class ReferenceProjector:
    """Tiny transactional projection used to prove cursor/idempotency semantics."""

    def __init__(self) -> None:
        self.cursor = 0
        self.rows = {}

    async def load_cursor(self, application_ref, root_trace_id):
        del application_ref, root_trace_id
        return self.cursor

    async def project(self, event, expected_cursor):
        if expected_cursor != self.cursor:
            raise ValueError("stale projection cursor")
        self.rows.setdefault(event.event_key, event.payload)
        self.cursor = event.event_id