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+// Boost.Units - A C++ library for zero-overhead dimensional analysis and
+// unit/quantity manipulation and conversion
+//
+// Copyright (C) 2003-2008 Matthias Christian Schabel
+// Copyright (C) 2008 Steven Watanabe
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_UNITS_DETAIL_PUSH_FRONT_OR_ADD_HPP
+#define BOOST_UNITS_DETAIL_PUSH_FRONT_OR_ADD_HPP
+
+#include <boost/mpl/plus.hpp>
+#include <boost/mpl/front.hpp>
+#include <boost/mpl/push_front.hpp>
+#include <boost/mpl/pop_front.hpp>
+#include <boost/type_traits/is_same.hpp>
+
+#include <boost/units/units_fwd.hpp>
+#include <boost/units/detail/push_front_if.hpp>
+
+namespace boost {
+
+namespace units {
+
+template<class Item, class Next>
+struct list;
+
+namespace detail {
+
+template<class T>
+struct is_empty_dim;
+
+/// add an instantiation of dim to Sequence.
+template<bool>
+struct push_front_or_add_impl;
+
+template<>
+struct push_front_or_add_impl<true>
+{
+ template<typename Sequence, typename T>
+ struct apply
+ {
+ typedef typename mpl::plus<T, typename Sequence::item>::type item;
+ typedef typename push_front_if<!is_empty_dim<item>::value>::template apply<
+ typename Sequence::next,
+ item
+ > type;
+ };
+};
+
+template<>
+struct push_front_or_add_impl<false>
+{
+ template<typename Sequence, typename T>
+ struct apply
+ {
+ typedef list<T, Sequence> type;
+ };
+};
+
+template<typename Sequence, typename T>
+struct push_front_or_add
+{
+ typedef typename push_front_or_add_impl<boost::is_same<typename T::tag_type, typename Sequence::item::tag_type>::value>::template apply<
+ Sequence,
+ T
+ >::type type;
+};
+
+template<typename T>
+struct push_front_or_add<dimensionless_type, T>
+{
+ typedef list<T, dimensionless_type> type;
+};
+
+} // namespace detail
+
+} // namespace units
+
+} // namespace boost
+
+#endif