add SentenceParser.OnBaseSentence

sentence.go

@@ -103,6 +103,10 @@ type SentenceParser struct {
 	// \g:2-3-1234*hh\!ABVDM,1,1,1,B,.....,0*hh
 	// \g:3-3-1234*hh\$ABVSI,r3669961,1,013536.96326433,1386,-98,,*hh
 	OnTagBlock func(tagBlock TagBlock) error
+
+	// OnBaseSentence is a callback for accessing/modifying the base sentence
+	// before further parsing is done.
+	OnBaseSentence func(sentence *BaseSentence) error
 }
 
 func (p *SentenceParser) parseBaseSentence(raw string) (BaseSentence, error) {
@@ -264,6 +268,12 @@ func (p *SentenceParser) Parse(raw string) (Sentence, error) {
 		return nil, err
 	}
 
+	if p.OnBaseSentence != nil {
+		if err := p.OnBaseSentence(&s); err != nil {
+			return nil, err
+		}
+	}
+
 	// Custom parser allow overriding of existing parsers
 	if parser, ok := p.CustomParsers[s.Type]; ok {
 		return parser(s)

sentence_test.go

@@ -3,6 +3,7 @@ package nmea
 import (
 	"encoding/hex"
 	"errors"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -613,3 +614,66 @@ func TestSentenceParser_CheckCRC(t *testing.T) {
 		})
 	}
 }
+
+func TestSentenceParser_OnBaseSentence(t *testing.T) {
+	testErr := errors.New("this is a test")
+	tests := []struct {
+		name     string
+		fn       func(*BaseSentence) error
+		raw      string
+		sentence Sentence
+		err      error
+	}{
+		{
+			name: "can modify prefix",
+			fn: func(s *BaseSentence) error {
+				if s.Type == "VDM" && strings.HasPrefix(s.Raw, "$") {
+					s.Raw = "!" + s.Raw[1:]
+				}
+				return nil
+			},
+			raw: "\\s:somewhere,c:1720289719*4D\\$AIVDM,1,1,,A,,0*26",
+			sentence: VDMVDO{
+				BaseSentence: BaseSentence{
+					Talker:   "AI",
+					Type:     "VDM",
+					Fields:   []string{"1", "1", "", "A", "", "0"},
+					Checksum: "26",
+					Raw:      "!AIVDM,1,1,,A,,0*26",
+					TagBlock: TagBlock{
+						Time:         1720289719,
+						RelativeTime: 0,
+						Destination:  "",
+						Grouping:     "",
+						LineCount:    0,
+						Source:       "somewhere",
+						Text:         "",
+					},
+				},
+				NumFragments:   1,
+				FragmentNumber: 1,
+				MessageID:      0,
+				Channel:        "A",
+				Payload:        []uint8{},
+			},
+		},
+		{
+			name: "should return error",
+			fn:   func(_ *BaseSentence) error { return testErr },
+			raw:  "$GNRMC,142754.0,A,4302.539570,N,07920.379823,W,0.0,,070617,0.0,E,A*21",
+			err:  testErr,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			p := SentenceParser{OnBaseSentence: tt.fn}
+			s, err := p.Parse(tt.raw)
+			if tt.err == nil {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.sentence, s)
+			} else {
+				assert.Equal(t, tt.err, err)
+			}
+		})
+	}
+}