format/delta/apply: Move core delta apply algorithm here

1 files changed, 49 insertions, 0 deletions

diff --git a/format/delta/apply/header.go b/format/delta/apply/header.go
new file mode 100644
index 00000000..f6aadea3
--- /dev/null
+++ b/format/delta/apply/header.go
@@ -0,0 +1,49 @@
+package apply
+
+import (
+	"fmt"
+	"io"
+)
+
+// ReadHeaderSizes reads the first two varints in one inflated delta stream.
+func ReadHeaderSizes(reader io.Reader) (int, int, error) {
+	// Two Git varints are read here. Each can take up to 10 bytes.
+	var buf [20]byte
+	n := 0
+
+	for {
+		if n >= len(buf) {
+			return 0, 0, fmt.Errorf("format/delta/apply: malformed delta varint")
+		}
+		if _, err := io.ReadFull(reader, buf[n:n+1]); err != nil {
+			return 0, 0, fmt.Errorf("format/delta/apply: malformed delta varint: %w", err)
+		}
+		n++
+		if buf[n-1]&0x80 == 0 {
+			break
+		}
+	}
+	pos := 0
+	srcSize, err := readVarint(buf[:n], &pos)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	for {
+		if n >= len(buf) {
+			return 0, 0, fmt.Errorf("format/delta/apply: malformed delta varint")
+		}
+		if _, err := io.ReadFull(reader, buf[n:n+1]); err != nil {
+			return 0, 0, fmt.Errorf("format/delta/apply: malformed delta varint: %w", err)
+		}
+		n++
+		if buf[n-1]&0x80 == 0 {
+			break
+		}
+	}
+	dstSize, err := readVarint(buf[:n], &pos)
+	if err != nil {
+		return 0, 0, err
+	}
+	return srcSize, dstSize, nil
+}