1 files changed, 59 insertions, 0 deletions
diff --git a/objectstore/packed/delta_resolve_chain_start.go b/objectstore/packed/delta_resolve_chain_start.go
new file mode 100644
index 00000000..70266565
--- /dev/null
+++ b/objectstore/packed/delta_resolve_chain_start.go
@@ -0,0 +1,59 @@
+package packed
+
+import (
+	"fmt"
+
+	packfmt "codeberg.org/lindenii/furgit/format/pack"
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// deltaResolveChainStart finds the nearest cached chain node or inflates the
+// innermost base object. It returns the starting bytes and the next delta index
+// to apply in reverse order.
+func (store *Store) deltaResolveChainStart(chain deltaChain) (objecttype.Type, []byte, int, error) {
+	for i, node := range chain.deltas {
+		store.cacheMu.RLock()
+		ty, out, ok := store.deltaCache.get(
+			deltaBaseKey{packName: node.loc.packName, offset: node.loc.offset},
+		)
+		store.cacheMu.RUnlock()
+
+		if ok {
+			return ty, out, i - 1, nil
+		}
+	}
+
+	store.cacheMu.RLock()
+	ty, out, ok := store.deltaCache.get(
+		deltaBaseKey{packName: chain.baseLoc.packName, offset: chain.baseLoc.offset},
+	)
+	store.cacheMu.RUnlock()
+
+	if ok {
+		return ty, out, len(chain.deltas) - 1, nil
+	}
+
+	pack, meta, err := store.entryMetaAt(chain.baseLoc)
+	if err != nil {
+		return objecttype.TypeInvalid, nil, 0, err
+	}
+
+	if !packfmt.IsBaseObjectType(meta.ty) {
+		return objecttype.TypeInvalid, nil, 0, fmt.Errorf("objectstore/packed: delta chain base is not a base object")
+	}
+
+	base, err := inflateAt(pack, meta.dataOffset, meta.size)
+	if err != nil {
+		return objecttype.TypeInvalid, nil, 0, err
+	}
+
+	store.cacheMu.Lock()
+	store.deltaCache.add(
+		deltaBaseKey{packName: chain.baseLoc.packName, offset: chain.baseLoc.offset},
+		meta.ty,
+		base,
+	)
+	store.cacheMu.Unlock()
+
+	return meta.ty, base, len(chain.deltas) - 1, nil
+}