron schedule is never subject to it."""

    max_concurrent_runs: int = 1
    """Ceiling on active scheduled executions across ALL tasks. Long runs
    accumulate across polls, so each poll may only claim into what is left."""

    lease_seconds: int = 60
    """How long a claim on a task stays valid. Bounds how quickly a task
    orphaned between claim and dispatch becomes reachable again."""


@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")
            normalized = " ".join(fields)
            # Counting fields is not parsing: five fields of garbage would
            # otherwise construct fine and surface later, deep in next_after,
            # as a croniter error outside the ScheduleError family the router
            # maps -- an input error turned unclassified 500.
            try:
                croniter(normalized)
            except CroniterError as exc:
                raise InvalidScheduleError(f"Invalid cron expression: {normalized}") from exc
            object.__setattr__(self, "cron", normalized)

        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)

    @classmethod
    def from_primitives(cls, schedule_type: str, *, cron: str | None, run_at: str | None, timezone: str) -> ScheduleSpec:
        """Build from the loose strings both boundaries arrive as.

        The HTTP body and the stored JSON column both carry a schedule type
        plus a mapping, so the rule for turning that into a value object lives
        here instead of being written out once per adapter. Each adapter keeps
        only what is genuinely its own — which keys its own format uses — and
        the errors below stay one family the router maps uniformly.

        Four strings rather than a `Mapping[str, Any]` on purpose: a mapping in
        this signature would mean the domain is handling a transport or storage
        format, which is the thing this class exists to avoid.

        `cron` and `run_at` are annotated as the contract expects them but
        checked rather than trusted — both callers read from data a client can
        influence, so a non-string must be rejected here instead of reaching
        `fromisoformat` or the cron parser. The field the given type does not
        use is ignored: a boundary that carries both keys is not an error.

        Raises:
            InvalidScheduleError: unknown schedule type, the type's field
                missing or not a string, or an unparseable `run_at`. Value
                rules — 5-field cron, resolvable timezone — are left to
                __post_init__ and surface as the same error.
        """
        try:
            kind = ScheduleType(schedule_type)
        except ValueError as exc:
            raise InvalidScheduleError(f"Unsupported schedule_type: {schedule_type}") from exc

        if kind is ScheduleType.CRON:
            if not isinstance(cron, str):
                raise InvalidScheduleError("cron schedule requires schedule_spec.cron")
            return cls.cron_schedule(cron, timezone)

        if not isinstance(run_at, str):
            raise InvalidScheduleError("once schedule requires run_at")
        try:
            parsed = datetime.fromisoformat(run_at)
        except ValueError as exc:
            raise InvalidScheduleError(f"once schedule has an unparseable run_at: {run_at!r}") from exc
        return cls.once_at(parsed, timezone)

    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).

        **Both branches return UTC** (#4607). `run_at` keeps the zone it was
        declared in, so returning it unconverted handed the caller a non-UTC
        offset while CRON handed back UTC. The value is persisted into
        `scheduled_tasks.next_run_at`, and SQLAlchemy's SQLite dialect discards
        tzinfo on bind -- so a once task declared in Asia/Shanghai fired eight
        hours late, and a negative offset fired early, slipping past the
        `min_once_delay_seconds` floor on the way. Postgres timestamptz
        normalizes on write, which is why only SQLite deployments saw it.
        """
        if now.tzinfo is None:
            now = now.replace(tzinfo=UTC)

        if self.schedule_type is ScheduleType.ONCE:
            return self.run_at.astimezone(UTC) 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:
            # Unreachable with an aware input on today's croniter, and kept
            # verbatim from schedules.py:51-52 rather than dropped: it costs
            # one branch and guards a library detail we do not control.
            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