1 files changed, 39 insertions, 0 deletions
diff --git a/formats/packfile/common.go b/formats/packfile/common.go
new file mode 100644
index 0000000..4a97dc7
--- /dev/null
+++ b/formats/packfile/common.go
@@ -0,0 +1,39 @@
+package packfile
+
+import (
+	"fmt"
+	"io"
+)
+
+type trackingReader struct {
+	r io.Reader
+	n int
+}
+
+func (t *trackingReader) Pos() int { return t.n }
+
+func (t *trackingReader) Read(p []byte) (n int, err error) {
+	n, err = t.r.Read(p)
+	if err != nil {
+		return 0, err
+	}
+
+	t.n += n
+
+	return n, err
+}
+
+func (t *trackingReader) ReadByte() (c byte, err error) {
+	var p [1]byte
+	n, err := t.r.Read(p[:])
+	if err != nil {
+		return 0, err
+	}
+
+	if n > 1 {
+		return 0, fmt.Errorf("read %d bytes, should have read just 1", n)
+	}
+
+	t.n += n // n is 1
+	return p[0], nil
+}