Finalized the implementation

impl_generator.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <unordered_set>
+
+class ImplGenerator
+{
+    public:
+    ImplGenerator(int nr_entries);
+    bool has_more_numbers();
+    int get_next_num();
+
+    private:
+    int m_nr_entries;
+    int internal_counter = 0;
+    std::unordered_set<int> already_generated_numbers;
+
+    virtual int get_next_num_internal() = 0;
+};