diff --git a/histogram.cpp b/histogram.cpp
index 0c923b3..ca1dd20 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -4,7 +4,7 @@
 
 using namespace std;
 
-static void find_minmax(const vector <double> &numbers, double &min, double &max)
+void find_minmax(const vector <double> &numbers, double &min, double &max)
 {
     min = numbers[0];
     max = numbers[0];
diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..ac2f4af
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,4 @@
+#pragma once
+#include <vector>
+
+void find_minmax(const std::vector <double> &numbers, double &min, double &max);
diff --git a/unittest.cpp b/unittest.cpp
new file mode 100644
index 0000000..c6d4f3f
--- /dev/null
+++ b/unittest.cpp
@@ -0,0 +1,45 @@
+#define DOCTEST_CONFIG_NO_MULTITHREADING
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include "doctest.h"
+#include "histogram_internal.h"
+
+TEST_CASE("distinct positive numbers")
+{
+    double min = 0;
+    double max = 0;
+    find_minmax({1, 2}, min, max);
+    CHECK(min == 1);
+    CHECK(max == 2);
+}
+TEST_CASE("negative numbers")
+{
+    double min = 0;
+    double max = 0;
+    find_minmax({-1, -2}, min, max);
+    CHECK(min == -2);
+    CHECK(max == -1);
+}
+TEST_CASE("vector of the same elements")
+{
+    double min = 0;
+    double max = 0;
+    find_minmax({3,3,3}, min, max);
+    CHECK(min == 3);
+    CHECK(max == 3);
+
+}
+TEST_CASE("vector of the one elements")
+{
+    double min = 0;
+    double max = 0;
+    find_minmax({3}, min, max);
+    CHECK(min == max);
+}
+TEST_CASE("positive and negative numbers")
+{
+    double min = 0;
+    double max = 0;
+    find_minmax({-1, 2, 0, -3, 5}, min, max);
+    CHECK(min == -3);
+    CHECK(max == 5);
+}