diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..bbe5577
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,5 @@
+#pragma once
+#include <vector>
+
+void
+find_minmax(const std::vector<double>& numbers, double& min, double& max, bool& res);
diff --git a/unittest.cbp b/unittest.cbp
new file mode 100644
index 0000000..bffa065
--- /dev/null
+++ b/unittest.cbp
@@ -0,0 +1,44 @@
+<?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>
+		<Unit filename="doctest.h">
+			<Option target="&lt;{~None~}&gt;" />
+		</Unit>
+		<Unit filename="histogram.cpp" />
+		<Unit filename="histogram_internal.h" />
+		<Unit filename="unittest.cpp" />
+		<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..6f2588a
--- /dev/null
+++ b/unittest.cpp
@@ -0,0 +1,48 @@
+#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;
+    bool res = true;
+    find_minmax({1, 2}, min, max ,res);
+    CHECK(res == true);
+    CHECK(min == 1);
+    CHECK(max == 2);
+}
+TEST_CASE("only one number") {
+    double min = 0;
+    double max = 0;
+    bool res = true;
+    find_minmax({1}, min, max, res);
+    CHECK(res == true);
+    CHECK(min == 1);
+    CHECK(max == 1);
+}
+TEST_CASE("distinct negative elements") {
+    double min = 0;
+    double max = 0;
+    bool res = true;
+    find_minmax({-1, -5}, min, max, res);
+    CHECK(res == true);
+    CHECK(min == -5);
+    CHECK(max == -1);
+}
+TEST_CASE("equal numbers") {
+    double min = 0;
+    double max = 0;
+    bool res = true;
+    find_minmax({4, 4, 4}, min, max, res);
+    CHECK(res == true);
+    CHECK(min == 4);
+    CHECK(max == 4);
+}
+TEST_CASE("no elements in vector") {
+    double min = 0;
+    double max = 0;
+    bool res = true;
+    find_minmax({}, min, max, res);
+    CHECK(res == false);
+}