Method PacketToWire

lib/wire.go:2220–2296

PacketToWire converts a Packet to wire format, which is wholly little-endian See notehub-defs.go for binary wire format of PB payload

(payload Packet, secureData bool, downlinksPending bool)

// PacketToWire converts a Packet to wire format, which is wholly little-endian
// See notehub-defs.go for binary wire format of PB payload
func (h *PacketHandler) PacketToWire(payload Packet, secureData bool, downlinksPending bool) (msg []byte, err error) {

	// Determine whether or not this packet will be encrypted
	encrypted := secureData
	if !h.Notehub.MayEncrypt {
		encrypted = false
	}
	if h.Notehub.MustEncrypt {
		encrypted = true
	}

	// Bail if too much data
	if len(payload.Data) > h.PacketMaxDownlinkData(encrypted) {
		err = fmt.Errorf("data length %d is greater than max allowed %d", len(payload.Data), h.PacketMaxDownlinkData(encrypted))
		return
	}

	// Attempt to encode the data using Snappy, and only use the compressed data if it shrinks.
	// This doesn't give us extra room to pack data within the MTU because it's coming too late,
	// but this does save us over-the-air bytes and thus dollars.  Note that the data, but not
	// the port, is compressed.  There's no reason for this other than code flow.
	compressed := false
	if len(payload.Data) > 0 {
		data := snappy.Encode(nil, payload.Data)
		if len(data) < len(payload.Data) {
			payload.Data = data
			compressed = true
		}
	}

	// Construct the flag byte and insert it
	packetHeader := h.Notecard.PacketCidType & PacketTypeMask
	if encrypted {
		packetHeader |= PacketFlagEncrypted
	}
	if compressed {
		packetHeader |= PacketFlagCompressed
	}
	if downlinksPending {
		packetHeader |= PacketFlagDownlinksPending
	}
	msg = append(msg, packetHeader)

	// Append the connection ID
	if h.Notecard.PacketCidType != CidNone {
		msg = append(msg, h.Notehub.Cid...)
	}

	// If unencrypted, converting to wire is trivial
	if !encrypted {
		msg = append(msg, payload.MessagePort)
		msg = append(msg, payload.Data...)
		return
	}

	// Generate the cleartext by appending the data to the port
	cleartext := append([]byte{payload.MessagePort}, payload.Data...)