sphero: calculate checksum of response packets

a little more robust.

platforms/sphero/sphero_driver.go

@@ -106,11 +106,16 @@ func (s *SpheroDriver) Start() bool {
 			header := s.readHeader()
 			if header != nil && len(header) != 0 {
 				body := s.readBody(header[4])
-				if header[1] == 0xFE {
+				data := append(header, body...)
-					async := append(header, body...)
+				checksum := data[len(data)-1]
-					s.asyncResponse = append(s.asyncResponse, async)
+				if checksum != calculateChecksum(data[2:len(data)-1]) {
-				} else {
+					continue
-					s.responseChannel <- append(header, body...)
+				}
+				switch header[1] {
+				case 0xFE:
+					s.asyncResponse = append(s.asyncResponse, data)
+				case 0xFF:
+					s.responseChannel <- data
 				}
 			}
 		}
@@ -230,7 +235,10 @@ func (s *SpheroDriver) write(packet *packet) {
 
 func (s *SpheroDriver) calculateChecksum(packet *packet) uint8 {
 	buf := append(packet.header, packet.body...)
-	buf = buf[2:]
+	return calculateChecksum(buf[2:])
+}
+
+func calculateChecksum(buf []byte) byte {
 	var calculatedChecksum uint16
 	for i := range buf {
 		calculatedChecksum += uint16(buf[i])

platforms/sphero/sphero_driver_test.go

@@ -1,8 +1,9 @@
 package sphero
 
 import (
-	"github.com/hybridgroup/gobot"
 	"testing"
+
+	"github.com/hybridgroup/gobot"
 )
 
 func initTestSpheroDriver() *SpheroDriver {
@@ -20,3 +21,20 @@ func TestSpheroDriverHalt(t *testing.T) {
 	d := initTestSpheroDriver()
 	gobot.Assert(t, d.Halt(), true)
 }
+
+func TestCalculateChecksum(t *testing.T) {
+	tests := []struct {
+		data     []byte
+		checksum byte
+	}{
+		{[]byte{0x00}, 0xff},
+		{[]byte{0xf0, 0x0f}, 0x00},
+	}
+
+	for _, tt := range tests {
+		actual := calculateChecksum(tt.data)
+		if actual != tt.checksum {
+			t.Errorf("Expected %x, got %x for data %x.", tt.checksum, actual, tt.data)
+		}
+	}
+}