"""Moving a task between stages — the only place the stage column changes.

Two rules live here rather than in the scheduler:

1. **Optimistic moves.** ``advance`` refuses to guess: it takes the stage the
   caller believes the task is at, and the repository's compare-and-set either
   applies the move or raises :class:`~app.core.errors.ConflictError` because
   another worker got there first. A lost race is a normal outcome, not a
   crash — the caller re-reads and continues.
2. **Dependents never get stuck.** When a task fails or is cancelled, every
   task that declared it in ``depends_on`` is parked in
   ``pending_user_decision`` instead of waiting for a dependency that will
   never finish (agybot bug M1: a dead task deadlocked its children).

The coarse ``tasks.status`` is written by the caller in the same transaction
(see ``engine/scheduler.py`` in a later phase): the stage is the engine's
truth, the status column is the summary existing APIs keep reading.
"""

from __future__ import annotations

from app.core.errors import ConflictError, NotFoundError
from app.domain.workforce.pipeline import (
    SIDE_STAGES,
    TERMINAL_STAGES,
    Stage,
    TaskPipelineState,
    can_transition_stage,
)
from app.infrastructure.repositories.engine import (
    InvalidTransitionError,
    PipelineStateRepository,
    StaleStageError,
    UnknownTaskError,
)


async def advance(
    states: PipelineStateRepository,
    *,
    task_id: str,
    expected_from: Stage,
    to: Stage,
) -> TaskPipelineState:
    """Move one task from ``expected_from`` to ``to``, or fail cleanly.

    Raises ``ConflictError`` when the transition is illegal or when another
    worker moved the task first, and ``NotFoundError`` when the task has no
    engine state row at all.
    """
    if not can_transition_stage(expected_from, to):
        raise ConflictError(
            f"گذار نابجا از {expected_from.value} به {to.value}."
        )
    try:
        return await states.advance(task_id=task_id, expected_from=expected_from, to=to)
    except UnknownTaskError as exc:
        raise NotFoundError("این کار وضعیت موتور ندارد.") from exc
    except InvalidTransitionError as exc:
        raise ConflictError(str(exc)) from exc
    except StaleStageError as exc:
        raise ConflictError(
            "این کار هم‌اکنون توسط موتور دیگری در حرکت است؛ دوباره تلاش کنید."
        ) from exc


async def park_dependents(
    states: PipelineStateRepository,
    *,
    task_id: str,
) -> list[TaskPipelineState]:
    """Park every dependent of a failed or cancelled task with a human.

    A dependency that failed leaves its dependents unable to ever finish, so
    they move to ``pending_user_decision`` instead of waiting for work that
    will never happen (agybot bug M1: a dead task deadlocked its children).
    Dependents that already sit in another side state are left alone — a task
    a human already paused must not be re-parked.

    The guard is deliberate: a task that merely *exists* is not a reason to
    park anything. Only a dependency that is actually ``failed`` or
    ``cancelled`` strands its children, so the function refuses to act on a
    healthy task however often a tick calls it.
    """
    state = await states.get(task_id)
    if state is None or state.stage not in (Stage.FAILED, Stage.CANCELLED):
        return []
    parked: list[TaskPipelineState] = []
    for dependent in await states.list_dependents(task_id):
        # A dependent that already waits on a human (paused, another decision)
        # is left alone: it is already parked, just elsewhere.
        if dependent.stage in SIDE_STAGES:
            continue
        if not can_transition_stage(dependent.stage, Stage.PENDING_USER_DECISION):
            continue
        try:
            parked.append(
                await advance(
                    states,
                    task_id=dependent.task_id,
                    expected_from=dependent.stage,
                    to=Stage.PENDING_USER_DECISION,
                )
            )
        except ConflictError:
            # A worker is already moving it; the next tick will see the truth.
            continue
    return parked


async def is_terminal_state(states: PipelineStateRepository, task_id: str) -> bool:
    """True when the task's stage can no longer move (done/cancelled/failed)."""
    state = await states.get(task_id)
    return state is None or state.stage in TERMINAL_STAGES