Add original element names of WebM aliases (#88)

Matroska: Timestamp (https://matroska.org/technical/specs/index.html#Timestamp)
WebM: Timecode (https://www.webmproject.org/docs/container/#Timecode)

elementtype.go

@@ -47,15 +47,15 @@ const (
 	ElementSeekPosition
 
 	ElementInfo
-	ElementTimecodeScale
+	ElementTimestampScale
 	ElementDuration
 	ElementDateUTC
 	ElementTitle
 	ElementMuxingApp
 	ElementWritingApp
 
 	ElementCluster
-	ElementTimecode
+	ElementTimestamp
 	ElementPrevSize
 	ElementSimpleBlock
 	ElementBlockGroup
@@ -125,6 +125,12 @@ const (
 	elementMax
 )
 
+// WebM aliases
+const (
+	ElementTimecodeScale = ElementTimestampScale
+	ElementTimecode      = ElementTimestamp
+)
+
 func (i ElementType) String() string {
 	switch i {
 	case ElementEBML:
@@ -157,8 +163,8 @@ func (i ElementType) String() string {
 		return "SeekPosition"
 	case ElementInfo:
 		return "Info"
-	case ElementTimecodeScale:
-		return "TimecodeScale"
+	case ElementTimestampScale:
+		return "TimestampScale"
 	case ElementDuration:
 		return "Duration"
 	case ElementDateUTC:
@@ -171,8 +177,8 @@ func (i ElementType) String() string {
 		return "WritingApp"
 	case ElementCluster:
 		return "Cluster"
-	case ElementTimecode:
-		return "Timecode"
+	case ElementTimestamp:
+		return "Timestamp"
 	case ElementPrevSize:
 		return "PrevSize"
 	case ElementSimpleBlock:
@@ -342,8 +348,8 @@ func ElementTypeFromString(s string) (ElementType, error) {
 		return ElementSeekPosition, nil
 	case "Info":
 		return ElementInfo, nil
-	case "TimecodeScale":
-		return ElementTimecodeScale, nil
+	case "TimestampScale":
+		return ElementTimestampScale, nil
 	case "Duration":
 		return ElementDuration, nil
 	case "DateUTC":
@@ -356,8 +362,8 @@ func ElementTypeFromString(s string) (ElementType, error) {
 		return ElementWritingApp, nil
 	case "Cluster":
 		return ElementCluster, nil
-	case "Timecode":
-		return ElementTimecode, nil
+	case "Timestamp":
+		return ElementTimestamp, nil
 	case "PrevSize":
 		return ElementPrevSize, nil
 	case "SimpleBlock":
@@ -484,6 +490,13 @@ func ElementTypeFromString(s string) (ElementType, error) {
 		return ElementCueClusterPosition, nil
 	case "CueBlockNumber":
 		return ElementCueBlockNumber, nil
+
+		// WebM aliases
+	case "TimecodeScale":
+		return ElementTimecodeScale, nil
+	case "Timecode":
+		return ElementTimecode, nil
+
 	default:
 		return 0, errUnknownElementType
 	}

unmarshal.go

@@ -60,11 +60,7 @@ func readElement(r0 io.Reader, n int64, vo reflect.Value, depth int, pos uint64,
 		r = r0
 	}
 
-	type field struct {
-		v reflect.Value
-		t reflect.Type
-	}
-	fieldMap := make(map[string]field)
+	fieldMap := make(map[ElementType]reflect.Value)
 	if vo.IsValid() {
 		for i := 0; i < vo.NumField(); i++ {
 			var nn []string
@@ -77,7 +73,11 @@ func readElement(r0 io.Reader, n int64, vo reflect.Value, depth int, pos uint64,
 			} else {
 				name = vo.Type().Field(i).Name
 			}
-			fieldMap[name] = field{vo.Field(i), vo.Type().Field(i).Type}
+			t, err := ElementTypeFromString(name)
+			if err != nil {
+				return nil, err
+			}
+			fieldMap[t] = vo.Field(i)
 		}
 	}
 
@@ -102,8 +102,8 @@ func readElement(r0 io.Reader, n int64, vo reflect.Value, depth int, pos uint64,
 			return nil, err
 		}
 		var vnext reflect.Value
-		if fm, ok := fieldMap[v.e.String()]; ok {
-			vnext = fm.v
+		if vn, ok := fieldMap[v.e]; ok {
+			vnext = vn
 		}
 
 		var elem *Element

unmarshal_test.go

@@ -258,6 +258,15 @@ func TestUnmarshal_Error(t *testing.T) {
 			t.Errorf("Unexpected error, %v, got %v\n", errIndefiniteType, err)
 		}
 	})
+	t.Run("UnknownElement", func(t *testing.T) {
+		input := &struct {
+			Header struct {
+			} `ebml:"Unknown"`
+		}{}
+		if err := Unmarshal(bytes.NewBuffer([]byte{}), input); err != errUnknownElementType {
+			t.Errorf("Unexpected error, %v, got %v\n", errUnknownElementType, err)
+		}
+	})
 	t.Run("Short", func(t *testing.T) {
 		TestBinaries := map[string][]byte{
 			"ElementID": {0x1a, 0x45, 0xdf},