diff --git a/src/diff_tform.c b/src/diff_tform.c
index 2c2e1fb19..dbcdc277f 100644
--- a/src/diff_tform.c
+++ b/src/diff_tform.c
@@ -7,6 +7,7 @@
 #include "common.h"
 #include "diff.h"
 #include "git2/config.h"
+#include "buf_text.h"
 
 static git_diff_delta *diff_delta__dup(
 	const git_diff_delta *d, git_pool *pool)
@@ -297,10 +298,15 @@ on_error:
 	return -1;
 }
 
+typedef struct {
+	/* array of delta index * 2 + (old_file/new_file) -> file hashes */
+	git_buf_text_hashsig *sigs;
+} diff_similarity_cache;
+
 static unsigned int calc_similarity(
-	void *cache, git_diff_file *old_file, git_diff_file *new_file)
+	void *ref, git_diff_file *old_file, git_diff_file *new_file)
 {
-	GIT_UNUSED(cache);
+	diff_similarity_cache *cache = ref;
 
 	if (git_oid_cmp(&old_file->oid, &new_file->oid) == 0)
 		return 100;