gtsocial-umbx

ifd_enumerate.go (45458B)

---

 package exif
 
 import (
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"strconv"
 	"strings"
 	"time"
 
 	"encoding/binary"
 
 	"github.com/dsoprea/go-logging"
 
 	"github.com/dsoprea/go-exif/v3/common"
 	"github.com/dsoprea/go-exif/v3/undefined"
 )
 
 var (
 	ifdEnumerateLogger = log.NewLogger("exif.ifd_enumerate")
 )
 
 var (
 	// ErrNoThumbnail means that no thumbnail was found.
 	ErrNoThumbnail = errors.New("no thumbnail")
 
 	// ErrNoGpsTags means that no GPS info was found.
 	ErrNoGpsTags = errors.New("no gps tags")
 
 	// ErrTagTypeNotValid means that the tag-type is not valid.
 	ErrTagTypeNotValid = errors.New("tag type invalid")
 
 	// ErrOffsetInvalid means that the file offset is not valid.
 	ErrOffsetInvalid = errors.New("file offset invalid")
 )
 
 var (
 	// ValidGpsVersions is the list of recognized EXIF GPS versions/signatures.
 	ValidGpsVersions = [][4]byte{
 		// 2.0.0.0 appears to have a very similar format to 2.2.0.0, so enabling
 		// it under that assumption.
 		//
 		// IFD-PATH=[IFD] ID=(0x8825) NAME=[GPSTag] COUNT=(1) TYPE=[LONG] VALUE=[114]
 		// IFD-PATH=[IFD/GPSInfo] ID=(0x0000) NAME=[GPSVersionID] COUNT=(4) TYPE=[BYTE] VALUE=[02 00 00 00]
 		// IFD-PATH=[IFD/GPSInfo] ID=(0x0001) NAME=[GPSLatitudeRef] COUNT=(2) TYPE=[ASCII] VALUE=[S]
 		// IFD-PATH=[IFD/GPSInfo] ID=(0x0002) NAME=[GPSLatitude] COUNT=(3) TYPE=[RATIONAL] VALUE=[38/1...]
 		// IFD-PATH=[IFD/GPSInfo] ID=(0x0003) NAME=[GPSLongitudeRef] COUNT=(2) TYPE=[ASCII] VALUE=[E]
 		// IFD-PATH=[IFD/GPSInfo] ID=(0x0004) NAME=[GPSLongitude] COUNT=(3) TYPE=[RATIONAL] VALUE=[144/1...]
 		// IFD-PATH=[IFD/GPSInfo] ID=(0x0012) NAME=[GPSMapDatum] COUNT=(7) TYPE=[ASCII] VALUE=[WGS-84]
 		//
 		{2, 0, 0, 0},
 
 		{2, 2, 0, 0},
 
 		// Suddenly appeared at the default in 2.31: https://home.jeita.or.jp/tsc/std-pdf/CP-3451D.pdf
 		//
 		// Note that the presence of 2.3.0.0 doesn't seem to guarantee
 		// coordinates. In some cases, we seen just the following:
 		//
 		// GPS Tag Version     |2.3.0.0
 		// GPS Receiver Status |V
 		// Geodetic Survey Data|WGS-84
 		// GPS Differential Cor|0
 		//
 		{2, 3, 0, 0},
 	}
 )
 
 // byteParser knows how to decode an IFD and all of the tags it
 // describes.
 //
 // The IFDs and the actual values can float throughout the EXIF block, but the
 // IFD itself is just a minor header followed by a set of repeating,
 // statically-sized records. So, the tags (though notnecessarily their values)
 // are fairly simple to enumerate.
 type byteParser struct {
 	byteOrder     binary.ByteOrder
 	rs            io.ReadSeeker
 	ifdOffset     uint32
 	currentOffset uint32
 }
 
 // newByteParser returns a new byteParser struct.
 //
 // initialOffset is for arithmetic-based tracking of where we should be at in
 // the stream.
 func newByteParser(rs io.ReadSeeker, byteOrder binary.ByteOrder, initialOffset uint32) (bp *byteParser, err error) {
 	// TODO(dustin): Add test
 
 	bp = &byteParser{
 		rs:            rs,
 		byteOrder:     byteOrder,
 		currentOffset: initialOffset,
 	}
 
 	return bp, nil
 }
 
 // getUint16 reads a uint16 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 (bp *byteParser) getUint16() (value uint16, raw []byte, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add test
 
 	needBytes := 2
 
 	raw = make([]byte, needBytes)
 
 	_, err = io.ReadFull(bp.rs, raw)
 	log.PanicIf(err)
 
 	value = bp.byteOrder.Uint16(raw)
 
 	bp.currentOffset += uint32(needBytes)
 
 	return value, raw, nil
 }
 
 // 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 (bp *byteParser) getUint32() (value uint32, raw []byte, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add test
 
 	needBytes := 4
 
 	raw = make([]byte, needBytes)
 
 	_, err = io.ReadFull(bp.rs, raw)
 	log.PanicIf(err)
 
 	value = bp.byteOrder.Uint32(raw)
 
 	bp.currentOffset += uint32(needBytes)
 
 	return value, raw, nil
 }
 
 // CurrentOffset returns the starting offset but the number of bytes that we
 // have parsed. This is arithmetic-based tracking, not a seek(0) operation.
 func (bp *byteParser) CurrentOffset() uint32 {
 	return bp.currentOffset
 }
 
 // IfdEnumerate is the main enumeration type. It knows how to parse the IFD
 // containers in the EXIF blob.
 type IfdEnumerate struct {
 	ebs            ExifBlobSeeker
 	byteOrder      binary.ByteOrder
 	tagIndex       *TagIndex
 	ifdMapping     *exifcommon.IfdMapping
 	furthestOffset uint32
 
 	visitedIfdOffsets map[uint32]struct{}
 }
 
 // NewIfdEnumerate returns a new instance of IfdEnumerate.
 func NewIfdEnumerate(ifdMapping *exifcommon.IfdMapping, tagIndex *TagIndex, ebs ExifBlobSeeker, byteOrder binary.ByteOrder) *IfdEnumerate {
 	return &IfdEnumerate{
 		ebs:        ebs,
 		byteOrder:  byteOrder,
 		ifdMapping: ifdMapping,
 		tagIndex:   tagIndex,
 
 		visitedIfdOffsets: make(map[uint32]struct{}),
 	}
 }
 
 func (ie *IfdEnumerate) getByteParser(ifdOffset uint32) (bp *byteParser, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	initialOffset := ExifAddressableAreaStart + ifdOffset
 
 	rs, err := ie.ebs.GetReadSeeker(int64(initialOffset))
 	log.PanicIf(err)
 
 	bp, err =
 		newByteParser(
 			rs,
 			ie.byteOrder,
 			initialOffset)
 
 	if err != nil {
 		if err == ErrOffsetInvalid {
 			return nil, err
 		}
 
 		log.Panic(err)
 	}
 
 	return bp, nil
 }
 
 func (ie *IfdEnumerate) parseTag(ii *exifcommon.IfdIdentity, tagPosition int, bp *byteParser) (ite *IfdTagEntry, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	tagId, _, err := bp.getUint16()
 	log.PanicIf(err)
 
 	tagTypeRaw, _, err := bp.getUint16()
 	log.PanicIf(err)
 
 	tagType := exifcommon.TagTypePrimitive(tagTypeRaw)
 
 	unitCount, _, err := bp.getUint32()
 	log.PanicIf(err)
 
 	valueOffset, rawValueOffset, err := bp.getUint32()
 	log.PanicIf(err)
 
 	// Check whether the embedded type indicator is valid.
 
 	if tagType.IsValid() == false {
 		// Technically, we have the type on-file in the tags-index, but
 		// if the type stored alongside the data disagrees with it,
 		// which it apparently does, all bets are off.
 		ifdEnumerateLogger.Warningf(nil,
 			"Tag (0x%04x) in IFD [%s] at position (%d) has invalid type (0x%04x) and will be skipped.",
 			tagId, ii, tagPosition, int(tagType))
 
 		ite = &IfdTagEntry{
 			tagId:   tagId,
 			tagType: tagType,
 		}
 
 		return ite, ErrTagTypeNotValid
 	}
 
 	// Check whether the embedded type is listed among the supported types for
 	// the registered tag. If not, skip processing the tag.
 
 	it, err := ie.tagIndex.Get(ii, tagId)
 	if err != nil {
 		if log.Is(err, ErrTagNotFound) == true {
 			ifdEnumerateLogger.Warningf(nil, "Tag (0x%04x) is not known and will be skipped.", tagId)
 
 			ite = &IfdTagEntry{
 				tagId: tagId,
 			}
 
 			return ite, ErrTagNotFound
 		}
 
 		log.Panic(err)
 	}
 
 	// If we're trying to be as forgiving as possible then use whatever type was
 	// reported in the format. Otherwise, only accept a type that's expected for
 	// this tag.
 	if ie.tagIndex.UniversalSearch() == false && it.DoesSupportType(tagType) == false {
 		// The type in the stream disagrees with the type that this tag is
 		// expected to have. This can present issues with how we handle the
 		// special-case tags (e.g. thumbnails, GPS, etc..) when those tags
 		// suddenly have data that we no longer manipulate correctly/
 		// accurately.
 		ifdEnumerateLogger.Warningf(nil,
 			"Tag (0x%04x) in IFD [%s] at position (%d) has unsupported type (0x%02x) and will be skipped.",
 			tagId, ii, tagPosition, int(tagType))
 
 		return nil, ErrTagTypeNotValid
 	}
 
 	// Construct tag struct.
 
 	rs, err := ie.ebs.GetReadSeeker(0)
 	log.PanicIf(err)
 
 	ite = newIfdTagEntry(
 		ii,
 		tagId,
 		tagPosition,
 		tagType,
 		unitCount,
 		valueOffset,
 		rawValueOffset,
 		rs,
 		ie.byteOrder)
 
 	ifdPath := ii.UnindexedString()
 
 	// If it's an IFD but not a standard one, it'll just be seen as a LONG
 	// (the standard IFD tag type), later, unless we skip it because it's
 	// [likely] not even in the standard list of known tags.
 	mi, err := ie.ifdMapping.GetChild(ifdPath, tagId)
 	if err == nil {
 		currentIfdTag := ii.IfdTag()
 
 		childIt := exifcommon.NewIfdTag(&currentIfdTag, tagId, mi.Name)
 		iiChild := ii.NewChild(childIt, 0)
 		ite.SetChildIfd(iiChild)
 
 		// We also need to set `tag.ChildFqIfdPath` but can't do it here
 		// because we don't have the IFD index.
 	} else if log.Is(err, exifcommon.ErrChildIfdNotMapped) == false {
 		log.Panic(err)
 	}
 
 	return ite, nil
 }
 
 // TagVisitorFn is called for each tag when enumerating through the EXIF.
 type TagVisitorFn func(ite *IfdTagEntry) (err error)
 
 // tagPostParse do some tag-level processing here following the parse of each.
 func (ie *IfdEnumerate) tagPostParse(ite *IfdTagEntry, med *MiscellaneousExifData) (err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add test
 
 	ii := ite.IfdIdentity()
 
 	tagId := ite.TagId()
 	tagType := ite.TagType()
 
 	it, err := ie.tagIndex.Get(ii, tagId)
 	if err == nil {
 		ite.setTagName(it.Name)
 	} else {
 		if err != ErrTagNotFound {
 			log.Panic(err)
 		}
 
 		// This is an unknown tag.
 
 		originalBt := exifcommon.BasicTag{
 			FqIfdPath: ii.String(),
 			IfdPath:   ii.UnindexedString(),
 			TagId:     tagId,
 		}
 
 		if med != nil {
 			med.unknownTags[originalBt] = exifcommon.BasicTag{}
 		}
 
 		utilityLogger.Debugf(nil,
 			"Tag (0x%04x) is not valid for IFD [%s]. Attempting secondary "+
 				"lookup.", tagId, ii.String())
 
 		// This will overwrite the existing `it` and `err`. Since `FindFirst()`
 		// might generate different Errors than `Get()`, the log message above
 		// is import to try and mitigate confusion in that case.
 		it, err = ie.tagIndex.FindFirst(tagId, tagType, nil)
 		if err != nil {
 			if err != ErrTagNotFound {
 				log.Panic(err)
 			}
 
 			// This is supposed to be a convenience function and if we were
 			// to keep the name empty or set it to some placeholder, it
 			// might be mismanaged by the package that is calling us. If
 			// they want to specifically manage these types of tags, they
 			// can use more advanced functionality to specifically -handle
 			// unknown tags.
 			utilityLogger.Warningf(nil,
 				"Tag with ID (0x%04x) in IFD [%s] is not recognized and "+
 					"will be ignored.", tagId, ii.String())
 
 			return ErrTagNotFound
 		}
 
 		ite.setTagName(it.Name)
 
 		utilityLogger.Warningf(nil,
 			"Tag with ID (0x%04x) is not valid for IFD [%s], but it *is* "+
 				"valid as tag [%s] under IFD [%s] and has the same type "+
 				"[%s], so we will use that. This EXIF blob was probably "+
 				"written by a buggy implementation.",
 			tagId, ii.UnindexedString(), it.Name, it.IfdPath,
 			tagType)
 
 		if med != nil {
 			med.unknownTags[originalBt] = exifcommon.BasicTag{
 				IfdPath: it.IfdPath,
 				TagId:   tagId,
 			}
 		}
 	}
 
 	// This is a known tag (from the standard, unless the user did
 	// something different).
 
 	// Skip any tags that have a type that doesn't match the type in the
 	// index (which is loaded with the standard and accept tag
 	// information unless configured otherwise).
 	//
 	// We've run into multiple instances of the same tag, where a) no
 	// tag should ever be repeated, and b) all but one had an incorrect
 	// type and caused parsing/conversion woes. So, this is a quick fix
 	// for those scenarios.
 	if ie.tagIndex.UniversalSearch() == false && it.DoesSupportType(tagType) == false {
 		ifdEnumerateLogger.Warningf(nil,
 			"Skipping tag [%s] (0x%04x) [%s] with an unexpected type: %v ∉ %v",
 			ii.UnindexedString(), tagId, it.Name,
 			tagType, it.SupportedTypes)
 
 		return ErrTagNotFound
 	}
 
 	return nil
 }
 
 // parseIfd decodes the IFD block that we're currently sitting on the first
 // byte of.
 func (ie *IfdEnumerate) parseIfd(ii *exifcommon.IfdIdentity, bp *byteParser, visitor TagVisitorFn, doDescend bool, med *MiscellaneousExifData) (nextIfdOffset uint32, entries []*IfdTagEntry, thumbnailData []byte, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	tagCount, _, err := bp.getUint16()
 	log.PanicIf(err)
 
 	ifdEnumerateLogger.Debugf(nil, "IFD [%s] tag-count: (%d)", ii.String(), tagCount)
 
 	entries = make([]*IfdTagEntry, 0)
 
 	var enumeratorThumbnailOffset *IfdTagEntry
 	var enumeratorThumbnailSize *IfdTagEntry
 
 	for i := 0; i < int(tagCount); i++ {
 		ite, err := ie.parseTag(ii, i, bp)
 		if err != nil {
 			if log.Is(err, ErrTagNotFound) == true || log.Is(err, ErrTagTypeNotValid) == true {
 				// These tags should've been fully logged in parseTag(). The
 				// ITE returned is nil so we can't print anything about them, now.
 				continue
 			}
 
 			log.Panic(err)
 		}
 
 		err = ie.tagPostParse(ite, med)
 		if err == nil {
 			if err == ErrTagNotFound {
 				continue
 			}
 
 			log.PanicIf(err)
 		}
 
 		tagId := ite.TagId()
 
 		if visitor != nil {
 			err := visitor(ite)
 			log.PanicIf(err)
 		}
 
 		if ite.IsThumbnailOffset() == true {
 			ifdEnumerateLogger.Debugf(nil, "Skipping the thumbnail offset tag (0x%04x). Use accessors to get it or set it.", tagId)
 
 			enumeratorThumbnailOffset = ite
 			entries = append(entries, ite)
 
 			continue
 		} else if ite.IsThumbnailSize() == true {
 			ifdEnumerateLogger.Debugf(nil, "Skipping the thumbnail size tag (0x%04x). Use accessors to get it or set it.", tagId)
 
 			enumeratorThumbnailSize = ite
 			entries = append(entries, ite)
 
 			continue
 		}
 
 		if ite.TagType() != exifcommon.TypeUndefined {
 			// If this tag's value is an offset, bump our max-offset value to
 			// what that offset is plus however large that value is.
 
 			vc := ite.getValueContext()
 
 			farOffset, err := vc.GetFarOffset()
 			if err == nil {
 				candidateOffset := farOffset + uint32(vc.SizeInBytes())
 				if candidateOffset > ie.furthestOffset {
 					ie.furthestOffset = candidateOffset
 				}
 			} else if err != exifcommon.ErrNotFarValue {
 				log.PanicIf(err)
 			}
 		}
 
 		// If it's an IFD but not a standard one, it'll just be seen as a LONG
 		// (the standard IFD tag type), later, unless we skip it because it's
 		// [likely] not even in the standard list of known tags.
 		if ite.ChildIfdPath() != "" {
 			if doDescend == true {
 				ifdEnumerateLogger.Debugf(nil, "Descending from IFD [%s] to IFD [%s].", ii, ite.ChildIfdPath())
 
 				currentIfdTag := ii.IfdTag()
 
 				childIfdTag :=
 					exifcommon.NewIfdTag(
 						&currentIfdTag,
 						ite.TagId(),
 						ite.ChildIfdName())
 
 				iiChild := ii.NewChild(childIfdTag, 0)
 
 				err := ie.scan(iiChild, ite.getValueOffset(), visitor, med)
 				log.PanicIf(err)
 
 				ifdEnumerateLogger.Debugf(nil, "Ascending from IFD [%s] to IFD [%s].", ite.ChildIfdPath(), ii)
 			}
 		}
 
 		entries = append(entries, ite)
 	}
 
 	if enumeratorThumbnailOffset != nil && enumeratorThumbnailSize != nil {
 		thumbnailData, err = ie.parseThumbnail(enumeratorThumbnailOffset, enumeratorThumbnailSize)
 		if err != nil {
 			ifdEnumerateLogger.Errorf(
 				nil, err,
 				"We tried to bump our furthest-offset counter but there was an issue first seeking past the thumbnail.")
 		} else {
 			// In this case, the value is always an offset.
 			offset := enumeratorThumbnailOffset.getValueOffset()
 
 			// This this case, the value is always a length.
 			length := enumeratorThumbnailSize.getValueOffset()
 
 			ifdEnumerateLogger.Debugf(nil, "Found thumbnail in IFD [%s]. Its offset is (%d) and is (%d) bytes.", ii, offset, length)
 
 			furthestOffset := offset + length
 
 			if furthestOffset > ie.furthestOffset {
 				ie.furthestOffset = furthestOffset
 			}
 		}
 	}
 
 	nextIfdOffset, _, err = bp.getUint32()
 	log.PanicIf(err)
 
 	_, alreadyVisited := ie.visitedIfdOffsets[nextIfdOffset]
 
 	if alreadyVisited == true {
 		ifdEnumerateLogger.Warningf(nil, "IFD at offset (0x%08x) has been linked-to more than once. There might be a cycle in the IFD chain. Not reparsing.", nextIfdOffset)
 		nextIfdOffset = 0
 	}
 
 	if nextIfdOffset != 0 {
 		ie.visitedIfdOffsets[nextIfdOffset] = struct{}{}
 		ifdEnumerateLogger.Debugf(nil, "[%s] Next IFD at offset: (0x%08x)", ii.String(), nextIfdOffset)
 	} else {
 		ifdEnumerateLogger.Debugf(nil, "[%s] IFD chain has terminated.", ii.String())
 	}
 
 	return nextIfdOffset, entries, thumbnailData, nil
 }
 
 func (ie *IfdEnumerate) parseThumbnail(offsetIte, lengthIte *IfdTagEntry) (thumbnailData []byte, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	vRaw, err := lengthIte.Value()
 	log.PanicIf(err)
 
 	vList := vRaw.([]uint32)
 	if len(vList) != 1 {
 		log.Panicf("not exactly one long: (%d)", len(vList))
 	}
 
 	length := vList[0]
 
 	// The tag is official a LONG type, but it's actually an offset to a blob of bytes.
 	offsetIte.updateTagType(exifcommon.TypeByte)
 	offsetIte.updateUnitCount(length)
 
 	thumbnailData, err = offsetIte.GetRawBytes()
 	log.PanicIf(err)
 
 	return thumbnailData, nil
 }
 
 // scan parses and enumerates the different IFD blocks and invokes a visitor
 // callback for each tag. No information is kept or returned.
 func (ie *IfdEnumerate) scan(iiGeneral *exifcommon.IfdIdentity, ifdOffset uint32, visitor TagVisitorFn, med *MiscellaneousExifData) (err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add test
 
 	for ifdIndex := 0; ; ifdIndex++ {
 		iiSibling := iiGeneral.NewSibling(ifdIndex)
 
 		ifdEnumerateLogger.Debugf(nil, "Parsing IFD [%s] at offset (0x%04x) (scan).", iiSibling.String(), ifdOffset)
 
 		bp, err := ie.getByteParser(ifdOffset)
 		if err != nil {
 			if err == ErrOffsetInvalid {
 				ifdEnumerateLogger.Errorf(nil, nil, "IFD [%s] at offset (0x%04x) is unreachable. Terminating scan.", iiSibling.String(), ifdOffset)
 				break
 			}
 
 			log.Panic(err)
 		}
 
 		nextIfdOffset, _, _, err := ie.parseIfd(iiSibling, bp, visitor, true, med)
 		log.PanicIf(err)
 
 		currentOffset := bp.CurrentOffset()
 		if currentOffset > ie.furthestOffset {
 			ie.furthestOffset = currentOffset
 		}
 
 		if nextIfdOffset == 0 {
 			break
 		}
 
 		ifdOffset = nextIfdOffset
 	}
 
 	return nil
 }
 
 // MiscellaneousExifData is reports additional data collected during the parse.
 type MiscellaneousExifData struct {
 	// UnknownTags contains all tags that were invalid for their containing
 	// IFDs. The values represent alternative IFDs that were correctly matched
 	// to those tags and used instead.
 	unknownTags map[exifcommon.BasicTag]exifcommon.BasicTag
 }
 
 // UnknownTags returns the unknown tags encountered during the scan.
 func (med *MiscellaneousExifData) UnknownTags() map[exifcommon.BasicTag]exifcommon.BasicTag {
 	return med.unknownTags
 }
 
 // ScanOptions tweaks parser behavior/choices.
 type ScanOptions struct {
 	// NOTE(dustin): Reserved for future usage.
 }
 
 // Scan enumerates the different EXIF blocks (called IFDs). `rootIfdName` will
 // be "IFD" in the TIFF standard.
 func (ie *IfdEnumerate) Scan(iiRoot *exifcommon.IfdIdentity, ifdOffset uint32, visitor TagVisitorFn, so *ScanOptions) (med *MiscellaneousExifData, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add test
 
 	med = &MiscellaneousExifData{
 		unknownTags: make(map[exifcommon.BasicTag]exifcommon.BasicTag),
 	}
 
 	err = ie.scan(iiRoot, ifdOffset, visitor, med)
 	log.PanicIf(err)
 
 	ifdEnumerateLogger.Debugf(nil, "Scan: It looks like the furthest offset that contained EXIF data in the EXIF blob was (%d) (Scan).", ie.FurthestOffset())
 
 	return med, nil
 }
 
 // Ifd represents a single, parsed IFD.
 type Ifd struct {
 	ifdIdentity *exifcommon.IfdIdentity
 
 	ifdMapping *exifcommon.IfdMapping
 	tagIndex   *TagIndex
 
 	offset    uint32
 	byteOrder binary.ByteOrder
 	id        int
 
 	parentIfd *Ifd
 
 	// ParentTagIndex is our tag position in the parent IFD, if we had a parent
 	// (if `ParentIfd` is not nil and we weren't an IFD referenced as a sibling
 	// instead of as a child).
 	parentTagIndex int
 
 	entries        []*IfdTagEntry
 	entriesByTagId map[uint16][]*IfdTagEntry
 
 	children      []*Ifd
 	childIfdIndex map[string]*Ifd
 
 	thumbnailData []byte
 
 	nextIfdOffset uint32
 	nextIfd       *Ifd
 }
 
 // IfdIdentity returns IFD identity that this struct represents.
 func (ifd *Ifd) IfdIdentity() *exifcommon.IfdIdentity {
 	return ifd.ifdIdentity
 }
 
 // Entries returns a flat list of all tags for this IFD.
 func (ifd *Ifd) Entries() []*IfdTagEntry {
 
 	// TODO(dustin): Add test
 
 	return ifd.entries
 }
 
 // EntriesByTagId returns a map of all tags for this IFD.
 func (ifd *Ifd) EntriesByTagId() map[uint16][]*IfdTagEntry {
 
 	// TODO(dustin): Add test
 
 	return ifd.entriesByTagId
 }
 
 // Children returns a flat list of all child IFDs of this IFD.
 func (ifd *Ifd) Children() []*Ifd {
 
 	// TODO(dustin): Add test
 
 	return ifd.children
 }
 
 // ChildWithIfdPath returns a map of all child IFDs of this IFD.
 func (ifd *Ifd) ChildIfdIndex() map[string]*Ifd {
 
 	// TODO(dustin): Add test
 
 	return ifd.childIfdIndex
 }
 
 // ParentTagIndex returns the position of this IFD's tag in its parent IFD (*if*
 // there is a parent).
 func (ifd *Ifd) ParentTagIndex() int {
 
 	// TODO(dustin): Add test
 
 	return ifd.parentTagIndex
 }
 
 // Offset returns the offset of the IFD in the stream.
 func (ifd *Ifd) Offset() uint32 {
 
 	// TODO(dustin): Add test
 
 	return ifd.offset
 }
 
 // Offset returns the offset of the IFD in the stream.
 func (ifd *Ifd) ByteOrder() binary.ByteOrder {
 
 	// TODO(dustin): Add test
 
 	return ifd.byteOrder
 }
 
 // NextIfd returns the Ifd struct for the next IFD in the chain.
 func (ifd *Ifd) NextIfd() *Ifd {
 
 	// TODO(dustin): Add test
 
 	return ifd.nextIfd
 }
 
 // ChildWithIfdPath returns an `Ifd` struct for the given child of the current
 // IFD.
 func (ifd *Ifd) ChildWithIfdPath(iiChild *exifcommon.IfdIdentity) (childIfd *Ifd, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): This is a bridge while we're introducing the IFD type-system. We should be able to use the (IfdIdentity).Equals() method for this.
 	ifdPath := iiChild.UnindexedString()
 
 	for _, childIfd := range ifd.children {
 		if childIfd.ifdIdentity.UnindexedString() == ifdPath {
 			return childIfd, nil
 		}
 	}
 
 	log.Panic(ErrTagNotFound)
 	return nil, nil
 }
 
 // FindTagWithId returns a list of tags (usually just zero or one) that match
 // the given tag ID. This is efficient.
 func (ifd *Ifd) FindTagWithId(tagId uint16) (results []*IfdTagEntry, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	results, found := ifd.entriesByTagId[tagId]
 	if found != true {
 		log.Panic(ErrTagNotFound)
 	}
 
 	return results, nil
 }
 
 // FindTagWithName returns a list of tags (usually just zero or one) that match
 // the given tag name. This is not efficient (though the labor is trivial).
 func (ifd *Ifd) FindTagWithName(tagName string) (results []*IfdTagEntry, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	it, err := ifd.tagIndex.GetWithName(ifd.ifdIdentity, tagName)
 	if log.Is(err, ErrTagNotFound) == true {
 		log.Panic(ErrTagNotKnown)
 	} else if err != nil {
 		log.Panic(err)
 	}
 
 	results = make([]*IfdTagEntry, 0)
 	for _, ite := range ifd.entries {
 		if ite.TagId() == it.Id {
 			results = append(results, ite)
 		}
 	}
 
 	if len(results) == 0 {
 		log.Panic(ErrTagNotFound)
 	}
 
 	return results, nil
 }
 
 // String returns a description string.
 func (ifd *Ifd) String() string {
 	parentOffset := uint32(0)
 	if ifd.parentIfd != nil {
 		parentOffset = ifd.parentIfd.offset
 	}
 
 	return fmt.Sprintf("Ifd<ID=(%d) IFD-PATH=[%s] INDEX=(%d) COUNT=(%d) OFF=(0x%04x) CHILDREN=(%d) PARENT=(0x%04x) NEXT-IFD=(0x%04x)>", ifd.id, ifd.ifdIdentity.UnindexedString(), ifd.ifdIdentity.Index(), len(ifd.entries), ifd.offset, len(ifd.children), parentOffset, ifd.nextIfdOffset)
 }
 
 // Thumbnail returns the raw thumbnail bytes. This is typically directly
 // readable by any standard image viewer.
 func (ifd *Ifd) Thumbnail() (data []byte, err error) {
 
 	if ifd.thumbnailData == nil {
 		return nil, ErrNoThumbnail
 	}
 
 	return ifd.thumbnailData, nil
 }
 
 // dumpTags recursively builds a list of tags from an IFD.
 func (ifd *Ifd) dumpTags(tags []*IfdTagEntry) []*IfdTagEntry {
 	if tags == nil {
 		tags = make([]*IfdTagEntry, 0)
 	}
 
 	// Now, print the tags while also descending to child-IFDS as we encounter them.
 
 	ifdsFoundCount := 0
 
 	for _, ite := range ifd.entries {
 		tags = append(tags, ite)
 
 		childIfdPath := ite.ChildIfdPath()
 		if childIfdPath != "" {
 			ifdsFoundCount++
 
 			childIfd, found := ifd.childIfdIndex[childIfdPath]
 			if found != true {
 				log.Panicf("alien child IFD referenced by a tag: [%s]", childIfdPath)
 			}
 
 			tags = childIfd.dumpTags(tags)
 		}
 	}
 
 	if len(ifd.children) != ifdsFoundCount {
 		log.Panicf("have one or more dangling child IFDs: (%d) != (%d)", len(ifd.children), ifdsFoundCount)
 	}
 
 	if ifd.nextIfd != nil {
 		tags = ifd.nextIfd.dumpTags(tags)
 	}
 
 	return tags
 }
 
 // DumpTags prints the IFD hierarchy.
 func (ifd *Ifd) DumpTags() []*IfdTagEntry {
 	return ifd.dumpTags(nil)
 }
 
 func (ifd *Ifd) printTagTree(populateValues bool, index, level int, nextLink bool) {
 	indent := strings.Repeat(" ", level*2)
 
 	prefix := " "
 	if nextLink {
 		prefix = ">"
 	}
 
 	fmt.Printf("%s%sIFD: %s\n", indent, prefix, ifd)
 
 	// Now, print the tags while also descending to child-IFDS as we encounter them.
 
 	ifdsFoundCount := 0
 
 	for _, ite := range ifd.entries {
 		if ite.ChildIfdPath() != "" {
 			fmt.Printf("%s - TAG: %s\n", indent, ite)
 		} else {
 			// This will just add noise to the output (byte-tags are fully
 			// dumped).
 			if ite.IsThumbnailOffset() == true || ite.IsThumbnailSize() == true {
 				continue
 			}
 
 			it, err := ifd.tagIndex.Get(ifd.ifdIdentity, ite.TagId())
 
 			tagName := ""
 			if err == nil {
 				tagName = it.Name
 			}
 
 			var valuePhrase string
 			if populateValues == true {
 				var err error
 
 				valuePhrase, err = ite.Format()
 				if err != nil {
 					if log.Is(err, exifcommon.ErrUnhandledUndefinedTypedTag) == true {
 						ifdEnumerateLogger.Warningf(nil, "Skipping non-standard undefined tag: [%s] (%04x)", ifd.ifdIdentity.UnindexedString(), ite.TagId())
 						continue
 					} else if err == exifundefined.ErrUnparseableValue {
 						ifdEnumerateLogger.Warningf(nil, "Skipping unparseable undefined tag: [%s] (%04x) [%s]", ifd.ifdIdentity.UnindexedString(), ite.TagId(), it.Name)
 						continue
 					}
 
 					log.Panic(err)
 				}
 			} else {
 				valuePhrase = "!UNRESOLVED"
 			}
 
 			fmt.Printf("%s - TAG: %s NAME=[%s] VALUE=[%v]\n", indent, ite, tagName, valuePhrase)
 		}
 
 		childIfdPath := ite.ChildIfdPath()
 		if childIfdPath != "" {
 			ifdsFoundCount++
 
 			childIfd, found := ifd.childIfdIndex[childIfdPath]
 			if found != true {
 				log.Panicf("alien child IFD referenced by a tag: [%s]", childIfdPath)
 			}
 
 			childIfd.printTagTree(populateValues, 0, level+1, false)
 		}
 	}
 
 	if len(ifd.children) != ifdsFoundCount {
 		log.Panicf("have one or more dangling child IFDs: (%d) != (%d)", len(ifd.children), ifdsFoundCount)
 	}
 
 	if ifd.nextIfd != nil {
 		ifd.nextIfd.printTagTree(populateValues, index+1, level, true)
 	}
 }
 
 // PrintTagTree prints the IFD hierarchy.
 func (ifd *Ifd) PrintTagTree(populateValues bool) {
 	ifd.printTagTree(populateValues, 0, 0, false)
 }
 
 func (ifd *Ifd) printIfdTree(level int, nextLink bool) {
 	indent := strings.Repeat(" ", level*2)
 
 	prefix := " "
 	if nextLink {
 		prefix = ">"
 	}
 
 	fmt.Printf("%s%s%s\n", indent, prefix, ifd)
 
 	// Now, print the tags while also descending to child-IFDS as we encounter them.
 
 	ifdsFoundCount := 0
 
 	for _, ite := range ifd.entries {
 		childIfdPath := ite.ChildIfdPath()
 		if childIfdPath != "" {
 			ifdsFoundCount++
 
 			childIfd, found := ifd.childIfdIndex[childIfdPath]
 			if found != true {
 				log.Panicf("alien child IFD referenced by a tag: [%s]", childIfdPath)
 			}
 
 			childIfd.printIfdTree(level+1, false)
 		}
 	}
 
 	if len(ifd.children) != ifdsFoundCount {
 		log.Panicf("have one or more dangling child IFDs: (%d) != (%d)", len(ifd.children), ifdsFoundCount)
 	}
 
 	if ifd.nextIfd != nil {
 		ifd.nextIfd.printIfdTree(level, true)
 	}
 }
 
 // PrintIfdTree prints the IFD hierarchy.
 func (ifd *Ifd) PrintIfdTree() {
 	ifd.printIfdTree(0, false)
 }
 
 func (ifd *Ifd) dumpTree(tagsDump []string, level int) []string {
 	if tagsDump == nil {
 		tagsDump = make([]string, 0)
 	}
 
 	indent := strings.Repeat(" ", level*2)
 
 	var ifdPhrase string
 	if ifd.parentIfd != nil {
 		ifdPhrase = fmt.Sprintf("[%s]->[%s]:(%d)", ifd.parentIfd.ifdIdentity.UnindexedString(), ifd.ifdIdentity.UnindexedString(), ifd.ifdIdentity.Index())
 	} else {
 		ifdPhrase = fmt.Sprintf("[ROOT]->[%s]:(%d)", ifd.ifdIdentity.UnindexedString(), ifd.ifdIdentity.Index())
 	}
 
 	startBlurb := fmt.Sprintf("%s> IFD %s TOP", indent, ifdPhrase)
 	tagsDump = append(tagsDump, startBlurb)
 
 	ifdsFoundCount := 0
 	for _, ite := range ifd.entries {
 		tagsDump = append(tagsDump, fmt.Sprintf("%s  - (0x%04x)", indent, ite.TagId()))
 
 		childIfdPath := ite.ChildIfdPath()
 		if childIfdPath != "" {
 			ifdsFoundCount++
 
 			childIfd, found := ifd.childIfdIndex[childIfdPath]
 			if found != true {
 				log.Panicf("alien child IFD referenced by a tag: [%s]", childIfdPath)
 			}
 
 			tagsDump = childIfd.dumpTree(tagsDump, level+1)
 		}
 	}
 
 	if len(ifd.children) != ifdsFoundCount {
 		log.Panicf("have one or more dangling child IFDs: (%d) != (%d)", len(ifd.children), ifdsFoundCount)
 	}
 
 	finishBlurb := fmt.Sprintf("%s< IFD %s BOTTOM", indent, ifdPhrase)
 	tagsDump = append(tagsDump, finishBlurb)
 
 	if ifd.nextIfd != nil {
 		siblingBlurb := fmt.Sprintf("%s* LINKING TO SIBLING IFD [%s]:(%d)", indent, ifd.nextIfd.ifdIdentity.UnindexedString(), ifd.nextIfd.ifdIdentity.Index())
 		tagsDump = append(tagsDump, siblingBlurb)
 
 		tagsDump = ifd.nextIfd.dumpTree(tagsDump, level)
 	}
 
 	return tagsDump
 }
 
 // DumpTree returns a list of strings describing the IFD hierarchy.
 func (ifd *Ifd) DumpTree() []string {
 	return ifd.dumpTree(nil, 0)
 }
 
 // GpsInfo parses and consolidates the GPS info. This can only be called on the
 // GPS IFD.
 func (ifd *Ifd) GpsInfo() (gi *GpsInfo, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	gi = new(GpsInfo)
 
 	if ifd.ifdIdentity.Equals(exifcommon.IfdGpsInfoStandardIfdIdentity) == false {
 		log.Panicf("GPS can only be read on GPS IFD: [%s]", ifd.ifdIdentity.UnindexedString())
 	}
 
 	if tags, found := ifd.entriesByTagId[TagGpsVersionId]; found == false {
 		// We've seen this. We'll just have to default to assuming we're in a
 		// 2.2.0.0 format.
 		ifdEnumerateLogger.Warningf(nil, "No GPS version tag (0x%04x) found.", TagGpsVersionId)
 	} else {
 		versionBytes, err := tags[0].GetRawBytes()
 		log.PanicIf(err)
 
 		hit := false
 		for _, acceptedGpsVersion := range ValidGpsVersions {
 			if bytes.Compare(versionBytes, acceptedGpsVersion[:]) == 0 {
 				hit = true
 				break
 			}
 		}
 
 		if hit != true {
 			ifdEnumerateLogger.Warningf(nil, "GPS version not supported: %v", versionBytes)
 			log.Panic(ErrNoGpsTags)
 		}
 	}
 
 	tags, found := ifd.entriesByTagId[TagLatitudeId]
 	if found == false {
 		ifdEnumerateLogger.Warningf(nil, "latitude not found")
 		log.Panic(ErrNoGpsTags)
 	}
 
 	latitudeValue, err := tags[0].Value()
 	log.PanicIf(err)
 
 	// Look for whether North or South.
 	tags, found = ifd.entriesByTagId[TagLatitudeRefId]
 	if found == false {
 		ifdEnumerateLogger.Warningf(nil, "latitude-ref not found")
 		log.Panic(ErrNoGpsTags)
 	}
 
 	latitudeRefValue, err := tags[0].Value()
 	log.PanicIf(err)
 
 	tags, found = ifd.entriesByTagId[TagLongitudeId]
 	if found == false {
 		ifdEnumerateLogger.Warningf(nil, "longitude not found")
 		log.Panic(ErrNoGpsTags)
 	}
 
 	longitudeValue, err := tags[0].Value()
 	log.PanicIf(err)
 
 	// Look for whether West or East.
 	tags, found = ifd.entriesByTagId[TagLongitudeRefId]
 	if found == false {
 		ifdEnumerateLogger.Warningf(nil, "longitude-ref not found")
 		log.Panic(ErrNoGpsTags)
 	}
 
 	longitudeRefValue, err := tags[0].Value()
 	log.PanicIf(err)
 
 	// Parse location.
 
 	latitudeRaw := latitudeValue.([]exifcommon.Rational)
 
 	gi.Latitude, err = NewGpsDegreesFromRationals(latitudeRefValue.(string), latitudeRaw)
 	log.PanicIf(err)
 
 	longitudeRaw := longitudeValue.([]exifcommon.Rational)
 
 	gi.Longitude, err = NewGpsDegreesFromRationals(longitudeRefValue.(string), longitudeRaw)
 	log.PanicIf(err)
 
 	// Parse altitude.
 
 	altitudeTags, foundAltitude := ifd.entriesByTagId[TagAltitudeId]
 	altitudeRefTags, foundAltitudeRef := ifd.entriesByTagId[TagAltitudeRefId]
 
 	if foundAltitude == true && foundAltitudeRef == true {
 		altitudePhrase, err := altitudeTags[0].Format()
 		log.PanicIf(err)
 
 		ifdEnumerateLogger.Debugf(nil, "Altitude is [%s].", altitudePhrase)
 
 		altitudeValue, err := altitudeTags[0].Value()
 		log.PanicIf(err)
 
 		altitudeRefPhrase, err := altitudeRefTags[0].Format()
 		log.PanicIf(err)
 
 		ifdEnumerateLogger.Debugf(nil, "Altitude-reference is [%s].", altitudeRefPhrase)
 
 		altitudeRefValue, err := altitudeRefTags[0].Value()
 		log.PanicIf(err)
 
 		altitudeRaw := altitudeValue.([]exifcommon.Rational)
 		if altitudeRaw[0].Denominator > 0 {
 			altitude := int(altitudeRaw[0].Numerator / altitudeRaw[0].Denominator)
 
 			if altitudeRefValue.([]byte)[0] == 1 {
 				altitude *= -1
 			}
 
 			gi.Altitude = altitude
 		}
 	}
 
 	// Parse timestamp from separate date and time tags.
 
 	timestampTags, foundTimestamp := ifd.entriesByTagId[TagTimestampId]
 	datestampTags, foundDatestamp := ifd.entriesByTagId[TagDatestampId]
 
 	if foundTimestamp == true && foundDatestamp == true {
 		datestampValue, err := datestampTags[0].Value()
 		log.PanicIf(err)
 
 		datePhrase := datestampValue.(string)
 		ifdEnumerateLogger.Debugf(nil, "Date tag value is [%s].", datePhrase)
 
 		// Normalize the separators.
 		datePhrase = strings.ReplaceAll(datePhrase, "-", ":")
 
 		dateParts := strings.Split(datePhrase, ":")
 
 		year, err1 := strconv.ParseUint(dateParts[0], 10, 16)
 		month, err2 := strconv.ParseUint(dateParts[1], 10, 8)
 		day, err3 := strconv.ParseUint(dateParts[2], 10, 8)
 
 		if err1 == nil && err2 == nil && err3 == nil {
 			timestampValue, err := timestampTags[0].Value()
 			log.PanicIf(err)
 
 			timePhrase, err := timestampTags[0].Format()
 			log.PanicIf(err)
 
 			ifdEnumerateLogger.Debugf(nil, "Time tag value is [%s].", timePhrase)
 
 			timestampRaw := timestampValue.([]exifcommon.Rational)
 
 			hour := int(timestampRaw[0].Numerator / timestampRaw[0].Denominator)
 			minute := int(timestampRaw[1].Numerator / timestampRaw[1].Denominator)
 			second := int(timestampRaw[2].Numerator / timestampRaw[2].Denominator)
 
 			gi.Timestamp = time.Date(int(year), time.Month(month), int(day), hour, minute, second, 0, time.UTC)
 		}
 	}
 
 	return gi, nil
 }
 
 // ParsedTagVisitor is a callback used if wanting to visit through all tags and
 // child IFDs from the current IFD and going down.
 type ParsedTagVisitor func(*Ifd, *IfdTagEntry) error
 
 // EnumerateTagsRecursively calls the given visitor function for every tag and
 // IFD in the current IFD, recursively.
 func (ifd *Ifd) EnumerateTagsRecursively(visitor ParsedTagVisitor) (err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	for ptr := ifd; ptr != nil; ptr = ptr.nextIfd {
 		for _, ite := range ifd.entries {
 			childIfdPath := ite.ChildIfdPath()
 			if childIfdPath != "" {
 				childIfd := ifd.childIfdIndex[childIfdPath]
 
 				err := childIfd.EnumerateTagsRecursively(visitor)
 				log.PanicIf(err)
 			} else {
 				err := visitor(ifd, ite)
 				log.PanicIf(err)
 			}
 		}
 	}
 
 	return nil
 }
 
 // QueuedIfd is one IFD that has been identified but yet to be processed.
 type QueuedIfd struct {
 	IfdIdentity *exifcommon.IfdIdentity
 
 	Offset uint32
 	Parent *Ifd
 
 	// ParentTagIndex is our tag position in the parent IFD, if we had a parent
 	// (if `ParentIfd` is not nil and we weren't an IFD referenced as a sibling
 	// instead of as a child).
 	ParentTagIndex int
 }
 
 // IfdIndex collects a bunch of IFD and tag information stored in several
 // different ways in order to provide convenient lookups.
 type IfdIndex struct {
 	RootIfd *Ifd
 	Ifds    []*Ifd
 	Tree    map[int]*Ifd
 	Lookup  map[string]*Ifd
 }
 
 // Collect enumerates the different EXIF blocks (called IFDs) and builds out an
 // index struct for referencing all of the parsed data.
 func (ie *IfdEnumerate) Collect(rootIfdOffset uint32) (index IfdIndex, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add MiscellaneousExifData to IfdIndex
 
 	tree := make(map[int]*Ifd)
 	ifds := make([]*Ifd, 0)
 	lookup := make(map[string]*Ifd)
 
 	queue := []QueuedIfd{
 		{
 			IfdIdentity: exifcommon.IfdStandardIfdIdentity,
 			Offset:      rootIfdOffset,
 		},
 	}
 
 	edges := make(map[uint32]*Ifd)
 
 	for {
 		if len(queue) == 0 {
 			break
 		}
 
 		qi := queue[0]
 		ii := qi.IfdIdentity
 
 		offset := qi.Offset
 		parentIfd := qi.Parent
 
 		queue = queue[1:]
 
 		ifdEnumerateLogger.Debugf(nil, "Parsing IFD [%s] (%d) at offset (0x%04x) (Collect).", ii.String(), ii.Index(), offset)
 
 		bp, err := ie.getByteParser(offset)
 		if err != nil {
 			if err == ErrOffsetInvalid {
 				return index, err
 			}
 
 			log.Panic(err)
 		}
 
 		// TODO(dustin): We don't need to pass the index in as a separate argument. Get from the II.
 
 		nextIfdOffset, entries, thumbnailData, err := ie.parseIfd(ii, bp, nil, false, nil)
 		log.PanicIf(err)
 
 		currentOffset := bp.CurrentOffset()
 		if currentOffset > ie.furthestOffset {
 			ie.furthestOffset = currentOffset
 		}
 
 		id := len(ifds)
 
 		entriesByTagId := make(map[uint16][]*IfdTagEntry)
 		for _, ite := range entries {
 			tagId := ite.TagId()
 
 			tags, found := entriesByTagId[tagId]
 			if found == false {
 				tags = make([]*IfdTagEntry, 0)
 			}
 
 			entriesByTagId[tagId] = append(tags, ite)
 		}
 
 		ifd := &Ifd{
 			ifdIdentity: ii,
 
 			byteOrder: ie.byteOrder,
 
 			id: id,
 
 			parentIfd:      parentIfd,
 			parentTagIndex: qi.ParentTagIndex,
 
 			offset:         offset,
 			entries:        entries,
 			entriesByTagId: entriesByTagId,
 
 			// This is populated as each child is processed.
 			children: make([]*Ifd, 0),
 
 			nextIfdOffset: nextIfdOffset,
 			thumbnailData: thumbnailData,
 
 			ifdMapping: ie.ifdMapping,
 			tagIndex:   ie.tagIndex,
 		}
 
 		// Add ourselves to a big list of IFDs.
 		ifds = append(ifds, ifd)
 
 		// Install ourselves into a by-id lookup table (keys are unique).
 		tree[id] = ifd
 
 		// Install into by-name buckets.
 		lookup[ii.String()] = ifd
 
 		// Add a link from the previous IFD in the chain to us.
 		if previousIfd, found := edges[offset]; found == true {
 			previousIfd.nextIfd = ifd
 		}
 
 		// Attach as a child to our parent (where we appeared as a tag in
 		// that IFD).
 		if parentIfd != nil {
 			parentIfd.children = append(parentIfd.children, ifd)
 		}
 
 		// Determine if any of our entries is a child IFD and queue it.
 		for i, ite := range entries {
 			if ite.ChildIfdPath() == "" {
 				continue
 			}
 
 			tagId := ite.TagId()
 			childIfdName := ite.ChildIfdName()
 
 			currentIfdTag := ii.IfdTag()
 
 			childIfdTag :=
 				exifcommon.NewIfdTag(
 					&currentIfdTag,
 					tagId,
 					childIfdName)
 
 			iiChild := ii.NewChild(childIfdTag, 0)
 
 			qi := QueuedIfd{
 				IfdIdentity: iiChild,
 
 				Offset:         ite.getValueOffset(),
 				Parent:         ifd,
 				ParentTagIndex: i,
 			}
 
 			queue = append(queue, qi)
 		}
 
 		// If there's another IFD in the chain.
 		if nextIfdOffset != 0 {
 			iiSibling := ii.NewSibling(ii.Index() + 1)
 
 			// Allow the next link to know what the previous link was.
 			edges[nextIfdOffset] = ifd
 
 			qi := QueuedIfd{
 				IfdIdentity: iiSibling,
 				Offset:      nextIfdOffset,
 			}
 
 			queue = append(queue, qi)
 		}
 	}
 
 	index.RootIfd = tree[0]
 	index.Ifds = ifds
 	index.Tree = tree
 	index.Lookup = lookup
 
 	err = ie.setChildrenIndex(index.RootIfd)
 	log.PanicIf(err)
 
 	ifdEnumerateLogger.Debugf(nil, "Collect: It looks like the furthest offset that contained EXIF data in the EXIF blob was (%d).", ie.FurthestOffset())
 
 	return index, nil
 }
 
 func (ie *IfdEnumerate) setChildrenIndex(ifd *Ifd) (err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	childIfdIndex := make(map[string]*Ifd)
 	for _, childIfd := range ifd.children {
 		childIfdIndex[childIfd.ifdIdentity.UnindexedString()] = childIfd
 	}
 
 	ifd.childIfdIndex = childIfdIndex
 
 	for _, childIfd := range ifd.children {
 		err := ie.setChildrenIndex(childIfd)
 		log.PanicIf(err)
 	}
 
 	return nil
 }
 
 // FurthestOffset returns the furthest offset visited in the EXIF blob. This
 // *does not* account for the locations of any undefined tags since we always
 // evaluate the furthest offset, whether or not the user wants to know it.
 //
 // We are not willing to incur the cost of actually parsing those tags just to
 // know their length when there are still undefined tags that are out there
 // that we still won't have any idea how to parse, thus making this an
 // approximation regardless of how clever we get.
 func (ie *IfdEnumerate) FurthestOffset() uint32 {
 
 	// TODO(dustin): Add test
 
 	return ie.furthestOffset
 }
 
 // parseOneIfd is a hack to use an IE to parse a raw IFD block. Can be used for
 // testing. The fqIfdPath ("fully-qualified IFD path") will be less qualified
 // in that the numeric index will always be zero (the zeroth child) rather than
 // the proper number (if its actually a sibling to the first child, for
 // instance).
 func parseOneIfd(ifdMapping *exifcommon.IfdMapping, tagIndex *TagIndex, ii *exifcommon.IfdIdentity, byteOrder binary.ByteOrder, ifdBlock []byte, visitor TagVisitorFn) (nextIfdOffset uint32, entries []*IfdTagEntry, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add test
 
 	ebs := NewExifReadSeekerWithBytes(ifdBlock)
 
 	rs, err := ebs.GetReadSeeker(0)
 	log.PanicIf(err)
 
 	bp, err := newByteParser(rs, byteOrder, 0)
 	if err != nil {
 		if err == ErrOffsetInvalid {
 			return 0, nil, err
 		}
 
 		log.Panic(err)
 	}
 
 	dummyEbs := NewExifReadSeekerWithBytes([]byte{})
 	ie := NewIfdEnumerate(ifdMapping, tagIndex, dummyEbs, byteOrder)
 
 	nextIfdOffset, entries, _, err = ie.parseIfd(ii, bp, visitor, true, nil)
 	log.PanicIf(err)
 
 	return nextIfdOffset, entries, nil
 }
 
 // parseOneTag is a hack to use an IE to parse a raw tag block.
 func parseOneTag(ifdMapping *exifcommon.IfdMapping, tagIndex *TagIndex, ii *exifcommon.IfdIdentity, byteOrder binary.ByteOrder, tagBlock []byte) (ite *IfdTagEntry, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): Add test
 
 	ebs := NewExifReadSeekerWithBytes(tagBlock)
 
 	rs, err := ebs.GetReadSeeker(0)
 	log.PanicIf(err)
 
 	bp, err := newByteParser(rs, byteOrder, 0)
 	if err != nil {
 		if err == ErrOffsetInvalid {
 			return nil, err
 		}
 
 		log.Panic(err)
 	}
 
 	dummyEbs := NewExifReadSeekerWithBytes([]byte{})
 	ie := NewIfdEnumerate(ifdMapping, tagIndex, dummyEbs, byteOrder)
 
 	ite, err = ie.parseTag(ii, 0, bp)
 	log.PanicIf(err)
 
 	err = ie.tagPostParse(ite, nil)
 	if err != nil {
 		if err == ErrTagNotFound {
 			return nil, err
 		}
 
 		log.Panic(err)
 	}
 
 	return ite, nil
 }
 
 // FindIfdFromRootIfd returns the given `Ifd` given the root-IFD and path of the
 // desired IFD.
 func FindIfdFromRootIfd(rootIfd *Ifd, ifdPath string) (ifd *Ifd, err error) {
 	defer func() {
 		if state := recover(); state != nil {
 			err = log.Wrap(state.(error))
 		}
 	}()
 
 	// TODO(dustin): !! Add test.
 
 	lineage, err := rootIfd.ifdMapping.ResolvePath(ifdPath)
 	log.PanicIf(err)
 
 	// Confirm the first IFD is our root IFD type, and then prune it because
 	// from then on we'll be searching down through our children.
 
 	if len(lineage) == 0 {
 		log.Panicf("IFD path must be non-empty.")
 	} else if lineage[0].Name != exifcommon.IfdStandardIfdIdentity.Name() {
 		log.Panicf("First IFD path item must be [%s].", exifcommon.IfdStandardIfdIdentity.Name())
 	}
 
 	desiredRootIndex := lineage[0].Index
 	lineage = lineage[1:]
 
 	// TODO(dustin): !! This is a poorly conceived fix that just doubles the work we already have to do below, which then interacts badly with the indices not being properly represented in the IFD-phrase.
 	// TODO(dustin): !! <-- However, we're not sure whether we shouldn't store a secondary IFD-path with the indices. Some IFDs may not necessarily restrict which IFD indices they can be a child of (only the IFD itself matters). Validation should be delegated to the caller.
 	thisIfd := rootIfd
 	for currentRootIndex := 0; currentRootIndex < desiredRootIndex; currentRootIndex++ {
 		if thisIfd.nextIfd == nil {
 			log.Panicf("Root-IFD index (%d) does not exist in the data.", currentRootIndex)
 		}
 
 		thisIfd = thisIfd.nextIfd
 	}
 
 	for _, itii := range lineage {
 		var hit *Ifd
 		for _, childIfd := range thisIfd.children {
 			if childIfd.ifdIdentity.TagId() == itii.TagId {
 				hit = childIfd
 				break
 			}
 		}
 
 		// If we didn't find the child, add it.
 		if hit == nil {
 			log.Panicf("IFD [%s] in [%s] not found: %s", itii.Name, ifdPath, thisIfd.children)
 		}
 
 		thisIfd = hit
 
 		// If we didn't find the sibling, add it.
 		for i := 0; i < itii.Index; i++ {
 			if thisIfd.nextIfd == nil {
 				log.Panicf("IFD [%s] does not have (%d) occurrences/siblings", thisIfd.ifdIdentity.UnindexedString(), itii.Index)
 			}
 
 			thisIfd = thisIfd.nextIfd
 		}
 	}
 
 	return thisIfd, nil
 }