from __future__ import annotations

from django.db import transaction

from control_plane.events.bus import EventBus
from control_plane.events.models import EventType
from control_plane.projects.models import Task, TaskStatus


class TaskScheduler:
    def __init__(self, bus: EventBus | None = None) -> None:
        self.bus = bus or EventBus()

    def claim_next_ready_task(self) -> Task | None:
        with transaction.atomic():
            candidates = (
                Task.objects.select_for_update(skip_locked=True)
                .filter(status=TaskStatus.READY)
                .order_by("-priority", "created_at")
            )
            for task in candidates:
                blocking_dependencies = task.dependency_edges.exclude(depends_on__status=TaskStatus.COMPLETE)
                if blocking_dependencies.exists():
                    continue
                task.status = TaskStatus.RUNNING
                task.save(update_fields=["status", "updated_at"])
                self.bus.publish(EventType.TASK_STARTED, project=task.project, task=task, payload={"task_id": str(task.id)})
                return task
        return None