formats/packfile: new reader API (wip)

1 files changed, 23 insertions, 0 deletions

diff --git a/formats/packfile/delta.go b/formats/packfile/delta.go
index 86b556f..30703eb 100644
--- a/formats/packfile/delta.go
+++ b/formats/packfile/delta.go
@@ -1,5 +1,7 @@
 package packfile
 
+import "io"
+
 const delta_size_min = 4
 
 func deltaHeaderSize(b []byte) (uint, []byte) {
@@ -91,3 +93,24 @@ func PatchDelta(src, delta []byte) []byte {
 	}
 	return dest
 }
+
+func decodeOffset(src io.ByteReader, steps int) (int, error) {
+	b, err := src.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	var offset = int(b & 0x7f)
+	for (b & 0x80) != 0 {
+		offset++ // WHY?
+		b, err = src.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+
+		offset = (offset << 7) + int(b&0x7f)
+	}
+
+	// offset needs to be aware of the bytes we read for `o.typ` and `o.size`
+	offset += steps
+	return -offset, nil
+}