"""Server-side workflow state machine. Every stage change flows through
`transition()`, which validates against ALLOWED_TRANSITIONS and records both
a StageTransition row (timestamps + acting user, for aging/cycle-time
reporting) and an audit entry."""
from app.domain import ALLOWED_TRANSITIONS, Stage
from app.models import Ncr, StageTransition
from app.models.base import utcnow
from app.services.audit import audit_event

from sqlalchemy.ext.asyncio import AsyncSession


class InvalidTransitionError(Exception):
    def __init__(self, from_stage: str, to_stage: str):
        self.from_stage = from_stage
        self.to_stage = to_stage
        super().__init__(f"Invalid stage transition: {from_stage} -> {to_stage}")


def transition(
    db: AsyncSession,
    ncr: Ncr,
    to_stage: Stage,
    *,
    action: str,
    actor_id: int,
    note: str | None = None,
) -> None:
    from_stage = Stage(ncr.stage)
    if to_stage not in ALLOWED_TRANSITIONS.get(from_stage, set()):
        raise InvalidTransitionError(from_stage.value, to_stage.value)

    now = utcnow()
    ncr.stage = to_stage.value
    ncr.stage_entered_at = now
    db.add(
        StageTransition(
            ncr_id=ncr.id,
            from_stage=from_stage.value,
            to_stage=to_stage.value,
            action=action,
            acted_by_id=actor_id,
            acted_at=now,
            note=note,
        )
    )
    audit_event(
        db,
        ncr_id=ncr.id,
        user_id=actor_id,
        action=action,
        field_name="stage",
        old_value=from_stage.value,
        new_value=to_stage.value,
        detail=note,
    )


def record_creation(db: AsyncSession, ncr: Ncr, actor_id: int) -> None:
    db.add(
        StageTransition(
            ncr_id=ncr.id,
            from_stage=None,
            to_stage=Stage.NEW_REQUEST.value,
            action="create",
            acted_by_id=actor_id,
            acted_at=ncr.created_at,
        )
    )
    audit_event(
        db,
        ncr_id=ncr.id,
        user_id=actor_id,
        action="create",
        detail=f"NCR {ncr.ncr_number} created",
    )