Merge pull request #228 from observerly/feature/astrotiff/writeIFD

feat: add writeIFD() utility to astrotiff module in @observerly/iris

pkg/astrotiff/writer.go

@@ -11,11 +11,20 @@ package astrotiff
 /*****************************************************************************************************************/
 
 import (
+	"encoding/binary"
 	"io"
+	"sort"
+
+	metadata "github.com/observerly/iris/pkg/ifd"
 )
 
 /*****************************************************************************************************************/
 
+// We only write little-endian TIFF files.
+var enc = binary.LittleEndian
+
+/*****************************************************************************************************************/
+
 // writePixels writes the internal byte array of an image to w. It is less general
 // but much faster then encode. writePixels is used when pix directly
 // corresponds to one of the TIFF image types.
@@ -36,3 +45,66 @@ func writePixels(w io.Writer, pix []byte, nrows, length, stride int) error {
 }
 
 /*****************************************************************************************************************/
+
+// writeIFD writes the Image File Directory to w. The IFD is written at the given offset.
+// The IFD is written in ascending order of the tag values.
+func writeIFD(w io.Writer, ifdOffset int, d []metadata.IFDEntry) error {
+	var buf [metadata.IFDLengthInBytes]byte
+	// Make space for "pointer area" containing IFD entry data
+	// longer than 4 bytes.
+	parea := make([]byte, 1024)
+	pstart := ifdOffset + metadata.IFDLengthInBytes*len(d) + 6
+	var o int // Current offset in parea.
+
+	// The IFD has to be written with the tags in ascending order.
+	sort.Sort(metadata.SortByTagInterface(d))
+
+	// Write the number of entries in this IFD.
+	if err := binary.Write(w, enc, uint16(len(d))); err != nil {
+		return err
+	}
+
+	for _, entry := range d {
+		enc.PutUint16(buf[0:2], uint16(entry.Tag))
+		enc.PutUint16(buf[2:4], uint16(entry.DataType))
+		count := uint32(len(entry.Data))
+		if entry.DataType == metadata.DataTypeRational {
+			count /= 2
+		}
+		enc.PutUint32(buf[4:8], count)
+		datalen := int(count * uint32(entry.DataType.ByteSize()))
+
+		if datalen <= 4 {
+			entry.PutData(buf[8:12])
+		} else {
+			if (o + datalen) > len(parea) {
+				newlen := len(parea) + 1024
+				for (o + datalen) > newlen {
+					newlen += 1024
+				}
+				newarea := make([]byte, newlen)
+				copy(newarea, parea)
+				parea = newarea
+			}
+			entry.PutData(parea[o : o+datalen])
+			enc.PutUint32(buf[8:12], uint32(pstart+o))
+			o += datalen
+		}
+
+		if _, err := w.Write(buf[:]); err != nil {
+			return err
+		}
+	}
+
+	// The IFD ends with the offset of the next IFD in the file,
+	// or zero if it is the last one (page 14).
+	if err := binary.Write(w, enc, uint32(0)); err != nil {
+		return err
+	}
+
+	_, err := w.Write(parea[:o])
+
+	return err
+}
+
+/*****************************************************************************************************************/