From 984da7f1a144a8dd178711a24dba14bd83e18f53 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sun, 22 Mar 2020 18:50:55 +0200
Subject: [PATCH] Add mp_iterate

---
 include/boost/mp11/algorithm.hpp | 28 +++++++++++++
 test/Jamfile                     |  1 +
 test/mp_iterate.cpp              | 68 ++++++++++++++++++++++++++++++++
 3 files changed, 97 insertions(+)
 create mode 100644 test/mp_iterate.cpp

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index 168cfdf..f0a700d 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -1192,6 +1192,34 @@ template<template<class...> class F> struct mp_partial_sum_impl_f
 template<class L, class V, template<class...> class F> using mp_partial_sum = mp_second<mp_fold_q<L, mp_list<V, mp_clear<L>>, detail::mp_partial_sum_impl_f<F>> >;
 template<class L, class V, class Q> using mp_partial_sum_q = mp_partial_sum<L, V, Q::template fn>;
 
+// mp_iterate<V, F, R>
+namespace detail
+{
+
+template<class V, template<class...> class F, template<class...> class R, class N> struct mp_iterate_impl;
+
+} // namespace detail
+
+template<class V, template<class...> class F, template<class...> class R> using mp_iterate = typename detail::mp_iterate_impl<V, F, R, mp_valid<R, V>>::type;
+
+namespace detail
+{
+
+template<class V, template<class...> class F, template<class...> class R> struct mp_iterate_impl<V, F, R, mp_false>
+{
+    template<class X> using _f = mp_list<F<X>>;
+    using type = mp_eval_or<mp_list<>, _f, V>;
+};
+
+template<class V, template<class...> class F, template<class...> class R> struct mp_iterate_impl<V, F, R, mp_true>
+{
+    using type = mp_push_front<mp_iterate<R<V>, F, R>, F<V>>;
+};
+
+} // namespace detail
+
+template<class V, class Qf, class Qr> using mp_iterate_q = mp_iterate<V, Qf::template fn, Qr::template fn>;
+
 } // namespace mp11
 } // namespace boost
 
diff --git a/test/Jamfile b/test/Jamfile
index 09afb45..dc2d062 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -121,6 +121,7 @@ run mp_rotate_left.cpp ;
 run mp_rotate_right.cpp ;
 run mp_power_set.cpp ;
 run mp_partial_sum.cpp ;
+run mp_iterate.cpp ;
 
 # integral
 run integral.cpp ;
diff --git a/test/mp_iterate.cpp b/test/mp_iterate.cpp
new file mode 100644
index 0000000..a0d1cb2
--- /dev/null
+++ b/test/mp_iterate.cpp
@@ -0,0 +1,68 @@
+// Copyright 2020 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+#include <tuple>
+#include <cstddef>
+
+struct X1 {};
+struct X2 { using first_type = double; using next_type = X1; };
+struct X3 { using first_type = float; using next_type = X2; };
+struct X4 { using first_type = int; using next_type = X3; };
+
+template<class T> using first_type = typename T::first_type;
+template<class T> using next_type = typename T::next_type;
+
+template<class T1, class T2> struct cons {};
+
+template<class T1, class T2> struct cons2
+{
+    using first_type = T1;
+    using next_type = T2;
+};
+
+using boost::mp11::mp_reverse_fold;
+using boost::mp11::mp_iterate;
+using boost::mp11::mp_first;
+using boost::mp11::mp_second;
+using boost::mp11::mp_rest;
+
+template<class L1> void test()
+{
+    using R1 = mp_iterate<L1, mp_first, mp_rest>;
+    BOOST_TEST_TRAIT_TRUE((std::is_same<L1, R1>));
+
+    using V2 = mp_reverse_fold<L1, void, cons>;
+    using R2 = mp_iterate<V2, mp_first, mp_second>;
+    BOOST_TEST_TRAIT_TRUE((std::is_same<L1, R2>));
+
+#if !BOOST_MP11_WORKAROUND( BOOST_MP11_MSVC, < 1900 )
+
+    using V3 = mp_reverse_fold<L1, void, cons2>;
+    using R3 = mp_iterate<V3, first_type, next_type>;
+    BOOST_TEST_TRAIT_TRUE((std::is_same<L1, R3>));
+
+#endif
+}
+
+int main()
+{
+    using boost::mp11::mp_list;
+
+    test< mp_list<> >();
+    test< mp_list<int> >();
+    test< mp_list<int, void> >();
+    test< mp_list<int, void, float> >();
+    test< mp_list<int, void, float, double> >();
+
+    using boost::mp11::mp_identity_t;
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_iterate<X4, mp_identity_t, next_type>, mp_list<X4, X3, X2, X1>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_iterate<X4, first_type, next_type>, mp_list<int, float, double>>));
+
+    return boost::report_errors();
+}