More work on writing

bufferedwriter.go

@@ -4,12 +4,60 @@ import (
 	"io"
 )
 
-type bufferedWriter struct {
-	w      io.Writer
-	buffer []bufferedBytes
+type bufferedBytes struct {
+	data []byte
+	r    offsetRange
 }
 
-type bufferedBytes struct {
-	data   []byte
-	offset int
+type offsetRange struct {
+	beg int
+	end int
+}
+
+func (o *offsetRange) offset(off int) {
+	o.beg += off
+	o.end += off
+}
+
+func (o offsetRange) within(check int) bool {
+	return check >= o.beg || check <= o.end
+}
+
+type bufferedWriter struct {
+	w          io.Writer
+	buffer     []bufferedBytes
+	mainOffset int
+}
+
+func newBufferedWriter(w io.Writer) *bufferedWriter {
+	var out bufferedWriter
+	out.w = w
+	return &out
+}
+
+func (b *bufferedWriter) WriteTo(data []byte, offset int64) (n int, err error) {
+	if int(offset) == b.mainOffset {
+		n, err = b.Write(data)
+		if err != nil {
+			return
+		}
+	}
+	newBuff := bufferedBytes{
+		data: data,
+		r: offsetRange{
+			beg: int(offset),
+			end: int(offset) + len(data),
+		},
+	}
+	b.buffer = append(b.buffer, newBuff)
+	return 0, nil
+}
+
+func (b *bufferedWriter) Write(data []byte) (int, error) {
+	n, err := b.w.Write(data)
+	b.mainOffset += n
+	if err != nil {
+		return n, err
+	}
+	return n, err
 }