package application

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

const (
	// StatusSubmitted reports that the application was created by the
	// applicant and awaits admin decision.
	StatusSubmitted Status = "submitted"

	// StatusApproved reports that the admin accepted the application and
	// a membership record was created for the applicant.
	StatusApproved Status = "approved"

	// StatusRejected reports that the admin declined the application.
	// The applicant may submit a new application while enrollment is open.
	StatusRejected Status = "rejected"
)

// IsKnown reports whether status belongs to the frozen application status
// vocabulary.
func (status Status) IsKnown() bool {
	switch status {
	case StatusSubmitted, StatusApproved, StatusRejected:
		return true
	default:
		return false
	}
}

// IsTerminal reports whether status can no longer accept lifecycle
// transitions.
func (status Status) IsTerminal() bool {
	switch status {
	case StatusApproved, StatusRejected:
		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 Application
// Lifecycle section.
var allowedTransitions = map[transitionKey]struct{}{
	{StatusSubmitted, StatusApproved}: {},
	{StatusSubmitted, StatusRejected}: {},
}

// 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
}