index.h

#ifndef PASTELSYS_INDEX_H
#define PASTELSYS_INDEX_H

namespace Pastel
{

    template <typename Key>
    class Index
    {
    public:
        using KeyToIndex = 
            std::unordered_set<Key, integer>;
        using Key_ConstIterator = 
            typename KeyToIndex::iterator;
        using IndexToKey = 
            std::vector<Key_ConstIterator>;

        template <
            typename Iterator>
        void assign(
            const Iterator& begin, 
            const Iterator& end)
        {
            for (Iterator i = begin;i != end;++i)
            {
                insert(*i);
            }
        }

        void clear()
        {
            keyToIndex_.clear();
            indexToKey_.clear();
        }

        integer n() const
        {
            return indexToKey_.size();
        }

        void reserve(integer n)
        {
            ENSURE_OP(n, >=, 0);
            indexToKey_.reserve(n);
            keyToIndex_.reserve(n);
        }

        template <typename Type>
        Key_ConstIterator insert(Type&& key)
        {
            Key_ConstIterator i = 
                keyToIndex_.emplace(
                    std::forward(key), 
                    (integer)indexToKey_.size());
            try
            {
                indexToKey_.push_back(i);
            }
            catch(...)
            {
                keyToIndex_.erase(i);
                throw;
            };

            return i;
        }

    private:
        KeyToIndex keyToIndex_;
        IndexToKey indexToKey_;
    };

}

#endif