Add mp_nth_element

doc/mp11/algorithm.adoc

@@ -441,6 +441,18 @@ using L1 = mp_list<std::ratio<1,2>, std::ratio<1,4>>;
 using R1 = mp_sort<L1, std::ratio_less>; // mp_list<ratio<1,4>, ratio<1,2>>
 ----
 
+## mp_nth_element_c<L, I, P>
+
+    template<class L, std::size_t I, template<class...> class P> using mp_nth_element_c = /*...*/;
+
+Returns the element at position `I` in `mp_sort<L, P>`.
+
+## mp_nth_element<L, I, P>
+
+    template<class L, class I, template<class...> class P> using mp_nth_element = /*...*/;
+
+Like `mp_nth_element_c`, but with a type argument `I`. `I::value` must be a nonnegative number.
+
 ## mp_min_element<L, P>
 
     template<class L, template<class...> class P> using mp_min_element = /*...*/;

include/boost/mp11/algorithm.hpp

@@ -508,6 +508,45 @@ template<template<class...> class L, class T1, class... T, template<class...> cl
 
 template<class L, template<class...> class P> using mp_sort = typename detail::mp_sort_impl<L, P>::type;
 
+// mp_nth_element(_c)<L, I, P>
+namespace detail
+{
+
+template<class L, std::size_t I, template<class...> class P> struct mp_nth_element_impl;
+
+template<template<class...> class L, class T1, std::size_t I, template<class...> class P> struct mp_nth_element_impl<L<T1>, I, P>
+{
+    static_assert( I == 0, "mp_nth_element index out of range" );
+    using type = T1;
+};
+
+template<template<class...> class L, class T1, class... T, std::size_t I, template<class...> class P> struct mp_nth_element_impl<L<T1, T...>, I, P>
+{
+    static_assert( I < 1 + sizeof...(T), "mp_nth_element index out of range" );
+
+    template<class U> using F = P<U, T1>;
+
+    using part = mp_partition<L<T...>, F>;
+
+    using L1 = mp_first<part>;
+    static std::size_t const N1 = mp_size<L1>::value;
+
+    using L2 = mp_second<part>;
+
+    using type = typename mp_cond<
+
+        mp_bool<(I < N1)>, mp_nth_element_impl<L1, I, P>,
+        mp_bool<(I == N1)>, mp_identity<T1>,
+        mp_true, mp_nth_element_impl<L2, I - N1 - 1, P>
+
+    >::type;
+};
+
+} // namespace detail
+
+template<class L, std::size_t I, template<class...> class P> using mp_nth_element_c = typename detail::mp_nth_element_impl<L, I, P>::type;
+template<class L, class I, template<class...> class P> using mp_nth_element = typename detail::mp_nth_element_impl<L, std::size_t{ I::value }, P>::type;
+
 // mp_find<L, V>
 namespace detail
 {

test/Jamfile

@@ -80,6 +80,7 @@ run mp_with_index_cx.cpp ;
 run mp_from_sequence.cpp ;
 run mp_min_element.cpp ;
 run mp_max_element.cpp ;
+run mp_nth_element.cpp ;
 
 # integral
 run integral.cpp ;

test/mp_nth_element.cpp

@@ -0,0 +1,72 @@
+
+// Copyright 2017 Peter Dimov.
+//
+// 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
+
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/mp11/integral.hpp>
+#include <boost/mp11/function.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+#include <tuple>
+
+int main()
+{
+    using boost::mp11::mp_nth_element;
+    using boost::mp11::mp_nth_element_c;
+    using boost::mp11::mp_list_c;
+    using boost::mp11::mp_sort;
+    using boost::mp11::mp_less;
+    using boost::mp11::mp_at_c;
+    using boost::mp11::mp_size_t;
+    using boost::mp11::mp_rename;
+
+    {
+        using L1 = mp_list_c<int, 7, 1, 11, 3, 2, 2, 4>;
+        using L2 = mp_sort<L1, mp_less>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 0, mp_less>, mp_at_c<L2, 0>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 1, mp_less>, mp_at_c<L2, 1>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 2, mp_less>, mp_at_c<L2, 2>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 3, mp_less>, mp_at_c<L2, 3>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 4, mp_less>, mp_at_c<L2, 4>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 5, mp_less>, mp_at_c<L2, 5>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 6, mp_less>, mp_at_c<L2, 6>>));
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<0>, mp_less>, mp_at_c<L2, 0>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<1>, mp_less>, mp_at_c<L2, 1>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<2>, mp_less>, mp_at_c<L2, 2>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<3>, mp_less>, mp_at_c<L2, 3>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<4>, mp_less>, mp_at_c<L2, 4>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<5>, mp_less>, mp_at_c<L2, 5>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<6>, mp_less>, mp_at_c<L2, 6>>));
+    }
+
+    {
+        using L1 = mp_rename<mp_list_c<int, 7, 1, 11, 3, 2, 2, 4>, std::tuple>;
+        using L2 = mp_sort<L1, mp_less>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 0, mp_less>, mp_at_c<L2, 0>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 1, mp_less>, mp_at_c<L2, 1>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 2, mp_less>, mp_at_c<L2, 2>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 3, mp_less>, mp_at_c<L2, 3>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 4, mp_less>, mp_at_c<L2, 4>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 5, mp_less>, mp_at_c<L2, 5>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element_c<L1, 6, mp_less>, mp_at_c<L2, 6>>));
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<0>, mp_less>, mp_at_c<L2, 0>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<1>, mp_less>, mp_at_c<L2, 1>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<2>, mp_less>, mp_at_c<L2, 2>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<3>, mp_less>, mp_at_c<L2, 3>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<4>, mp_less>, mp_at_c<L2, 4>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<5>, mp_less>, mp_at_c<L2, 5>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_nth_element<L1, mp_size_t<6>, mp_less>, mp_at_c<L2, 6>>));
+    }
+
+    return boost::report_errors();
+}