diff --git a/histogram_internal.h b/histogram_internal.h
index 34592d8..3a2e060 100644
--- a/histogram_internal.h
+++ b/histogram_internal.h
@@ -4,6 +4,6 @@
 #include <vector>
 
 void
-find_minmax(const std::vector<double>& numbers, double& min, double& max)
+find_minmax(const std::vector<double>& numbers, double& min, double& max);
 
 #endif // HISTOGRAM_INTERNAL_H_INCLUDED
diff --git a/unittest.cpp b/unittest.cpp
index e69de29..1ee1609 100644
--- a/unittest.cpp
+++ b/unittest.cpp
@@ -0,0 +1,37 @@
+#define DOCTEST_CONFIG_NO_MULTITHREADING
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include <vector>
+#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({ -10, 10 }, min, max);
+    CHECK(min == -10);
+    CHECK(max == 10);
+}
+
+TEST_CASE("empty numbers") {
+    double min = 0;
+    double max = 0;
+    find_minmax({ }, min, max);
+    CHECK(min != 0);
+    CHECK(max != 0);
+}
+
+TEST_CASE("one number") {
+    double min = 0;
+    double max = 0;
+    find_minmax({ 2 }, min, max);
+    CHECK(min == 2);
+    CHECK(max == 2);
+}