package transfer

const (
	StatePending     = "PENDING"
	StateAccepted    = "ACCEPTED"
	StateP2PActive   = "P2P_ACTIVE"
	StateRelayActive = "RELAY_ACTIVE"
	StateDone        = "DONE"
	StateFailed      = "FAILED"
	StateCancelled   = "CANCELLED"
)

const (
	ModeP2P   = "p2p"
	ModeRelay = "relay"
)

// AutoAcceptThreshold is the file size at/under which the receiver may
// silently auto-accept a transfer. brief §2: "> 100 MB 二次确认".
const AutoAcceptThreshold = 100 * 1024 * 1024

// IsValidTransition reports whether a transfer may move from→to.
//
// Forward-only: any state can advance to a "later" state, but never go back.
// We're permissive because client→server timing races are real:
//   - sender's /p2p can land before receiver's /accept commits → PENDING → P2P_ACTIVE
//   - either side may call /done before /p2p ever fired → ACCEPTED → DONE
// Strict ordering would surface as 409s the user can't recover from.
//
// Terminal states (DONE / FAILED / CANCELLED) accept no further transitions.
func IsValidTransition(from, to string) bool {
	switch from {
	case StatePending:
		return to == StateAccepted || to == StateP2PActive || to == StateRelayActive ||
			to == StateCancelled || to == StateFailed
	case StateAccepted:
		return to == StateP2PActive || to == StateRelayActive ||
			to == StateDone || to == StateCancelled || to == StateFailed
	case StateP2PActive:
		// P2P 失败自动 fallback 到 Relay (brief §2).
		return to == StateRelayActive || to == StateDone ||
			to == StateFailed || to == StateCancelled
	case StateRelayActive:
		return to == StateDone || to == StateFailed || to == StateCancelled
	case StateDone, StateFailed, StateCancelled:
		return false
	}
	return false
}

// IsTerminal reports whether the state is final (no further transitions).
func IsTerminal(state string) bool {
	return state == StateDone || state == StateFailed || state == StateCancelled
}