Add support for ASN.1 "choice"

.gitignore

@@ -0,0 +1 @@
+.DS_Store

asn1/asn1.go

@@ -572,7 +572,7 @@ func parseTagAndLength(bytes []byte, initOffset int) (ret tagAndLength, offset i
 // parseSequenceOf is used for SEQUENCE OF and SET OF values. It tries to parse
 // a number of ASN.1 values from the given byte slice and returns them as a
 // slice of Go values of the given type.
-func parseSequenceOf(bytes []byte, sliceType reflect.Type, elemType reflect.Type) (ret reflect.Value, err error) {
+func parseSequenceOf(bytes []byte, sliceType reflect.Type, elemType reflect.Type, params fieldParameters) (ret reflect.Value, err error) {
 	matchAny, expectedTag, compoundType, ok := getUniversalType(elemType)
 	if !ok {
 		err = StructuralError{"unknown Go type for slice"}
@@ -599,7 +599,7 @@ func parseSequenceOf(bytes []byte, sliceType reflect.Type, elemType reflect.Type
 			t.tag = TagUTCTime
 		}
 
-		if !matchAny && (t.class != ClassUniversal || t.isCompound != compoundType || t.tag != expectedTag) {
+		if !matchAny && (t.class != ClassUniversal || t.isCompound != compoundType || t.tag != expectedTag) && !params.choice {
 			err = StructuralError{"sequence tag mismatch"}
 			return
 		}
@@ -611,10 +611,10 @@ func parseSequenceOf(bytes []byte, sliceType reflect.Type, elemType reflect.Type
 		numElements++
 	}
 	ret = reflect.MakeSlice(sliceType, numElements, numElements)
-	params := fieldParameters{}
+	fp := fieldParameters{choice: params.choice}
 	offset := 0
 	for i := 0; i < numElements; i++ {
-		offset, err = parseField(ret.Index(i), bytes, offset, params)
+		offset, err = parseField(ret.Index(i), bytes, offset, fp)
 		if err != nil {
 			return
 		}
@@ -800,8 +800,8 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 	}
 
 	// We have unwrapped any explicit tagging at this point.
-	if !matchAnyClassAndTag && (t.class != expectedClass || t.tag != expectedTag) ||
-		(!matchAny && t.isCompound != compoundType) {
+	if (!matchAnyClassAndTag && (t.class != expectedClass || t.tag != expectedTag) ||
+		(!matchAny && t.isCompound != compoundType)) && !params.choice {
 		// Tags don't match. Again, it could be an optional element.
 		ok := setDefaultValue(v, params)
 		if ok {
@@ -913,6 +913,17 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 		innerOffset := 0
 		for i := 0; i < structType.NumField(); i++ {
 			field := structType.Field(i)
+			if params.choice {
+				tag := parseFieldParameters(field.Tag.Get("asn1")).tag
+				if tag != nil && t.tag == *tag || t.class != ClassContextSpecific && tag == nil {
+					if tag == nil || params.set {
+						innerBytes = bytes[initOffset:offset]
+					}
+					innerOffset, err = parseField(val.Field(i), innerBytes, innerOffset, fieldParameters{})
+					return
+				}
+				continue
+			}
 			if i == 0 && field.Type == rawContentsType {
 				continue
 			}
@@ -932,7 +943,7 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 			reflect.Copy(val, reflect.ValueOf(innerBytes))
 			return
 		}
-		newSlice, err1 := parseSequenceOf(innerBytes, sliceType, sliceType.Elem())
+		newSlice, err1 := parseSequenceOf(innerBytes, sliceType, sliceType.Elem(), params)
 		if err1 == nil {
 			val.Set(newSlice)
 		}

asn1/common.go

@@ -74,6 +74,7 @@ type tagAndLength struct {
 type fieldParameters struct {
 	optional     bool   // true iff the field is OPTIONAL
 	explicit     bool   // true iff an EXPLICIT tag is in use.
+	choice       bool   // true iff CHOICE tag is in use.
 	application  bool   // true iff an APPLICATION tag is in use.
 	private      bool   // true iff a PRIVATE tag is in use.
 	defaultValue *int64 // a default value for INTEGER typed fields (maybe nil).
@@ -100,6 +101,8 @@ func parseFieldParameters(str string) (ret fieldParameters) {
 			if ret.tag == nil {
 				ret.tag = new(int)
 			}
+		case part == "choice":
+			ret.choice = true
 		case part == "generalized":
 			ret.timeType = TagGeneralizedTime
 		case part == "utc":

asn1/marshal.go

@@ -480,7 +480,11 @@ func makeBody(value reflect.Value, params fieldParameters) (e encoder, err error
 		default:
 			m := make([]encoder, n1)
 			for i := 0; i < n1; i++ {
-				m[i], err = makeField(v.Field(i+startingField), parseFieldParameters(t.Field(i+startingField).Tag.Get("asn1")))
+				fp := parseFieldParameters(t.Field(i + startingField).Tag.Get("asn1"))
+				if params.explicit && params.choice {
+					fp.explicit = true
+				}
+				m[i], err = makeField(v.Field(i+startingField), fp)
 				if err != nil {
 					return nil, err
 				}
@@ -495,6 +499,10 @@ func makeBody(value reflect.Value, params fieldParameters) (e encoder, err error
 		}
 
 		var fp fieldParameters
+		fp.choice = params.choice
+		if params.choice && params.set {
+			fp.explicit = true
+		}
 
 		switch l := v.Len(); l {
 		case 0:
@@ -640,6 +648,10 @@ func makeField(v reflect.Value, params fieldParameters) (e encoder, err error) {
 		if params.explicit {
 			t.tag = bytesEncoder(appendTagAndLength(t.scratch[:0], tagAndLength{ClassUniversal, tag, bodyLen, isCompound}))
 
+			if params.choice {
+				t.tag = bytesEncoder(nil)
+			}
+
 			tt := new(taggedEncoder)
 
 			tt.body = t
@@ -660,6 +672,10 @@ func makeField(v reflect.Value, params fieldParameters) (e encoder, err error) {
 
 	t.tag = bytesEncoder(appendTagAndLength(t.scratch[:0], tagAndLength{class, tag, bodyLen, isCompound}))
 
+	if tag == TagSequence && params.choice {
+		t.tag = bytesEncoder(nil)
+	}
+
 	return t, nil
 }