Enable struct fields to be skipped in encode/decode

syntax/decode.go

@@ -32,6 +32,7 @@ type decOpts struct {
 	min    uint // minimum size in bytes
 	max    uint // maximum size in bytes
 	varint bool // whether to decode as a varint
+	none   bool // whether to decode at all
 }
 
 type decodeState struct {
@@ -287,6 +288,10 @@ func newSliceDecoder(t reflect.Type) decoderFunc {
 
 //////////
 
+func noopDecoder(d *decodeState, v reflect.Value, opts decOpts) int {
+	return 0
+}
+
 type structDecoder struct {
 	fieldOpts []decOpts
 	fieldDecs []decoderFunc
@@ -318,6 +323,12 @@ func newStructDecoder(t reflect.Type) decoderFunc {
 			max:    tagOpts["max"],
 			min:    tagOpts["min"],
 			varint: tagOpts[varintOption] > 0,
+			none:   tagOpts[noneOption] > 0,
+		}
+
+		if sd.fieldOpts[i].none {
+			sd.fieldDecs[i] = noopDecoder
+			continue
 		}
 
 		sd.fieldDecs[i] = typeDecoder(f.Type)

syntax/decode_test.go

@@ -20,6 +20,28 @@ func TestDecodeUnsupported(t *testing.T) {
 	}
 }
 
+func TestDecodeNone(t *testing.T) {
+	template := struct {
+		A uint16
+		B uint16 `tls:"none"`
+		C uint16
+	}{}
+	encoding := unhex("AAAACCCC")
+
+	template.B = 0xBBBB
+	n, err := Unmarshal(encoding, &template)
+	if n != len(encoding) {
+		t.Fatalf("Failed to use all of the encoded value")
+	}
+	if err != nil {
+		t.Fatalf("Error in unmarshal: %v", err)
+	}
+	if template.B != 0xBBBB {
+		t.Fatalf("Overwrote value of unencoded field")
+	}
+
+}
+
 func TestDecodeErrors(t *testing.T) {
 	vector0x20 := append([]byte{0x20}, buffer(0x20)...)
 	errorCases := map[string]struct {

syntax/encode.go

@@ -29,6 +29,7 @@ type encOpts struct {
 	min    uint // minimum size in bytes
 	max    uint // maximum size in bytes
 	varint bool // whether to encode as a varint
+	none   bool // whether to encode at all
 }
 
 type encodeState struct {
@@ -220,6 +221,8 @@ func newSliceEncoder(t reflect.Type) encoderFunc {
 
 //////////
 
+func noopEncoder(e *encodeState, v reflect.Value, opts encOpts) {}
+
 type structEncoder struct {
 	fieldOpts []encOpts
 	fieldEncs []encoderFunc
@@ -248,7 +251,14 @@ func newStructEncoder(t reflect.Type) encoderFunc {
 			max:    tagOpts["max"],
 			min:    tagOpts["min"],
 			varint: tagOpts[varintOption] > 0,
+			none:   tagOpts[noneOption] > 0,
+		}
+
+		if se.fieldOpts[i].none {
+			se.fieldEncs[i] = noopEncoder
+			continue
 		}
+
 		se.fieldEncs[i] = typeEncoder(f.Type)
 	}
 

syntax/success_test.go

@@ -174,6 +174,18 @@ func TestSuccessCases(t *testing.T) {
 			value:    struct{ V *uint16 }{V: &dummyUint16},
 			encoding: unhex("B0A0"),
 		},
+		"struct-none": {
+			value: struct {
+				A uint16
+				B uint16 `tls:"none"`
+				C uint16
+			}{
+				A: 0xAAAA,
+				B: 0x0000, // must be zero value in order for tests to pass
+				C: 0xCCCC,
+			},
+			encoding: unhex("AAAACCCC"),
+		},
 
 		// Marshaler
 		"marshaler": {

syntax/tags.go

@@ -10,6 +10,7 @@ import (
 type tagOptions map[string]uint
 
 var (
+	noneOption   = "none"
 	varintOption = "varint"
 
 	headOptionNone   = "none"
@@ -24,6 +25,11 @@ var (
 func parseTag(tag string) tagOptions {
 	opts := tagOptions{}
 	for _, token := range strings.Split(tag, ",") {
+		if token == noneOption {
+			opts[noneOption] = 1
+			continue
+		}
+
 		if token == varintOption {
 			opts[varintOption] = 1
 			continue