internal/format/packfile: Refactor ofs delta distances

1 files changed, 42 insertions, 4 deletions

diff --git a/internal/format/packfile/ofs.go b/internal/format/packfile/ofs.go
index ae9d8c22..5fb8ecdd 100644
--- a/internal/format/packfile/ofs.go
+++ b/internal/format/packfile/ofs.go
@@ -2,14 +2,25 @@ package packfile
 
 import (
 	"errors"
+	"fmt"
+	"math"
 )
 
-var ErrMalformedOfsDeltaDistance = errors.New("internal/format/packfile/delta: malformed ofs-delta distance")
+// 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, ErrMalformedOfsDeltaDistance
+		return 0, 0, fmt.Errorf("%w: truncated", ErrMalformedOfsDeltaDistance)
 	}
 
 	b := buf[0]
@@ -17,14 +28,41 @@ func ParseOfsDeltaDistance(buf []byte) (dist uint64, consumed int, err error) {
 
 	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, ErrMalformedOfsDeltaDistance
+			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)
+		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:]...)
+}