1 files changed, 89 insertions, 0 deletions
diff --git a/format/packfile/ingest/rewrite_header_trailer.go b/format/packfile/ingest/rewrite_header_trailer.go
new file mode 100644
index 00000000..f1f18a39
--- /dev/null
+++ b/format/packfile/ingest/rewrite_header_trailer.go
@@ -0,0 +1,89 @@
+package ingest
+
+import (
+	"encoding/binary"
+	"io"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+)
+
+// rewritePackHeaderAndTrailer rewrites object count and trailer hash using ReadAt/WriteAt.
+func rewritePackHeaderAndTrailer(state *ingestState) error {
+	var countRaw [4]byte
+
+	recordCountUint32, err := intconv.IntToUint32(len(state.records))
+	if err != nil {
+		return err
+	}
+
+	binary.BigEndian.PutUint32(countRaw[:], recordCountUint32)
+
+	_, err = state.packFile.WriteAt(countRaw[:], 8)
+	if err != nil {
+		return err
+	}
+
+	info, err := state.packFile.Stat()
+	if err != nil {
+		return err
+	}
+
+	endWithoutTrailer := info.Size()
+
+	hashImpl, err := state.algo.New()
+	if err != nil {
+		return err
+	}
+
+	var (
+		buf [128 << 10]byte
+		pos int64
+	)
+	for pos < endWithoutTrailer {
+		want := int64(len(buf))
+
+		remaining := endWithoutTrailer - pos
+		if remaining < want {
+			want = remaining
+		}
+
+		n, err := state.packFile.ReadAt(buf[:want], pos)
+		if err != nil && err != io.EOF {
+			return err
+		}
+
+		if n == 0 {
+			return io.ErrUnexpectedEOF
+		}
+
+		_, _ = hashImpl.Write(buf[:n])
+		pos += int64(n)
+	}
+
+	sum := hashImpl.Sum(nil)
+
+	_, err = state.packFile.WriteAt(sum, endWithoutTrailer)
+	if err != nil {
+		return err
+	}
+
+	packHash, err := objectid.FromBytes(state.algo, sum)
+	if err != nil {
+		return err
+	}
+
+	state.packHash = packHash
+	state.objectCountHeader = recordCountUint32
+
+	sumLenInt64 := int64(len(sum))
+
+	newConsumed, err := intconv.Int64ToUint64(endWithoutTrailer + sumLenInt64)
+	if err != nil {
+		return err
+	}
+
+	state.stream.consumed = newConsumed
+
+	return nil
+}