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+/******************************************************************************
+ *
+ * Copyright (C) 1997-2020 by Dimitri van Heesch.
+ *
+ * Permission to use, copy, modify, and distribute this software and its
+ * documentation under the terms of the GNU General Public License is hereby 
+ * granted. No representations are made about the suitability of this software 
+ * for any purpose. It is provided "as is" without express or implied warranty.
+ * See the GNU General Public License for more details.
+ *
+ * Documents produced by Doxygen are derivative works derived from the
+ * input used in their production; they are not affected by this license.
+ *
+ */
+
+#ifndef LINKEDMAP_H
+#define LINKEDMAP_H
+
+#include <unordered_map>
+#include <vector>
+#include <memory>
+#include <string>
+
+//! @brief Container class representing a vector of objects with unique keys.
+//! @details Elements can be quickly looked up given the key.
+//! When adding objects the order of addition is kept, and used while iterating.
+template<class T>
+class LinkedMap
+{
+  public:
+    using Ptr = std::unique_ptr<T>;
+    using Vec = std::vector<Ptr>;
+    using Map = std::unordered_map<std::string,T*>;
+    using iterator = typename Vec::iterator;
+
+    //! find an element given the key.
+    //! Returns a pointer to the element if found or nullptr if it is not found.
+    const T *find(const char *key) const
+    {
+      auto it = m_lookup.find(key);
+      return it!=m_lookup.end() ? it->second : nullptr;
+    }
+
+    //! non-const wrapper for find() const
+    T* find(const char *key)
+    {
+      return const_cast<T*>(static_cast<const LinkedMap&>(*this).find(key));
+    }
+
+    //! Adds a new element to the ordered set if it was not added already.
+    //! Return a non-owning pointer to the newly added item, or to the existing item if it was
+    //! already inserted before.
+    template<class...Args>
+    T *add(const char *key, Args&&... args)
+    {
+      T *result = find(key);
+      if (result==nullptr)
+      {
+        Ptr ptr = std::make_unique<T>(key,std::forward<Args>(args)...);
+        result = ptr.get();
+        m_lookup.insert({std::string(key),result});
+        m_entries.push_back(std::move(ptr));
+      }
+      return result;
+    }
+
+    iterator begin()    { return m_entries.begin(); }
+    iterator end()      { return m_entries.end();   }
+    bool empty() const  { return m_entries.empty(); }
+    int size() const    { return m_entries.size();  }
+
+    void clear()
+    {
+      m_entries.clear();
+      m_lookup.clear();
+    }
+
+  private:
+    Map m_lookup;
+    Vec m_entries;
+};
+
+
+#endif