1 files changed, 71 insertions, 0 deletions
diff --git a/object/store/packed/entry_parse.go b/object/store/packed/entry_parse.go
new file mode 100644
index 00000000..962e39df
--- /dev/null
+++ b/object/store/packed/entry_parse.go
@@ -0,0 +1,71 @@
+package packed
+
+import (
+	"fmt"
+
+	packfmt "codeberg.org/lindenii/furgit/format/packfile"
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// entryMeta describes one parsed pack entry header.
+type entryMeta struct {
+	// ty is the pack entry type tag.
+	ty objecttype.Type
+	// size is the declared resulting content size.
+	size int64
+	// dataOffset points to the zlib payload start.
+	dataOffset int
+	// baseRefID is set for ref-delta entries.
+	baseRefID objectid.ObjectID
+	// baseOfs is set for ofs-delta entries.
+	baseOfs uint64
+}
+
+// parseEntryMeta parses one pack entry header at offset.
+func parseEntryMeta(pack *packFile, algo objectid.Algorithm, offset uint64) (entryMeta, error) {
+	var zero entryMeta
+	if offset >= uint64(len(pack.data)) {
+		return zero, fmt.Errorf("objectstore/packed: pack %q offset %d out of bounds", pack.name, offset)
+	}
+
+	pos, err := intconv.Uint64ToInt(offset)
+	if err != nil {
+		return zero, fmt.Errorf("objectstore/packed: pack %q offset conversion: %w", pack.name, err)
+	}
+
+	entry, err := packfmt.ParseEntry(pack.data[pos:], algo.Size())
+	if err != nil {
+		return zero, fmt.Errorf("objectstore/packed: pack %q: %w", pack.name, err)
+	}
+
+	meta := entryMeta{
+		ty:         entry.Type,
+		size:       entry.Size,
+		dataOffset: pos + entry.DataOffset,
+	}
+	switch meta.ty {
+	case objecttype.TypeRefDelta:
+		baseID, err := objectid.FromBytes(algo, entry.RefBaseID)
+		if err != nil {
+			return zero, fmt.Errorf("objectstore/packed: pack %q invalid ref-delta base id: %w", pack.name, err)
+		}
+
+		meta.baseRefID = baseID
+	case objecttype.TypeOfsDelta:
+		if offset <= entry.OfsBaseDistance {
+			return zero, fmt.Errorf("objectstore/packed: pack %q has invalid ofs-delta base", pack.name)
+		}
+
+		meta.baseOfs = offset - entry.OfsBaseDistance
+	case objecttype.TypeCommit, objecttype.TypeTree, objecttype.TypeBlob, objecttype.TypeTag:
+		// Base object types do not have delta base metadata.
+	case objecttype.TypeInvalid, objecttype.TypeFuture:
+		return zero, fmt.Errorf("objectstore/packed: pack %q has unsupported entry type %d", pack.name, meta.ty)
+	default:
+		return zero, fmt.Errorf("objectstore/packed: pack %q has unsupported entry type %d", pack.name, meta.ty)
+	}
+
+	return meta, nil
+}