diff --git a/sparse-index.h b/sparse-index.h
index 633d4fb..b1f2cdb 100644
--- a/sparse-index.h
+++ b/sparse-index.h
@@ -23,4 +23,17 @@ void expand_to_path(struct index_state *istate,
 struct repository;
 int set_sparse_index_config(struct repository *repo, int enable);
 
+struct pattern_list;
+
+/**
+ * Scan the given index and compare its entries to the given pattern list.
+ * If the index is sparse and the pattern list uses cone mode patterns,
+ * then modify the index to contain the all of the file entries within that
+ * new pattern list. This expands sparse directories only as far as needed.
+ *
+ * If the pattern list is NULL or does not use cone mode patterns, then the
+ * index is expanded to a full index.
+ */
+void expand_index(struct index_state *istate, struct pattern_list *pl);
+
 #endif