package runtime

// transitionKey stores one `(from, to)` pair in the allowed-transitions
// table.
type transitionKey struct {
	from Status
	to   Status
}

// allowedTransitions stores the set of permitted `(from, to)` status
// pairs. The four pairs mirror the lifecycle flows frozen in
// `galaxy/rtmanager/README.md §Lifecycles`:
//
//   - running → stopped: graceful stop, observed Docker exit, or
//     reconcile observing an exited container.
//   - running → removed: reconcile_dispose when Docker no longer reports
//     the container at all.
//   - stopped → running: restart and patch inner start steps.
//   - stopped → removed: cleanup_container, both the periodic TTL worker
//     and the admin DELETE endpoint.
var allowedTransitions = map[transitionKey]struct{}{
	{StatusRunning, StatusStopped}: {},
	{StatusRunning, StatusRemoved}: {},
	{StatusStopped, StatusRunning}: {},
	{StatusStopped, StatusRemoved}: {},
}

// AllowedTransitions returns a copy of the `(from, to)` allowed
// transitions table used by Transition. The returned map is safe to
// mutate; callers should not rely on iteration order.
func AllowedTransitions() map[Status][]Status {
	result := make(map[Status][]Status)
	for key := range allowedTransitions {
		result[key.from] = append(result[key.from], key.to)
	}
	return result
}

// Transition reports whether from may transition to next. The function
// returns nil when the pair is permitted, and an *InvalidTransitionError
// wrapping ErrInvalidTransition otherwise. It does not touch any store
// and is safe to call from any layer.
func Transition(from Status, next Status) error {
	if !from.IsKnown() || !next.IsKnown() {
		return &InvalidTransitionError{From: from, To: next}
	}
	if _, ok := allowedTransitions[transitionKey{from: from, to: next}]; !ok {
		return &InvalidTransitionError{From: from, To: next}
	}
	return nil
}