1 files changed, 47 insertions, 0 deletions
diff --git a/packfile/delta/apply/header.go b/packfile/delta/apply/header.go
new file mode 100644
index 00000000..69c9659a
--- /dev/null
+++ b/packfile/delta/apply/header.go
@@ -0,0 +1,47 @@
+package apply
+
+import (
+	"fmt"
+	"io"
+)
+
+// ReadHeaderSizes reads the first two varints in one inflated delta stream.
+//
+// Callers that continue reading the same stream should pass their own buffered
+// byte reader and keep using that same reader afterwards.
+func ReadHeaderSizes(reader io.ByteReader) (int, int, error) {
+	srcSize, err := readVarintFromByteReader(reader)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	dstSize, err := readVarintFromByteReader(reader)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	return srcSize, dstSize, nil
+}
+
+// readVarintFromByteReader parses one Git delta varint from reader.
+func readVarintFromByteReader(reader io.ByteReader) (int, error) {
+	value := 0
+	shift := uint(0)
+
+	for {
+		b, err := reader.ReadByte()
+		if err != nil {
+			return 0, fmt.Errorf("delta/apply: malformed delta varint: %w", err)
+		}
+
+		value |= int(b&0x7f) << shift
+		if b&0x80 == 0 {
+			return value, nil
+		}
+
+		shift += 7
+		if shift > 63 {
+			return 0, fmt.Errorf("delta/apply: delta varint overflow")
+		}
+	}
+}