from __future__ import annotations

from datetime import datetime, UTC

from app.storage.models import StoredTask


class InvalidTransitionError(Exception):
    """Raised when task workflow transition is invalid."""


def start_task(task: StoredTask) -> StoredTask:
    """
    Transition task from backlog → in_progress.

    Rules:
    - Idempotent: if already started, do not change started_at
    - Cannot start a task that is already done
    """
    # already done → invalid
    if task.done_at is not None:
        raise InvalidTransitionError("invalid_transaction")

    # already started → idempotent (no change)
    if task.started_at is not None:
        return task

    return StoredTask(
        id=task.id,
        title=task.title,
        created_at=task.created_at,
        started_at=datetime.now(UTC),
        done_at=task.done_at,
    )


def complete_task(task: StoredTask) -> StoredTask:
    """
    Transition task from in_progress → done.

    Rules:
    - Allowed only if task is started
    - Idempotent: if already done, do not change done_at
    """
    # not started → invalid
    if task.started_at is None:
        raise InvalidTransitionError("invalid_transaction")

    # already done → idempotent
    if task.done_at is not None:
        return task

    return StoredTask(
        id=task.id,
        title=task.title,
        created_at=task.created_at,
        started_at=task.started_at,
        done_at=datetime.now(UTC),
    )