diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..8179497
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,6 @@
+#pragma once
+
+#include <vector>
+
+void
+find_minmax(const std::vector<double>& numbers, double& min, double& max);