code: Добавил тесты и исправил свою ошибку

histogram.cpp

@@ -1,3 +1,4 @@
+#include <iostream>
 #include "histogram.h"
 using namespace std;
 void find_minmax(const vector<double>& numbers, double& min, double& max)

histogram.h

@@ -5,6 +5,6 @@
 
 std::vector<size_t>
 
-make_histogram(const std::vector<double>& numbers, size_t bin_count);
+make_histogram(const std::vector<double>& numbers, size_t& bin_count);
 
 #endif // HISTOGRAM_H_INCLUDED

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

unittest.cpp

@@ -0,0 +1,39 @@
+#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);
+}
+
+