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1 // Copyright Louis Dionne 2013-2017
2 // Distributed under the Boost Software License, Version 1.0.
3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
5 #include <boost/hana/assert.hpp>
6 #include <boost/hana/first.hpp>
7 #include <boost/hana/fold_left.hpp>
8 #include <boost/hana/insert.hpp>
9 #include <boost/hana/integral_constant.hpp>
10 #include <boost/hana/map.hpp>
11 #include <boost/hana/pair.hpp>
12 #include <boost/hana/second.hpp>
13 #include <boost/hana/type.hpp>
14 namespace hana
= boost::hana
;
18 // Given a map of (key, value) pairs, returns a map of (value, key) pairs.
19 // This requires both the keys and the values to be compile-time Comparable.
20 auto invert
= [](auto map
) {
21 return hana::fold_left(map
, hana::make_map(), [](auto map
, auto pair
) {
22 return hana::insert(map
, hana::make_pair(hana::second(pair
), hana::first(pair
)));
26 auto m
= hana::make_map(
27 hana::make_pair(hana::type_c
<int>, hana::int_c
<1>),
28 hana::make_pair(hana::type_c
<float>, hana::int_c
<2>),
29 hana::make_pair(hana::int_c
<3>, hana::type_c
<void>)
32 BOOST_HANA_CONSTANT_CHECK(invert(m
) ==
34 hana::make_pair(hana::int_c
<1>, hana::type_c
<int>),
35 hana::make_pair(hana::int_c
<2>, hana::type_c
<float>),
36 hana::make_pair(hana::type_c
<void>, hana::int_c
<3>)