HashingWriter: Add documentation

backend/writer.go

@@ -46,12 +46,14 @@ func (h *HashAppendWriter) Write(p []byte) (n int, err error) {
 	return 0, errors.New("Write() called on closed HashAppendWriter")
 }
 
+// HashingWriter wraps an io.Writer to hashes all data that is written to it.
 type HashingWriter struct {
 	w    io.Writer
 	h    hash.Hash
 	size int
 }
 
+// NewHashAppendWriter wraps the writer w and feeds all data written to the hash h.
 func NewHashingWriter(w io.Writer, h hash.Hash) *HashingWriter {
 	return &HashingWriter{
 		h: h,
@@ -59,16 +61,19 @@ func NewHashingWriter(w io.Writer, h hash.Hash) *HashingWriter {
 	}
 }
 
+// Write wraps the write method of the underlying writer and also hashes all data.
 func (h *HashingWriter) Write(p []byte) (int, error) {
 	n, err := h.w.Write(p)
 	h.size += n
 	return n, err
 }
 
+// Sum returns the hash of all data written so far.
 func (h *HashingWriter) Sum(d []byte) []byte {
 	return h.h.Sum(d)
 }
 
+// Size returns the number of bytes written to the underlying writer.
 func (h *HashingWriter) Size() int {
 	return h.size
 }