diff --git a/histogram.h b/histogram.h
index 0536663..0d8dc8c 100644
--- a/histogram.h
+++ b/histogram.h
@@ -1,5 +1,9 @@
 #ifndef HISTOGRAM_H_INCLUDED
 #define HISTOGRAM_H_INCLUDED
 
+#include <vector>
+
+std::vector<size_t>
+make_histogram(const std::vector<double>& numbers, size_t bin_count);
 
 #endif // HISTOGRAM_H_INCLUDED