make simple text messages encodable

sds/sds.go

@@ -360,7 +360,7 @@ func ParseSDSTransfer(bytes []byte) (SDSTransfer, error) {
 }
 
 // NewTextMessageTransfer returns a new SDS-TRANSFER PDU for text messaging with the given parameters
-func NewTextMessageTransfer(messageReference MessageReference, immediate bool, deliveryReport DeliveryReportRequest, text string) SDSTransfer {
+func NewTextMessageTransfer(messageReference MessageReference, immediate bool, deliveryReport DeliveryReportRequest, encoding TextEncoding, text string) SDSTransfer {
 	var protocol ProtocolIdentifier
 	if immediate {
 		protocol = ImmediateTextMessaging
@@ -374,7 +374,7 @@ func NewTextMessageTransfer(messageReference MessageReference, immediate bool, d
 		DeliveryReportRequest: deliveryReport,
 		UserData: TextSDU{
 			TextHeader: TextHeader{
-				Encoding: ISO8859_1,
+				Encoding: encoding,
 			},
 			Text: text,
 		},
@@ -750,6 +750,7 @@ type ExternalSubscriberNumberDigit byte // its only 4 bits per digit
 
 /* Simple Text Messaging related types and functions */
 
+// ParseSimpleTextMessage parses a simple text message PDU
 func ParseSimpleTextMessage(bytes []byte) (SimpleTextMessage, error) {
 	if len(bytes) < 2 {
 		return SimpleTextMessage{}, fmt.Errorf("simple text message PDU too short: %d", len(bytes))
@@ -768,6 +769,22 @@ func ParseSimpleTextMessage(bytes []byte) (SimpleTextMessage, error) {
 	return result, nil
 }
 
+// NewSimpleTextMessage returns a new simple text message PDU according to the given parameters
+func NewSimpleTextMessage(immediate bool, encoding TextEncoding, text string) SimpleTextMessage {
+	var protocol ProtocolIdentifier
+	if immediate {
+		protocol = ImmediateTextMessaging
+	} else {
+		protocol = TextMessaging
+	}
+
+	return SimpleTextMessage{
+		protocol: protocol,
+		Encoding: encoding,
+		Text:     text,
+	}
+}
+
 // SimpleTextMessage represents the data of a simple text messaging PDU, according to [AI] 29.5.2.3
 type SimpleTextMessage struct {
 	protocol ProtocolIdentifier
@@ -780,6 +797,16 @@ func (m SimpleTextMessage) Immediate() bool {
 	return m.protocol == SimpleImmediateTextMessaging
 }
 
+// Encode this simple text message
+func (m SimpleTextMessage) Encode(bytes []byte, bits int) ([]byte, int) {
+	bytes, bits = m.protocol.Encode(bytes, bits)
+	bytes = append(bytes, byte(m.Encoding))
+	bits += 8
+	bytes, bits = AppendEncodedPayloadText(bytes, bits, m.Text, m.Encoding)
+
+	return bytes, bits
+}
+
 /* Text messaging related types and functions */
 
 // ParseTextSDU parses the user data of a text message.

sds/sds_test.go

@@ -32,10 +32,10 @@ func TestParseMessage(t *testing.T) {
 		},
 		{
 			desc:   "simple text message",
-			header: "+CTSDSR: 12,1234567,0,2345678,0,224",
+			header: "+CTSDSR: 12,1234567,0,2345678,0,104",
 			pdu:    "0201746573746D657373616765",
 			expected: IncomingMessage{
-				Header: Header{AIService: SDSTLService, Source: "1234567", Destination: "2345678", PDUBits: 224},
+				Header: Header{AIService: SDSTLService, Source: "1234567", Destination: "2345678", PDUBits: 104},
 				Payload: SimpleTextMessage{
 					protocol: SimpleTextMessaging,
 					Encoding: ISO8859_1,
@@ -493,6 +493,18 @@ func TestEncode(t *testing.T) {
 			expectedBytes: []byte{0x82, 0x06, 0xC9, 0x81, 0x44, 0x5A, 0x8F, 0x74, 0x65, 0x73, 0x74, 0x6D, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65},
 			expectedBits:  144,
 		},
+		{
+			desc: "simple text message",
+			values: []Encoder{
+				SimpleTextMessage{
+					protocol: SimpleTextMessaging,
+					Encoding: ISO8859_1,
+					Text:     "testmessage",
+				},
+			},
+			expectedBytes: []byte{0x02, 0x01, 0x74, 0x65, 0x73, 0x74, 0x6D, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65},
+			expectedBits:  104,
+		},
 	}
 	for _, tc := range tt {
 		t.Run(tc.desc, func(t *testing.T) {