1 files changed, 68 insertions, 0 deletions
diff --git a/internal/format/packfile/ofs.go b/internal/format/packfile/ofs.go
new file mode 100644
index 00000000..5fb8ecdd
--- /dev/null
+++ b/internal/format/packfile/ofs.go
@@ -0,0 +1,68 @@
+package packfile
+
+import (
+	"errors"
+	"fmt"
+	"math"
+)
+
+// ErrMalformedOfsDeltaDistance reports that
+// an ofs-delta backward distance encoding
+// is truncated, overlong, or overflows uint64.
+var ErrMalformedOfsDeltaDistance = errors.New("internal/format/packfile: malformed ofs-delta distance")
+
+// MaxOfsDeltaDistanceLen is the maximum encoded length
+// of an ofs-delta backward distance.
+// Every uint64 distance is encodable within this bound,
+// and [ParseOfsDeltaDistance] rejects longer encodings.
+const MaxOfsDeltaDistanceLen = 10
+
+// ParseOfsDeltaDistance parses an ofs-delta backward distance.
+func ParseOfsDeltaDistance(buf []byte) (dist uint64, consumed int, err error) {
+	if len(buf) == 0 {
+		return 0, 0, fmt.Errorf("%w: truncated", ErrMalformedOfsDeltaDistance)
+	}
+
+	b := buf[0]
+	dist = uint64(b & 0x7f)
+
+	consumed = 1
+	for b&0x80 != 0 {
+		if consumed >= MaxOfsDeltaDistanceLen {
+			return 0, 0, fmt.Errorf("%w: overlong encoding", ErrMalformedOfsDeltaDistance)
+		}
+
+		if consumed >= len(buf) {
+			return 0, 0, fmt.Errorf("%w: truncated", ErrMalformedOfsDeltaDistance)
+		}
+
+		if dist >= math.MaxUint64>>7 {
+			return 0, 0, fmt.Errorf("%w: overflows uint64", ErrMalformedOfsDeltaDistance)
+		}
+
+		b = buf[consumed]
+		consumed++
+		dist = ((dist + 1) << 7) | uint64(b&0x7f)
+	}
+
+	return dist, consumed, nil
+}
+
+// AppendOfsDeltaDistance appends the encoding of
+// an ofs-delta backward distance to dst.
+func AppendOfsDeltaDistance(dst []byte, dist uint64) []byte {
+	var buf [MaxOfsDeltaDistanceLen]byte
+
+	pos := len(buf) - 1
+	buf[pos] = byte(dist & 0x7f)
+
+	dist >>= 7
+	for dist != 0 {
+		dist--
+		pos--
+		buf[pos] = 0x80 | byte(dist&0x7f)
+		dist >>= 7
+	}
+
+	return append(dst, buf[pos:]...)
+}