From 07892c8d73ee29b97a9ae47545c17533c91d618b Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sun, 3 Feb 2019 19:48:16 +0200
Subject: [PATCH] Add mp_filter

---
 include/boost/mp11/algorithm.hpp | 21 +++++++++
 test/Jamfile                     |  1 +
 test/mp_filter.cpp               | 78 ++++++++++++++++++++++++++++++++
 3 files changed, 100 insertions(+)
 create mode 100644 test/mp_filter.cpp

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index ba2b181..31d5c4e 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -174,6 +174,27 @@ template<template<class...> class P, template<class...> class F, class... L> str
 template<template<class...> class P, template<class...> class F, class... L> using mp_transform_if = typename detail::mp_transform_if_impl<P, F, L...>::type;
 template<class Qp, class Qf, class... L> using mp_transform_if_q = typename detail::mp_transform_if_impl<Qp::template fn, Qf::template fn, L...>::type;
 
+// mp_filter<P, L...>
+namespace detail
+{
+
+template<template<class...> class P, class L1, class... L> struct mp_filter_impl
+{
+    using Qp = mp_quote<P>;
+
+    template<class T1, class... T> using _f = mp_if< mp_invoke_q<Qp, T1, T...>, mp_list<T1>, mp_list<> >;
+
+    using _t1 = mp_transform<_f, L1, L...>;
+    using _t2 = mp_apply<mp_append, _t1>;
+
+    using type = mp_assign<L1, _t2>;
+};
+
+} // namespace detail
+
+template<template<class...> class P, class... L> using mp_filter = typename detail::mp_filter_impl<P, L...>::type;
+template<class Q, class... L> using mp_filter_q = typename detail::mp_filter_impl<Q::template fn, L...>::type;
+
 // mp_fill<L, V>
 namespace detail
 {
diff --git a/test/Jamfile b/test/Jamfile
index b46dd24..7137c5f 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -48,6 +48,7 @@ run mp_transform_q.cpp ;
 run mp_transform_sf.cpp ;
 run mp_transform_if.cpp ;
 run mp_transform_if_q.cpp ;
+run mp_filter.cpp ;
 run mp_fill.cpp ;
 run mp_count.cpp ;
 run mp_count_if.cpp ;
diff --git a/test/mp_filter.cpp b/test/mp_filter.cpp
new file mode 100644
index 0000000..6a2ea13
--- /dev/null
+++ b/test/mp_filter.cpp
@@ -0,0 +1,78 @@
+
+// Copyright 2019 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/core/lightweight_test_trait.hpp>
+
+using boost::mp11::mp_int;
+
+template<class N> using mod_2 = mp_int<N::value % 2>;
+template<class N> using mod_3 = mp_int<N::value % 3>;
+template<class N> using mod_6 = mp_int<N::value % 6>;
+
+using boost::mp11::mp_not;
+using boost::mp11::mp_plus;
+
+template<class T> using P1 = mp_not<mod_6<T>>;
+template<class T1, class... T> using P2 = mp_not<mp_plus<T...>>;
+
+using boost::mp11::mp_bool;
+
+template<std::size_t N> struct second_is
+{
+    template<class T1, class T2> using fn = mp_bool< T2::value == N >;
+};
+
+using boost::mp11::mp_first;
+using boost::mp11::mp_filter_q;
+using boost::mp11::mp_iota;
+using boost::mp11::mp_size;
+
+template<class L, std::size_t N> using at_c = mp_first< mp_filter_q< second_is<N>, L, mp_iota<mp_size<L>> > >;
+
+int main()
+{
+    using boost::mp11::mp_iota_c;
+    using boost::mp11::mp_filter;
+    using boost::mp11::mp_list;
+    using boost::mp11::mp_size_t;
+    using boost::mp11::mp_transform;
+
+    {
+        int const N = 12;
+        using L1 = mp_iota_c<N>;
+
+        using R1 = mp_filter<P1, L1>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R1, mp_list<mp_size_t<0>, mp_size_t<6>>>));
+
+        using L2 = mp_transform<mod_2, L1>;
+        using L3 = mp_transform<mod_3, L1>;
+        using L6 = mp_transform<mod_6, L1>;
+
+        using R2 = mp_filter<P2, L1, L6>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R2, mp_list<mp_size_t<0>, mp_size_t<6>>>));
+
+        using R3 = mp_filter<P2, L1, L2, L3>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R3, mp_list<mp_size_t<0>, mp_size_t<6>>>));
+    }
+
+    {
+        int const N = 64;
+        int const M = 17;
+
+        using L1 = mp_iota_c<N>;
+        using R1 = at_c<L1, M>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R1, mp_size_t<M>>));
+    }
+
+    return boost::report_errors();
+}