ifd_enumerate: Create II when `Ifd` is constructed.

- ..rather than on-the-fly. Now efficient, especially now that it's used
  for find operations.

ifd_builder.go

@@ -235,7 +235,13 @@ func NewIfdBuilder(ii IfdIdentity, byteOrder binary.ByteOrder) (ib *IfdBuilder)
 // information as the given IFD.
 func NewIfdBuilderWithExistingIfd(ifd *Ifd) (ib *IfdBuilder) {
     ii := ifd.Identity()
-    ifdTagId := IfdTagIdWithIdentityOrFail(ii)
+
+    var ifdTagId uint16
+
+    // There is no tag-ID for the root IFD. It will never be a child IFD.
+    if ii != RootIi {
+        ifdTagId = IfdTagIdWithIdentityOrFail(ii)
+    }
 
     ib = &IfdBuilder{
         ii: ii,

ifd_builder_test.go

@@ -1162,14 +1162,15 @@ func TestNewIfdBuilderFromExistingChain(t *testing.T) {
 
 
 func TestNewIfdBuilderWithExistingIfd(t *testing.T) {
-    ii, _ := IfdIdOrFail(IfdStandard, IfdGps)
-    tagId := IfdTagIdWithIdentityOrFail(ii)
+    tagId := IfdTagIdWithIdentityOrFail(GpsIi)
 
     parentIfd := &Ifd{
+        Ii: RootIi,
         Name: IfdStandard,
     }
 
     ifd := &Ifd{
+        Ii: GpsIi,
         Name: IfdGps,
         ByteOrder: TestDefaultByteOrder,
         Offset: 0x123,

ifd_enumerate.go

@@ -273,6 +273,8 @@ func (ie *IfdEnumerate) Scan(ifdOffset uint32, visitor TagVisitor) (err error) {
 type Ifd struct {
     ByteOrder binary.ByteOrder
 
+    Ii IfdIdentity
+
     Id int
     ParentIfd *Ifd
     Name string
@@ -347,15 +349,7 @@ func (ifd Ifd) String() string {
 }
 
 func (ifd Ifd) Identity() IfdIdentity {
-    parentIfdName := ""
-    if ifd.ParentIfd != nil {
-        parentIfdName = ifd.ParentIfd.Name
-    }
-
-// TODO(dustin): !! We should be checking using the parent ID, not the parent name.
-    ii, _ := IfdIdOrFail(parentIfdName, ifd.Name)
-
-    return ii
+    return ifd.Ii
 }
 
 func (ifd Ifd) printNode(level int, nextLink bool) {
@@ -426,6 +420,8 @@ func (ie *IfdEnumerate) Collect(rootIfdOffset uint32) (index IfdIndex, err error
         }
 
         ii := queue[0].Ii
+
+
         name := ii.IfdName
         index := queue[0].Index
         offset := queue[0].Offset
@@ -453,6 +449,7 @@ func (ie *IfdEnumerate) Collect(rootIfdOffset uint32) (index IfdIndex, err error
 
         ifd := Ifd{
             ByteOrder: ie.byteOrder,
+            Ii: ii,
             Id: id,
             ParentIfd: parentIfd,
             Name: name,

tags.go

@@ -315,14 +315,14 @@ func IfdTagIdWithIdentityOrFail(ii IfdIdentity) (tagId uint16) {
         if tagId, found = tags[ii.IfdName]; found == true {
             if tagId == 0 {
                 // This IFD is not the type that can be linked to from a tag.
-                log.Panicf("not a child IFD")
+                log.Panicf("not a child IFD: [%s]", ii.IfdName)
             }
 
             return tagId
         }
     }
 
-    log.Panicf("no tag for invalid IFD identity")
+    log.Panicf("no tag for invalid IFD identity: %v", ii)
     return 0
 }
 

tags_test.go

@@ -108,7 +108,7 @@ func TestIfdTagIdWithIdentityOrFail_Miss(t *testing.T) {
     defer func() {
         if state := recover(); state != nil {
             err := log.Wrap(state.(error))
-            if err.Error() != "no tag for invalid IFD identity" {
+            if err.Error() != "no tag for invalid IFD identity: IfdIdentity<PARENT-NAME=[invalid-parent] NAME=[invalid-child]>" {
                 log.Panic(err)
             }
         }