gps: improve parsing and add tests to verify

Signed-off-by: deadprogram <ron@hybridgroup.com>

gps/gps.go

@@ -42,6 +42,12 @@ func (ge GPSError) Unwrap() error {
 	return ge.Err
 }
 
+const (
+	minimumNMEALength = 7
+	startingDelimiter = '$'
+	checksumDelimiter = '*'
+)
+
 // Device wraps a connection to a GPS device.
 type Device struct {
 	buffer   []byte
@@ -87,11 +93,11 @@ func (gps *Device) readNextSentence() (sentence string) {
 	gps.sentence.Reset()
 	var b byte = ' '
 
-	for b != '$' {
+	for b != startingDelimiter {
 		b = gps.readNextByte()
 	}
 
-	for b != '*' {
+	for b != checksumDelimiter {
 		gps.sentence.WriteByte(b)
 		b = gps.readNextByte()
 	}
@@ -153,8 +159,13 @@ func (gps *Device) WriteBytes(bytes []byte) {
 }
 
 // validSentence checks if a sentence has been received uncorrupted
+// For example, a valid NMEA sentence such as this:
+// $GPGLL,3751.65,S,14507.36,E*77
+// It has to start with a '$' character.
+// It has to have a 5 character long sentence identifier.
+// It has to end with a '*' character following by a checksum.
 func validSentence(sentence string) error {
-	if len(sentence) < 7 || sentence[0] != '$' || sentence[len(sentence)-3] != '*' {
+	if len(sentence) < minimumNMEALength || sentence[0] != startingDelimiter || sentence[len(sentence)-3] != checksumDelimiter {
 		return errInvalidNMEASentenceLength
 	}
 	var cs byte = 0

gps/gpsparser.go

@@ -60,11 +60,11 @@ func (parser *Parser) Parse(sentence string) (Fix, error) {
 			return fix, errInvalidGGASentence
 		}
 
-		fix.Altitude = findAltitude(fields[9])
-		fix.Satellites = findSatellites(fields[7])
-		fix.Longitude = findLongitude(fields[4], fields[5])
-		fix.Latitude = findLatitude(fields[2], fields[3])
 		fix.Time = findTime(fields[1])
+		fix.Latitude = findLatitude(fields[2], fields[3])
+		fix.Longitude = findLongitude(fields[4], fields[5])
+		fix.Satellites = findSatellites(fields[7])
+		fix.Altitude = findAltitude(fields[9])
 		fix.Valid = (fix.Altitude != -99999) && (fix.Satellites > 0)
 
 		return fix, nil
@@ -75,11 +75,11 @@ func (parser *Parser) Parse(sentence string) (Fix, error) {
 			return fix, errInvalidGLLSentence
 		}
 
-		fix.Latitude = findLatitude(fields[2], fields[3])
+		fix.Latitude = findLatitude(fields[1], fields[2])
-		fix.Longitude = findLongitude(fields[4], fields[5])
+		fix.Longitude = findLongitude(fields[3], fields[4])
-		fix.Time = findTime(fields[6])
+		fix.Time = findTime(fields[5])
 
-		fix.Valid = (fields[7] == "A")
+		fix.Valid = (fields[6] == "A")
 
 		return fix, nil
 	case "RMC":
@@ -89,12 +89,12 @@ func (parser *Parser) Parse(sentence string) (Fix, error) {
 			return fix, errInvalidRMCSentence
 		}
 
-		fix.Longitude = findLongitude(fields[5], fields[6])
-		fix.Latitude = findLatitude(fields[3], fields[4])
 		fix.Time = findTime(fields[1])
+		fix.Valid = (fields[2] == "A")
+		fix.Latitude = findLatitude(fields[3], fields[4])
+		fix.Longitude = findLongitude(fields[5], fields[6])
 		fix.Speed = findSpeed(fields[7])
 		fix.Heading = findHeading(fields[8])
-		fix.Valid = (len(fields[2]) > 0 && fields[2] == "A")
 
 		return fix, nil
 	}
@@ -113,8 +113,8 @@ func findTime(val string) time.Time {
 	m, _ := strconv.ParseInt(val[2:4], 10, 8)
 	s, _ := strconv.ParseInt(val[4:6], 10, 8)
 	ms := int64(0)
-	if len(val) == 10 {
+	if len(val) > 6 {
-		ms, _ = strconv.ParseInt(val[7:10], 10, 16)
+		ms, _ = strconv.ParseInt(val[7:], 10, 16)
 	}
 	t := time.Date(0, 0, 0, int(h), int(m), int(s), int(ms), time.UTC)
 
@@ -135,11 +135,11 @@ func findAltitude(val string) int32 {
 // $--GGA,,ddmm.mmmmm,x,,,,,,,,,,,*hh
 func findLatitude(val, hemi string) float32 {
 	if len(val) > 8 {
-		var dd = val[0:2]
+		dd := val[0:2]
-		var mm = val[2:]
+		mm := val[2:]
-		var d, _ = strconv.ParseFloat(dd, 32)
+		d, _ := strconv.ParseFloat(dd, 32)
-		var m, _ = strconv.ParseFloat(mm, 32)
+		m, _ := strconv.ParseFloat(mm, 32)
-		var v = float32(d + (m / 60))
+		v := float32(d + (m / 60))
 		if hemi == "S" {
 			v *= -1
 		}
@@ -148,7 +148,7 @@ func findLatitude(val, hemi string) float32 {
 	return 0.0
 }
 
-// findLatitude returns the longitude from an NMEA sentence:
+// findLongitude returns the longitude from an NMEA sentence:
 // $--GGA,,,,dddmm.mmmmm,x,,,,,,,,,*hh
 func findLongitude(val, hemi string) float32 {
 	if len(val) > 8 {

gps/gpsparser_test.go

@@ -0,0 +1,97 @@
+package gps
+
+import (
+	"testing"
+	"time"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestParseUnknownSentence(t *testing.T) {
+	c := qt.New(t)
+
+	p := NewParser()
+
+	val := "$GPGSV,3,1,09,07,14,317,22,08,31,284,25,10,32,133,39,16,85,232,29*7F"
+	_, err := p.Parse(val)
+	c.Assert(err.Error(), qt.Contains, "unsupported NMEA sentence type")
+}
+
+func TestParseGGA(t *testing.T) {
+	c := qt.New(t)
+
+	p := NewParser()
+
+	val := "$GPGGA,115739.00,4158.8441367,N,09147.4416929,"
+	fix, err := p.Parse(val)
+	if err != errInvalidGGASentence {
+		t.Error("should have errInvalidGGASentence error")
+	}
+
+	val = "$GPGGA,115739.00,4158.8441367,N,09147.4416929,W,4,13,0.9,255.747,M,-32.00,M,01,0000*6E"
+	fix, err = p.Parse(val)
+	if err != nil {
+		t.Error("should have parsed")
+	}
+	c.Assert(fix.Latitude, qt.Equals, float32(41.980735778808594))
+	c.Assert(fix.Longitude, qt.Equals, float32(-91.79069519042969))
+	c.Assert(fix.Altitude, qt.Equals, int32(255))
+}
+
+func TestParseGLL(t *testing.T) {
+	c := qt.New(t)
+
+	p := NewParser()
+
+	val := "$GPGLL,3953.88008971,N,10506.7531891"
+	_, err := p.Parse(val)
+	if err != errInvalidGLLSentence {
+		t.Error("should have errInvalidGLLSentence error")
+	}
+
+	val = "$GPGLL,5109.0262317,N,11401.8407304,W,202725.00,A,D*79"
+	fix, err := p.Parse(val)
+	if err != nil {
+		t.Error("should have parsed")
+	}
+
+	c.Assert(fix.Latitude, qt.Equals, float32(51.15043640136719))
+	c.Assert(fix.Longitude, qt.Equals, float32(-114.03067779541016))
+}
+
+func TestParseRMC(t *testing.T) {
+	c := qt.New(t)
+
+	p := NewParser()
+
+	val := "$GPRMC,203522.00,A,5109.0262308,N,11401.8407342,"
+	_, err := p.Parse(val)
+	if err != errInvalidRMCSentence {
+		t.Error("should have errInvalidRMCSentence error")
+	}
+
+	val = "$GPRMC,203522.00,A,5109.0262308,N,11401.8407342,W,0.004,133.4,130522,0.0,E,D*2B"
+	fix, err := p.Parse(val)
+	if err != nil {
+		t.Error("should have parsed")
+	}
+
+	c.Assert(fix.Latitude, qt.Equals, float32(51.15043640136719))
+	c.Assert(fix.Longitude, qt.Equals, float32(-114.03067779541016))
+}
+
+func TestTime(t *testing.T) {
+	c := qt.New(t)
+
+	val := ""
+	tm := findTime(val)
+	c.Assert(tm, qt.Equals, time.Time{})
+
+	val = "225446"
+	tm = findTime(val)
+	c.Assert(tm, qt.Equals, time.Date(0, 0, 0, 22, 54, 46, 0, time.UTC))
+
+	val = "124326.02752"
+	tm = findTime(val)
+	c.Assert(tm, qt.Equals, time.Date(0, 0, 0, 12, 43, 26, 2752, time.UTC))
+}