ifd: Implemented several layers of value processing. Done.

- It's going to be laborious to write unit-tests for, though.
pull/3/head
Dustin Oprea 2018-04-15 01:19:56 -04:00
parent 315ca60f03
commit 1511788a4e
5 changed files with 689 additions and 33 deletions

View File

@ -38,7 +38,7 @@ func (e *Exif) Parse(data []byte, visitor TagVisitor) (err error) {
}()
if e.IsExif(data) == false {
return ErrNotExif
log.Panic(ErrNotExif)
}
// Good reference:

View File

@ -5,6 +5,7 @@ import (
"os"
"path"
"fmt"
"reflect"
"io/ioutil"
@ -64,8 +65,16 @@ func TestParse(t *testing.T) {
// Run the parse.
ti := NewTagIndex()
tags := make([]string, 0)
visitor := func(tagId uint16, tagType TagType, valueContext ValueContext) (err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
log.PrintErrorf(err, "The visitor encountered an error.")
}
}()
visitor := func(tagId, tagType uint16, tagCount, valueOffset uint32) (err error) {
it, err := ti.GetWithTagId(tagId)
if err != nil {
if err == ErrTagNotFound {
@ -75,21 +84,42 @@ func TestParse(t *testing.T) {
}
}
fmt.Printf("Tag: ID=(0x%04x) NAME=[%s] IFD=[%s] TYPE=(%d) COUNT=(%d) VALUE-OFFSET=(%d)\n", tagId, it.Name, it.Ifd, tagType, tagCount, valueOffset)
valueString, err := tagType.ValueString(valueContext, true)
log.PanicIf(err)
// Notes on the tag-value's value (we'll have to use this as a pointer if the type potentially requires more than four bytes):
//
// This tag records the offset from the start of the TIFF header to the position where the value itself is
// recorded. In cases where the value fits in 4 Bytes, the value itself is recorded. If the value is smaller
// than 4 Bytes, the value is stored in the 4-Byte area starting from the left, i.e., from the lower end of
// the byte offset area. For example, in big endian format, if the type is SHORT and the value is 1, it is
// recorded as 00010000.H
description := fmt.Sprintf("ID=(0x%04x) NAME=[%s] IFD=[%s] COUNT=(%d) TYPE=[%s] VALUE=[%s]", tagId, it.Name, it.Ifd, valueContext.UnitCount, tagType.Name(), valueString)
tags = append(tags, description)
return nil
}
err = e.Parse(data[foundAt:], visitor)
log.PanicIf(err)
expected := []string {
"ID=(0x010f) NAME=[Make] IFD=[Image] COUNT=(6) TYPE=[ASCII] VALUE=[Canon]",
"ID=(0x0110) NAME=[Model] IFD=[Image] COUNT=(22) TYPE=[ASCII] VALUE=[Canon EOS 5D Mark III]",
"ID=(0x0112) NAME=[Orientation] IFD=[Image] COUNT=(1) TYPE=[SHORT] VALUE=[1]",
"ID=(0x011a) NAME=[XResolution] IFD=[Image] COUNT=(1) TYPE=[RATIONAL] VALUE=[72/1]",
"ID=(0x011b) NAME=[YResolution] IFD=[Image] COUNT=(1) TYPE=[RATIONAL] VALUE=[72/1]",
"ID=(0x0128) NAME=[ResolutionUnit] IFD=[Image] COUNT=(1) TYPE=[SHORT] VALUE=[2]",
"ID=(0x0132) NAME=[DateTime] IFD=[Image] COUNT=(20) TYPE=[ASCII] VALUE=[2017:12:02 08:18:50]",
"ID=(0x013b) NAME=[Artist] IFD=[Image] COUNT=(1) TYPE=[ASCII] VALUE=[]",
"ID=(0x0213) NAME=[YCbCrPositioning] IFD=[Image] COUNT=(1) TYPE=[SHORT] VALUE=[2]",
"ID=(0x8298) NAME=[Copyright] IFD=[Image] COUNT=(1) TYPE=[ASCII] VALUE=[]",
"ID=(0x8769) NAME=[ExifTag] IFD=[Image] COUNT=(1) TYPE=[LONG] VALUE=[360]",
"ID=(0x8825) NAME=[GPSTag] IFD=[Image] COUNT=(1) TYPE=[LONG] VALUE=[9554]",
"ID=(0x0103) NAME=[Compression] IFD=[Image] COUNT=(1) TYPE=[SHORT] VALUE=[6]",
"ID=(0x011a) NAME=[XResolution] IFD=[Image] COUNT=(1) TYPE=[RATIONAL] VALUE=[72/1]",
"ID=(0x011b) NAME=[YResolution] IFD=[Image] COUNT=(1) TYPE=[RATIONAL] VALUE=[72/1]",
"ID=(0x0128) NAME=[ResolutionUnit] IFD=[Image] COUNT=(1) TYPE=[SHORT] VALUE=[2]",
"ID=(0x0201) NAME=[JPEGInterchangeFormat] IFD=[Image] COUNT=(1) TYPE=[LONG] VALUE=[11444]",
"ID=(0x0202) NAME=[JPEGInterchangeFormatLength] IFD=[Image] COUNT=(1) TYPE=[LONG] VALUE=[21491]",
}
if reflect.DeepEqual(tags, expected) == false {
t.Fatalf("tags not correct:\n%v", tags)
}
}
func init() {

59
ifd.go
View File

@ -9,24 +9,10 @@ import (
"github.com/dsoprea/go-logging"
)
const (
BigEndianByteOrder = iota
LittleEndianByteOrder = iota
)
var (
ifdLogger = log.NewLogger("exifjpeg.ifd")
)
type IfdByteOrder int
func (ibo IfdByteOrder) IsBigEndian() bool {
return ibo == BigEndianByteOrder
}
func (ibo IfdByteOrder) IsLittleEndian() bool {
return ibo == LittleEndianByteOrder
}
type Ifd struct {
data []byte
@ -86,23 +72,43 @@ func (ifd *Ifd) getUint16() (value uint16, err error) {
// getUint32 reads a uint32 and advances both our current and our current
// accumulator (which allows us to know how far to seek to the beginning of the
// next IFD when it's time to jump).
func (ifd *Ifd) getUint32() (value uint32, err error) {
func (ifd *Ifd) getUint32() (value uint32, raw []byte, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
err = ifd.read(ifd.buffer, &value)
raw = make([]byte, 4)
_, err = ifd.buffer.Read(raw)
log.PanicIf(err)
ifd.currentOffset += 4
return value, nil
if ifd.byteOrder.IsBigEndian() {
value = binary.BigEndian.Uint32(raw)
} else {
value = binary.LittleEndian.Uint32(raw)
}
return value, raw, nil
}
// ValueContext describes all of the parameters required to find and extract
// the actual tag value.
type ValueContext struct {
UnitCount uint32
ValueOffset uint32
RawValueOffset []byte
RawExif []byte
}
type TagVisitor func(tagId, tagType uint16, tagCount, valueOffset uint32) (err error)
// TagVisitor is an optional callback that can get hit for every tag we parse
// through. `rawExif` is the byte array startign after the EXIF header (where
// the offsets of all IFDs and values are calculated from).
type TagVisitor func(tagId uint16, tagType TagType, valueContext ValueContext) (err error)
// parseCurrentIfd decodes the IFD block that we're currently sitting on the
// first byte of.
@ -129,19 +135,28 @@ func (ifd *Ifd) parseCurrentIfd(visitor TagVisitor) (nextIfdOffset uint32, err e
tagType, err := ifd.getUint16()
log.PanicIf(err)
tagCount, err := ifd.getUint32()
unitCount, _, err := ifd.getUint32()
log.PanicIf(err)
valueOffset, err := ifd.getUint32()
valueOffset, rawValueOffset, err := ifd.getUint32()
log.PanicIf(err)
if visitor != nil {
err := visitor(tagId, tagType, tagCount, valueOffset)
tt := NewTagType(tagType, ifd.byteOrder)
vc := ValueContext{
UnitCount: unitCount,
ValueOffset: valueOffset,
RawValueOffset: rawValueOffset,
RawExif: ifd.data[ifd.ifdTopOffset:],
}
err := visitor(tagId, tt, vc)
log.PanicIf(err)
}
}
nextIfdOffset, err = ifd.getUint32()
nextIfdOffset, _, err = ifd.getUint32()
log.PanicIf(err)
ifdLogger.Debugf(nil, "Next IFD at offset: (%08x)", nextIfdOffset)

View File

@ -140,7 +140,7 @@ func (ti *TagIndex) GetWithTagId(id uint16) (it *IndexedTag, err error) {
it, found := ti.tagsById[id]
if found == false {
return nil, ErrTagNotFound
log.Panic(ErrTagNotFound)
}
return it, nil

611
type.go Normal file
View File

@ -0,0 +1,611 @@
package exif
import (
"bytes"
"errors"
"fmt"
"encoding/binary"
"github.com/dsoprea/go-logging"
)
const (
TypeByte = uint16(1)
TypeAscii = uint16(2)
TypeShort = uint16(3)
TypeLong = uint16(4)
TypeRational = uint16(5)
TypeUndefined = uint16(6)
TypeSignedLong = uint16(9)
TypeSignedRational = uint16(10)
)
var (
TypeNames = map[uint16]string {
TypeByte: "BYTE",
TypeAscii: "ASCII",
TypeShort: "SHORT",
TypeLong: "LONG",
TypeRational: "RATIONAL",
TypeUndefined: "UNDEFINED",
TypeSignedLong: "SLONG",
TypeSignedRational: "SRATIONAL",
}
)
var (
typeLogger = log.NewLogger("exif.type")
)
var (
// ErrCantDetermineTagValueSize is used when we're trying to determine a
//size for a non-standard/undefined type.
ErrCantDetermineTagValueSize = errors.New("can not determine tag-value size")
// ErrNotEnoughData is used when there isn't enough data to accomodate what
// we're trying to parse (sizeof(type) * unit_count).
ErrNotEnoughData = errors.New("not enough data for type")
// ErrWrongType is used when we try to parse anything other than the current type.
ErrWrongType = errors.New("wrong type, can not parse")
)
const (
BigEndianByteOrder = iota
LittleEndianByteOrder = iota
)
type IfdByteOrder int
func (ibo IfdByteOrder) IsBigEndian() bool {
return ibo == BigEndianByteOrder
}
func (ibo IfdByteOrder) IsLittleEndian() bool {
return ibo == LittleEndianByteOrder
}
type Rational struct {
Numerator uint32
Denominator uint32
}
type SignedRational struct {
Numerator int32
Denominator int32
}
type TagType struct {
tagType uint16
name string
byteOrder IfdByteOrder
}
func NewTagType(tagType uint16, byteOrder IfdByteOrder) TagType {
name, found := TypeNames[tagType]
if found == false {
log.Panicf("tag-type not valid: 0x%04x", tagType)
}
return TagType{
tagType: tagType,
name: name,
byteOrder: byteOrder,
}
}
func (tt TagType) String() string {
return fmt.Sprintf("TagType<NAME=[%s]>", tt.name)
}
func (tt TagType) Name() string {
return tt.name
}
func (tt TagType) Type() uint16 {
return tt.tagType
}
func (tt TagType) Size() int {
if tt.tagType == TypeByte {
return 1
} else if tt.tagType == TypeAscii {
return 1
} else if tt.tagType == TypeShort {
return 2
} else if tt.tagType == TypeLong {
return 4
} else if tt.tagType == TypeRational {
return 8
} else if tt.tagType == TypeSignedLong {
return 4
} else if tt.tagType == TypeSignedRational {
return 8
} else {
log.Panic(ErrCantDetermineTagValueSize)
// Never called.
return 0
}
}
// ValueIsEmbedded will return a boolean indicating whether the value should be
// found directly within the IFD entry or an offset to somewhere else.
func (tt TagType) ValueIsEmbedded(unitCount uint32) bool {
return (tt.Size() * int(unitCount)) <= 4
}
func (tt TagType) ParseBytes(data []byte, rawCount uint32) (value []uint8, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.tagType != TypeByte {
log.Panic(ErrWrongType)
}
count := int(rawCount)
if len(data) < (tt.Size() * count) {
log.Panic(ErrNotEnoughData)
}
value = make([]uint8, count)
for i := 0; i < count; i++ {
value[i] = uint8(data[i])
}
return value, nil
}
func (tt TagType) ParseAscii(data []byte, rawCount uint32) (value string, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.tagType != TypeAscii {
log.Panic(ErrWrongType)
}
count := int(rawCount)
if len(data) < (tt.Size() * count) {
log.Panic(ErrNotEnoughData)
}
return string(data[:count]), nil
}
func (tt TagType) ParseShorts(data []byte, rawCount uint32) (value []uint16, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.tagType != TypeShort {
log.Panic(ErrWrongType)
}
count := int(rawCount)
if len(data) < (tt.Size() * count) {
log.Panic(ErrNotEnoughData)
}
value = make([]uint16, count)
for i := 0; i < count; i++ {
if tt.byteOrder.IsBigEndian() {
value[i] = binary.BigEndian.Uint16(data[i*2:])
} else {
value[i] = binary.LittleEndian.Uint16(data[i*2:])
}
}
return value, nil
}
func (tt TagType) ParseLongs(data []byte, rawCount uint32) (value []uint32, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.tagType != TypeLong {
log.Panic(ErrWrongType)
}
count := int(rawCount)
if len(data) < (tt.Size() * count) {
log.Panic(ErrNotEnoughData)
}
value = make([]uint32, count)
for i := 0; i < count; i++ {
if tt.byteOrder.IsBigEndian() {
value[i] = binary.BigEndian.Uint32(data[i*4:])
} else {
value[i] = binary.LittleEndian.Uint32(data[i*4:])
}
}
return value, nil
}
func (tt TagType) ParseRationals(data []byte, rawCount uint32) (value []Rational, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.tagType != TypeRational {
log.Panic(ErrWrongType)
}
count := int(rawCount)
if len(data) < (tt.Size() * count) {
log.Panic(ErrNotEnoughData)
}
value = make([]Rational, count)
for i := 0; i < count; i++ {
if tt.byteOrder.IsBigEndian() {
value[i].Numerator = binary.BigEndian.Uint32(data[i*8:])
value[i].Denominator = binary.BigEndian.Uint32(data[i*8 + 4:])
} else {
value[i].Numerator = binary.LittleEndian.Uint32(data[i*8:])
value[i].Denominator = binary.LittleEndian.Uint32(data[i*8 + 4:])
}
}
return value, nil
}
func (tt TagType) ParseSignedLongs(data []byte, rawCount uint32) (value []int32, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.tagType != TypeSignedLong {
log.Panic(ErrWrongType)
}
count := int(rawCount)
if len(data) < (tt.Size() * count) {
log.Panic(ErrNotEnoughData)
}
b := bytes.NewBuffer(data)
value = make([]int32, count)
for i := 0; i < count; i++ {
if tt.byteOrder.IsBigEndian() {
err := binary.Read(b, binary.BigEndian, &value[i])
log.PanicIf(err)
} else {
err := binary.Read(b, binary.LittleEndian, &value[i])
log.PanicIf(err)
}
}
return value, nil
}
func (tt TagType) ParseSignedRationals(data []byte, rawCount uint32) (value []SignedRational, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.tagType != TypeSignedRational {
log.Panic(ErrWrongType)
}
count := int(rawCount)
if len(data) < (tt.Size() * count) {
log.Panic(ErrNotEnoughData)
}
b := bytes.NewBuffer(data)
value = make([]SignedRational, count)
for i := 0; i < count; i++ {
if tt.byteOrder.IsBigEndian() {
err = binary.Read(b, binary.BigEndian, &value[i].Numerator)
log.PanicIf(err)
err = binary.Read(b, binary.BigEndian, &value[i].Denominator)
log.PanicIf(err)
} else {
err = binary.Read(b, binary.LittleEndian, &value[i].Numerator)
log.PanicIf(err)
err = binary.Read(b, binary.LittleEndian, &value[i].Denominator)
log.PanicIf(err)
}
}
return value, nil
}
func (tt TagType) ReadByteValues(valueContext ValueContext) (value []byte, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.ValueIsEmbedded(valueContext.UnitCount) == true {
typeLogger.Debugf(nil, "Reading BYTE value (embedded).")
value, err = tt.ParseBytes(valueContext.RawValueOffset, valueContext.UnitCount)
log.PanicIf(err)
} else {
typeLogger.Debugf(nil, "Reading BYTE value (at offset).")
value, err = tt.ParseBytes(valueContext.RawExif[valueContext.ValueOffset:], valueContext.UnitCount)
log.PanicIf(err)
}
return value, nil
}
func (tt TagType) ReadAsciiValue(valueContext ValueContext) (value string, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.ValueIsEmbedded(valueContext.UnitCount) == true {
typeLogger.Debugf(nil, "Reading ASCII value (embedded).")
value, err = tt.ParseAscii(valueContext.RawValueOffset, valueContext.UnitCount)
log.PanicIf(err)
} else {
typeLogger.Debugf(nil, "Reading ASCII value (at offset).")
value, err = tt.ParseAscii(valueContext.RawExif[valueContext.ValueOffset:], valueContext.UnitCount)
log.PanicIf(err)
}
len_ := len(value)
if value[len_ - 1] != 0 {
typeLogger.Warningf(nil, "ascii value not terminated with nul: [%s]", value)
// TODO(dustin): !! Debugging
fmt.Printf("ascii value not terminated with nul: [%s]", value)
return value, nil
} else {
return value[:len_ - 1], nil
}
}
func (tt TagType) ReadShortValues(valueContext ValueContext) (value []uint16, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.ValueIsEmbedded(valueContext.UnitCount) == true {
typeLogger.Debugf(nil, "Reading SHORT value (embedded).")
value, err = tt.ParseShorts(valueContext.RawValueOffset, valueContext.UnitCount)
log.PanicIf(err)
} else {
typeLogger.Debugf(nil, "Reading SHORT value (at offset).")
value, err = tt.ParseShorts(valueContext.RawExif[valueContext.ValueOffset:], valueContext.UnitCount)
log.PanicIf(err)
}
return value, nil
}
func (tt TagType) ReadLongValues(valueContext ValueContext) (value []uint32, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.ValueIsEmbedded(valueContext.UnitCount) == true {
typeLogger.Debugf(nil, "Reading LONG value (embedded).")
value, err = tt.ParseLongs(valueContext.RawValueOffset, valueContext.UnitCount)
log.PanicIf(err)
} else {
typeLogger.Debugf(nil, "Reading LONG value (at offset).")
value, err = tt.ParseLongs(valueContext.RawExif[valueContext.ValueOffset:], valueContext.UnitCount)
log.PanicIf(err)
}
return value, nil
}
func (tt TagType) ReadRationalValues(valueContext ValueContext) (value []Rational, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.ValueIsEmbedded(valueContext.UnitCount) == true {
typeLogger.Debugf(nil, "Reading RATIONAL value (embedded).")
value, err = tt.ParseRationals(valueContext.RawValueOffset, valueContext.UnitCount)
log.PanicIf(err)
} else {
typeLogger.Debugf(nil, "Reading RATIONAL value (at offset).")
value, err = tt.ParseRationals(valueContext.RawExif[valueContext.ValueOffset:], valueContext.UnitCount)
log.PanicIf(err)
}
return value, nil
}
func (tt TagType) ReadSignedLongValues(valueContext ValueContext) (value []int32, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.ValueIsEmbedded(valueContext.UnitCount) == true {
typeLogger.Debugf(nil, "Reading SLONG value (embedded).")
value, err = tt.ParseSignedLongs(valueContext.RawValueOffset, valueContext.UnitCount)
log.PanicIf(err)
} else {
typeLogger.Debugf(nil, "Reading SLONG value (at offset).")
value, err = tt.ParseSignedLongs(valueContext.RawExif[valueContext.ValueOffset:], valueContext.UnitCount)
log.PanicIf(err)
}
return value, nil
}
func (tt TagType) ReadSignedRationalValues(valueContext ValueContext) (value []SignedRational, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.ValueIsEmbedded(valueContext.UnitCount) == true {
typeLogger.Debugf(nil, "Reading SRATIONAL value (embedded).")
value, err = tt.ParseSignedRationals(valueContext.RawValueOffset, valueContext.UnitCount)
log.PanicIf(err)
} else {
typeLogger.Debugf(nil, "Reading SRATIONAL value (at offset).")
value, err = tt.ParseSignedRationals(valueContext.RawExif[valueContext.ValueOffset:], valueContext.UnitCount)
log.PanicIf(err)
}
return value, nil
}
// ValueString extracts and parses the given value, and returns a flat string.
// Where the type is not ASCII, `justFirst` indicates whether to just stringify
// the first item in the slice (or return an empty string if the slice is
// empty).
func (tt TagType) ValueString(valueContext ValueContext, justFirst bool) (value string, err error) {
defer func() {
if state := recover(); state != nil {
err = log.Wrap(state.(error))
}
}()
if tt.Type() == TypeByte {
raw, err := tt.ReadByteValues(valueContext)
log.PanicIf(err)
if justFirst == false {
return fmt.Sprintf("%v", raw), nil
} else if valueContext.UnitCount > 0 {
return fmt.Sprintf("%v", raw[0]), nil
} else {
return "", nil
}
} else if tt.Type() == TypeAscii {
raw, err := tt.ReadAsciiValue(valueContext)
log.PanicIf(err)
return fmt.Sprintf("%s", raw), nil
} else if tt.Type() == TypeShort {
raw, err := tt.ReadShortValues(valueContext)
log.PanicIf(err)
if justFirst == false {
return fmt.Sprintf("%v", raw), nil
} else if valueContext.UnitCount > 0 {
return fmt.Sprintf("%v", raw[0]), nil
} else {
return "", nil
}
} else if tt.Type() == TypeLong {
raw, err := tt.ReadLongValues(valueContext)
log.PanicIf(err)
if justFirst == false {
return fmt.Sprintf("%v", raw), nil
} else if valueContext.UnitCount > 0 {
return fmt.Sprintf("%v", raw[0]), nil
} else {
return "", nil
}
} else if tt.Type() == TypeRational {
raw, err := tt.ReadRationalValues(valueContext)
log.PanicIf(err)
parts := make([]string, len(raw))
for i, r := range raw {
parts[i] = fmt.Sprintf("%d/%d", r.Numerator, r.Denominator)
}
if justFirst == false {
return fmt.Sprintf("%v", parts), nil
} else if valueContext.UnitCount > 0 {
return parts[0], nil
} else {
return "", nil
}
} else if tt.Type() == TypeSignedLong {
raw, err := tt.ReadSignedLongValues(valueContext)
log.PanicIf(err)
if justFirst == false {
return fmt.Sprintf("%v", raw), nil
} else if valueContext.UnitCount > 0 {
return fmt.Sprintf("%v", raw[0]), nil
} else {
return "", nil
}
} else if tt.Type() == TypeRational {
raw, err := tt.ReadSignedRationalValues(valueContext)
log.PanicIf(err)
parts := make([]string, len(raw))
for i, r := range raw {
parts[i] = fmt.Sprintf("%d/%d", r.Numerator, r.Denominator)
}
if justFirst == false {
return fmt.Sprintf("%v", raw), nil
} else if valueContext.UnitCount > 0 {
return parts[0], nil
} else {
return "", nil
}
} else {
log.Panicf("value of type [%s] is unparseable", tt)
// Never called.
return "", nil
}
}