1 files changed, 66 insertions, 0 deletions
diff --git a/object/store/packed/delta_build_chain.go b/object/store/packed/delta_build_chain.go
new file mode 100644
index 00000000..59f83542
--- /dev/null
+++ b/object/store/packed/delta_build_chain.go
@@ -0,0 +1,66 @@
+package packed
+
+import (
+	"fmt"
+
+	packfmt "codeberg.org/lindenii/furgit/format/packfile"
+	objecttype "codeberg.org/lindenii/furgit/object/type"
+)
+
+// deltaBuildChain walks one object's chain and builds a reconstruction chain.
+func (store *Store) deltaBuildChain(start location) (deltaChain, error) {
+	visited := make(map[location]struct{})
+	current := start
+
+	var chain deltaChain
+
+	for {
+		if _, ok := visited[current]; ok {
+			return deltaChain{}, fmt.Errorf("objectstore/packed: delta cycle while resolving object")
+		}
+
+		visited[current] = struct{}{}
+
+		_, meta, err := store.entryMetaAt(current)
+		if err != nil {
+			return deltaChain{}, err
+		}
+
+		if packfmt.IsBaseObjectType(meta.ty) {
+			chain.baseLoc = current
+			chain.baseType = meta.ty
+
+			return chain, nil
+		}
+
+		switch meta.ty {
+		case objecttype.TypeRefDelta:
+			chain.deltas = append(chain.deltas, deltaNode{
+				loc:        current,
+				dataOffset: meta.dataOffset,
+			})
+
+			next, err := store.lookup(meta.baseRefID)
+			if err != nil {
+				return deltaChain{}, err
+			}
+
+			current = next
+		case objecttype.TypeOfsDelta:
+			chain.deltas = append(chain.deltas, deltaNode{
+				loc:        current,
+				dataOffset: meta.dataOffset,
+			})
+			current = location{
+				packName: current.packName,
+				offset:   meta.baseOfs,
+			}
+		case objecttype.TypeCommit, objecttype.TypeTree, objecttype.TypeBlob, objecttype.TypeTag:
+			return deltaChain{}, fmt.Errorf("objectstore/packed: internal invariant violation for base type %d", meta.ty)
+		case objecttype.TypeInvalid, objecttype.TypeFuture:
+			return deltaChain{}, fmt.Errorf("objectstore/packed: unsupported pack type %d", meta.ty)
+		default:
+			return deltaChain{}, fmt.Errorf("objectstore/packed: unsupported pack type %d", meta.ty)
+		}
+	}
+}