diff --git a/histogram.cpp b/histogram.cpp
index 5867495..52c06d3 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -3,7 +3,7 @@
 #include <vector>
 using namespace std;
 
-void find_minmax(const vector<double> &numbers, double &min, double &max)
+void find_minmax( 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..d28f5fc
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,7 @@
+#ifndef HISTOGRAM_INTERNAL_H_INCLUDED
+#define HISTOGRAM_INTERNAL_H_INCLUDED
+#include <vector>
+
+bool find_minmax(std::vector<double> numbers, double& min, double& max);
+
+#endif // HISTOGRAM_INTERNAL_H_INCLUDED
diff --git a/unittest.cbp b/unittest.cbp
new file mode 100644
index 0000000..7f9621b
--- /dev/null
+++ b/unittest.cbp
@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
+<CodeBlocks_project_file>
+	<FileVersion major="1" minor="6" />
+	<Project>
+		<Option title="unittest" />
+		<Option pch_mode="2" />
+		<Option compiler="gcc" />
+		<Build>
+			<Target title="Debug">
+				<Option output="bin/Debug/unittest" prefix_auto="1" extension_auto="1" />
+				<Option object_output="obj/Debug/" />
+				<Option type="1" />
+				<Option compiler="gcc" />
+				<Compiler>
+					<Add option="-g" />
+				</Compiler>
+			</Target>
+			<Target title="Release">
+				<Option output="bin/Release/unittest" prefix_auto="1" extension_auto="1" />
+				<Option object_output="obj/Release/" />
+				<Option type="1" />
+				<Option compiler="gcc" />
+				<Compiler>
+					<Add option="-O2" />
+				</Compiler>
+				<Linker>
+					<Add option="-s" />
+				</Linker>
+			</Target>
+		</Build>
+		<Compiler>
+			<Add option="-Wall" />
+		</Compiler>
+		<Extensions>
+			<lib_finder disable_auto="1" />
+		</Extensions>
+	</Project>
+</CodeBlocks_project_file>
diff --git a/unittest.cpp b/unittest.cpp
new file mode 100644
index 0000000..62f954d
--- /dev/null
+++ b/unittest.cpp
@@ -0,0 +1,40 @@
+#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("distinct positive numbers 2") {
+    double min = 0;
+    double max = 0;
+    std::vector<double>v{2,1};
+    CHECK(v.size() != 0);
+    CHECK(v.size() != 1);
+    find_minmax({1, 2}, min, max);
+    CHECK(min == 1);
+    CHECK(max == 2);
+    CHECK(min != max);
+}
+
+TEST_CASE("vector with one element") {
+    double min = 0;
+    double max = 0;
+    find_minmax({1}, min, max);
+    CHECK(min == 1);
+    CHECK(max == 1);
+}
+
+TEST_CASE("vector with same elements") {
+    double min = 0;
+    double max = 0;
+    find_minmax({2,2,2}, min, max);
+    CHECK(min == 2);
+    CHECK(max == 2);
+}