exif: Added EXIF header builder and unit-test.

exif.go

@@ -22,6 +22,8 @@ const (
 
 var (
     exifLogger = log.NewLogger("exif.exif")
+
+    ExifHeaderPrefixBytes = []byte("Exif\000\000")
 )
 
 var (
@@ -31,7 +33,7 @@ var (
 
 
 func IsExif(data []byte) (ok bool) {
-    if bytes.Compare(data[:6], []byte("Exif\000\000")) == 0 {
+    if bytes.Compare(data[:6], ExifHeaderPrefixBytes) == 0 {
         return true
     }
 
@@ -39,6 +41,9 @@ func IsExif(data []byte) (ok bool) {
 }
 
 
+// TODO(dustin): Isolated this to its own packge breakout the methods as independent function. There's no use for a dedicated struct.
+
+
 type Exif struct {
 
 }
@@ -170,3 +175,32 @@ func (e *Exif) Collect(exifData []byte) (eh ExifHeader, index IfdIndex, err erro
 
     return eh, index, nil
 }
+
+func BuildExifHeader(byteOrder binary.ByteOrder, firstIfdOffset uint32) (headerBytes []byte, err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+    b := new(bytes.Buffer)
+
+    _, err = b.Write(ExifHeaderPrefixBytes)
+    log.PanicIf(err)
+
+    if byteOrder == binary.BigEndian {
+        _, err := b.WriteString("MM")
+        log.PanicIf(err)
+    } else {
+        _, err := b.WriteString("II")
+        log.PanicIf(err)
+    }
+
+    _, err = b.Write([]byte { 0x2a, 0x00 })
+    log.PanicIf(err)
+
+    err = binary.Write(b, byteOrder, firstIfdOffset)
+    log.PanicIf(err)
+
+    return b.Bytes(), nil
+}

exif_test.go

@@ -319,6 +319,22 @@ func TestCollect(t *testing.T) {
     }
 }
 
+func TestBuildAndParseExifHeader(t *testing.T) {
+    headerBytes, err := BuildExifHeader(TestDefaultByteOrder, 0x11223344)
+    log.PanicIf(err)
+
+    e := NewExif()
+
+    eh, err := e.ParseExifHeader(headerBytes)
+    log.PanicIf(err)
+
+    if eh.ByteOrder != TestDefaultByteOrder {
+        t.Fatalf("Byte-order of EXIF header not correct.")
+    } else if eh.FirstIfdOffset != 0x11223344 {
+        t.Fatalf("First IFD offset not correct.")
+    }
+}
+
 func init() {
     goPath := os.Getenv("GOPATH")
     if goPath == "" {