core: add emplace function in double flat list

lib/include/irritator/core.hpp

@@ -1604,6 +1604,8 @@ public:
             std::swap(list, other.list);
             std::swap(node, other.node);
         }
+
+        friend class flat_double_list<T>;
     };
 
     class const_iterator
@@ -1694,6 +1696,8 @@ public:
             std::swap(list, other.list);
             std::swap(node, other.node);
         }
+
+        friend class flat_double_list<T>;
     };
 
 private:
@@ -1814,6 +1818,33 @@ public:
         return m_back->value;
     }
 
+    /**
+     * @brief Inserts a new element into the container directly before pos.
+     * @tparam ...Args 
+     * @param pos 
+     * @param ...args 
+     * @return 
+    */
+    template<typename... Args>
+    iterator emplace(iterator pos, Args&&... args) noexcept
+    {
+        if (!pos.node)
+            return emplace_back(std::forward<Args>(args)...);
+
+        if (!pos.node->prev)
+            return emplace_front(std::forward<Args>(args)...);
+            
+        node_type* new_node = m_allocator->alloc();
+        new (&new_node->value) T(std::forward<Args>(args)...);
+        ++m_size;
+
+        new_node->prev = pos.node->prev;
+        new_node->next = pos.node;
+        pos.node->prev->next = new_node;
+        pos.node->prev = new_node;
+        return iterator(this, new_node);
+    }
+
     template<typename... Args>
     iterator emplace_front(Args&&... args) noexcept
     {

lib/test/public-api.cpp

@@ -597,6 +597,18 @@ main()
             --it;
             expect(*it == 7);
         }
+
+        lst.emplace(lst.begin(), 10);
+        expect(*lst.begin() == 10);
+
+        {
+            auto it = lst.begin();
+            ++it;
+
+            it = lst.emplace(it, 11);
+            expect(*it == 11);
+            expect(*lst.begin() == 10);
+        }
     };
 
     "data_array_api"_test = [] {