ceph/src/boost/libs/hana/example/misc/indexed_sort.cpp

   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)
   4
   5 #include <boost/hana/at.hpp>
   6 #include <boost/hana/back.hpp>
   7 #include <boost/hana/core/to.hpp>
   8 #include <boost/hana/front.hpp>
   9 #include <boost/hana/functional/on.hpp>
  10 #include <boost/hana/integral_constant.hpp>
  11 #include <boost/hana/length.hpp>
  12 #include <boost/hana/less.hpp>
  13 #include <boost/hana/pair.hpp>
  14 #include <boost/hana/range.hpp>
  15 #include <boost/hana/sort.hpp>
  16 #include <boost/hana/transform.hpp>
  17 #include <boost/hana/tuple.hpp>
  18 #include <boost/hana/type.hpp>
  19 #include <boost/hana/zip.hpp>
  20
  21 #include <type_traits>
  22 namespace hana = boost::hana;
  23 using namespace hana::literals;
  24
  25
  26 auto indexed_sort = [](auto list, auto predicate) {
  27     auto indexed_list = hana::zip(
  28         list,
  29         hana::to_tuple(hana::make_range(0_c, hana::length(list)))
  30     );
  31     auto sorted = hana::sort.by(predicate ^hana::on^ hana::front, indexed_list);
  32     return hana::make_pair(hana::transform(sorted, hana::front),
  33                            hana::transform(sorted, hana::back));
  34 };
  35
  36 int main() {
  37     auto types = hana::tuple_t<char[4], char[2], char[1], char[5], char[3]>;
  38     auto sorted = indexed_sort(types, [](auto t, auto u) {
  39         return hana::sizeof_(t) < hana::sizeof_(u);
  40     });
  41     using Tup = decltype(
  42         hana::unpack(hana::first(sorted), hana::template_<hana::tuple>)
  43     )::type;
  44     auto indices = hana::second(indexed_sort(hana::second(sorted), hana::less));
  45
  46     // When accessed through the indices sequence, the tuple appears to be
  47     // ordered as the `types` above. However, as can be seen in the
  48     // static_assert below, the tuple is actually ordered differently.
  49     Tup tup;
  50     char const(&a)[4] = tup[indices[0_c]];
  51     char const(&b)[2] = tup[indices[1_c]];
  52     char const(&c)[1] = tup[indices[2_c]];
  53     char const(&d)[5] = tup[indices[3_c]];
  54     char const(&e)[3] = tup[indices[4_c]];
  55
  56     static_assert(std::is_same<
  57         Tup,
  58         hana::tuple<char[1], char[2], char[3], char[4], char[5]>
  59     >{}, "");
  60
  61     (void)a; (void)b; (void)c; (void)d; (void)e;
  62 }