Thanks for bug report! Code is very new. Bug manifests itself when substring is in 0 position. Fix:
diff --git a/volnitsky.h b/volnitsky.h
index a7dbc1f..62d686d 100644
-- a/volnitsky.h
+++ b/volnitsky.h
@@ -26,7 +26,7 @@ const char* volnitsky(const char* S, const char* Se,
const char* SS, con
// step through text
- for (const char* p = S+step; p <= Se-W_size; p+=step) {
+ for (const char* p = S+step-W_size; p <= Se-W_size; p+=step) {
