"""Mode-conflict guard — assist vs practice mutual exclusivity (REQ-IDEATE-03, TASK-01-05).

D-061 states assist is a separate mode (not concurrent with practice). This
module enforces mutual exclusivity on the server side: starting an assist shift
while a practice session is active (or vice versa) raises ModeConflictError.

The existing /pipecat/webrtc endpoint (practice) calls enforce_mutual_exclusivity(
..., 'practice'); the new /api/assist/shift/start endpoint calls it with 'assist'.
"""

from __future__ import annotations

from db.store import PraxisStore


class ModeConflictError(Exception):
    """Raised when a learner tries to start a session of one type while an
    active session of the other type exists (REQ-IDEATE-03)."""


async def enforce_mutual_exclusivity(
    store: PraxisStore, learner_id: str, requested_type: str
) -> None:
    """Raise ModeConflictError if the learner has an active session of the
    *other* type.

    requested_type: 'assist' or 'practice'. Ended sessions don't trigger the
    conflict (only active sessions count — ended_at IS NULL).
    """
    other_type = "practice" if requested_type == "assist" else "assist"
    active = await store.get_active_session(learner_id, other_type)
    if active is not None:
        if requested_type == "assist":
            raise ModeConflictError(
                "Cannot start assist shift: a practice session is active. "
                "End the practice session first."
            )
        raise ModeConflictError(
            "Cannot start practice session: an assist shift is active. "
            "End the shift first."
        )


__all__ = ["enforce_mutual_exclusivity", "ModeConflictError"]