package membership

// Status identifies one lifecycle state of a Game Lobby membership record.
type Status string

const (
	// StatusActive reports that the member is a full participant and may
	// send commands through Game Master.
	StatusActive Status = "active"

	// StatusRemoved reports that the member was removed post-start. The
	// engine slot is deactivated; the race name reservation is preserved
	// until the game finishes.
	StatusRemoved Status = "removed"

	// StatusBlocked reports that the member is blocked at the platform
	// level. The engine slot is retained; commands are blocked.
	StatusBlocked Status = "blocked"
)

// IsKnown reports whether status belongs to the frozen membership status
// vocabulary.
func (status Status) IsKnown() bool {
	switch status {
	case StatusActive, StatusRemoved, StatusBlocked:
		return true
	default:
		return false
	}
}

// IsTerminal reports whether status can no longer accept lifecycle
// transitions.
func (status Status) IsTerminal() bool {
	switch status {
	case StatusRemoved, StatusBlocked:
		return true
	default:
		return false
	}
}

// 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.
// It mirrors the state machine frozen in lobby/README.md Membership Model
// section.
var allowedTransitions = map[transitionKey]struct{}{
	{StatusActive, StatusRemoved}: {},
	{StatusActive, StatusBlocked}: {},
}

// AllowedTransitions returns a copy of the `(from, to)` allowed-transitions
// table used by Transition. The returned map is safe to mutate.
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
}