1 files changed, 49 insertions, 0 deletions
diff --git a/format/packfile/ingest/header.go b/format/packfile/ingest/header.go
new file mode 100644
index 00000000..6a214828
--- /dev/null
+++ b/format/packfile/ingest/header.go
@@ -0,0 +1,49 @@
+package ingest
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+
+	"codeberg.org/lindenii/furgit/format/packfile"
+)
+
+const packHeaderSize = 12
+
+// readAndValidatePackHeader reads one PACK header from src and validates it.
+func readAndValidatePackHeader(src io.Reader) (HeaderInfo, [packHeaderSize]byte, error) {
+	var hdr [packHeaderSize]byte
+
+	_, err := io.ReadFull(src, hdr[:])
+	if err != nil {
+		return HeaderInfo{}, [packHeaderSize]byte{}, &InvalidPackHeaderError{
+			Reason: fmt.Sprintf("read header: %v", err),
+		}
+	}
+
+	header, err := parseAndValidatePackHeader(hdr)
+	if err != nil {
+		return HeaderInfo{}, [packHeaderSize]byte{}, err
+	}
+
+	return header, hdr, nil
+}
+
+// parseAndValidatePackHeader validates one already-read PACK header.
+func parseAndValidatePackHeader(hdr [packHeaderSize]byte) (HeaderInfo, error) {
+	if binary.BigEndian.Uint32(hdr[:4]) != packfile.Signature {
+		return HeaderInfo{}, &InvalidPackHeaderError{Reason: "signature mismatch"}
+	}
+
+	version := binary.BigEndian.Uint32(hdr[4:8])
+	if !packfile.VersionSupported(version) {
+		return HeaderInfo{}, &InvalidPackHeaderError{
+			Reason: fmt.Sprintf("unsupported version %d", version),
+		}
+	}
+
+	return HeaderInfo{
+		Version:     version,
+		ObjectCount: binary.BigEndian.Uint32(hdr[8:12]),
+	}, nil
+}