internal/format/packfile/delta: Parse ofs-delta backward distance

2 files changed, 33 insertions, 0 deletions

diff --git a/internal/format/packfile/delta/doc.go b/internal/format/packfile/delta/doc.go
new file mode 100644
index 00000000..f63c96a8
--- /dev/null
+++ b/internal/format/packfile/delta/doc.go
@@ -0,0 +1,2 @@
+// Package delta provides various routines to handle Git delta compression.
+package delta
diff --git a/internal/format/packfile/delta/ofs.go b/internal/format/packfile/delta/ofs.go
new file mode 100644
index 00000000..f11ec1cd
--- /dev/null
+++ b/internal/format/packfile/delta/ofs.go
@@ -0,0 +1,31 @@
+package delta
+
+import (
+	"errors"
+	"fmt"
+)
+
+var ErrMalformedOfsDeltaDistance = errors.New("internal/format/packfile/delta: malformed ofs-delta distance")
+
+// ParseOfsDeltaDistance parses an ofs-delta backward distance.
+func ParseOfsDeltaDistance(buf []byte) (dist uint64, consumed int, err error) {
+	if len(buf) == 0 {
+		return 0, 0, ErrMalformedOfsDeltaDistance
+	}
+
+	b := buf[0]
+	dist = uint64(b & 0x7f)
+
+	consumed = 1
+	for b&0x80 != 0 {
+		if consumed >= len(buf) {
+			return 0, 0, fmt.Errorf("")
+		}
+
+		b = buf[consumed]
+		consumed++
+		dist = ((dist + 1) << 7) + uint64(b&0x7f)
+	}
+
+	return dist, consumed, ErrMalformedOfsDeltaDistance
+}