1 files changed, 46 insertions, 0 deletions
diff --git a/object/fetch/peel_to_tree_id.go b/object/fetch/peel_to_tree_id.go
new file mode 100644
index 00000000..21dcfde4
--- /dev/null
+++ b/object/fetch/peel_to_tree_id.go
@@ -0,0 +1,46 @@
+package fetch
+
+import (
+	"fmt"
+
+	objectid "codeberg.org/lindenii/furgit/object/id"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// PeelToTreeID peels tags until it reaches a tree object ID, or a commit whose
+// root tree object ID is then returned.
+func (r *Fetcher) PeelToTreeID(id objectid.ObjectID) (objectid.ObjectID, error) {
+	for {
+		ty, _, err := r.store.ReadHeader(id)
+		if err != nil {
+			return objectid.ObjectID{}, err
+		}
+
+		switch ty {
+		case objecttype.TypeTree:
+			return id, nil
+		case objecttype.TypeCommit:
+			commit, err := r.ExactCommit(id)
+			if err != nil {
+				return objectid.ObjectID{}, err
+			}
+
+			return commit.Object().Tree, nil
+		case objecttype.TypeTag:
+			tag, err := r.ExactTag(id)
+			if err != nil {
+				return objectid.ObjectID{}, err
+			}
+
+			id = tag.Object().Target
+		case objecttype.TypeInvalid,
+			objecttype.TypeBlob,
+			objecttype.TypeFuture,
+			objecttype.TypeOfsDelta,
+			objecttype.TypeRefDelta:
+			return objectid.ObjectID{}, fmt.Errorf("object/fetch: expected tree-ish object %s, got %v", id, ty)
+		default:
+			return objectid.ObjectID{}, fmt.Errorf("object/fetch: expected tree-ish object %s, got %v", id, ty)
+		}
+	}
+}