From 080c3437edbc8ddf7332d4786ff4b08f6e10b008 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Fri, 12 Mar 2021 04:27:50 +0200
Subject: [PATCH] Add mp_intersperse

---
 include/boost/mp11/algorithm.hpp | 21 ++++++++++++++++++
 test/Jamfile                     |  1 +
 test/mp_intersperse.cpp          | 38 ++++++++++++++++++++++++++++++++
 3 files changed, 60 insertions(+)
 create mode 100644 test/mp_intersperse.cpp

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index 8289be5..1b94f7e 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -1240,6 +1240,27 @@ template<class L, class Q> using mp_pairwise_fold_impl = mp_transform_q<Q, mp_po
 template<class L, class Q> using mp_pairwise_fold_q = mp_eval_if<mp_empty<L>, mp_clear<L>, detail::mp_pairwise_fold_impl, L, Q>;
 template<class L, template<class...> class F> using mp_pairwise_fold = mp_pairwise_fold_q<L, mp_quote<F>>;
 
+namespace detail
+{
+
+template<class L, class S> struct mp_intersperse_impl
+{
+};
+
+template<template<class...> class L, class S> struct mp_intersperse_impl<L<>, S>
+{
+    using type = L<>;
+};
+
+template<template<class...> class L, class T1, class... T, class S> struct mp_intersperse_impl<L<T1, T...>, S>
+{
+    using type = mp_append<L<T1>, L<S, T>...>;
+};
+
+} // namespace detail
+
+template<class L, class S> using mp_intersperse = typename detail::mp_intersperse_impl<L, S>::type;
+
 } // namespace mp11
 } // namespace boost
 
diff --git a/test/Jamfile b/test/Jamfile
index 8e46ac5..25e7d6b 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -124,6 +124,7 @@ run mp_partial_sum.cpp ;
 run mp_iterate.cpp ;
 run mp_pairwise_fold.cpp ;
 run mp_pairwise_fold_q.cpp ;
+run mp_intersperse.cpp ;
 
 # integral
 run integral.cpp ;
diff --git a/test/mp_intersperse.cpp b/test/mp_intersperse.cpp
new file mode 100644
index 0000000..5a5ef16
--- /dev/null
+++ b/test/mp_intersperse.cpp
@@ -0,0 +1,38 @@
+// Copyright 2021 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 <utility>
+
+struct X1 {};
+struct X2 {};
+struct X3 {};
+struct X4 {};
+struct X5 {};
+
+int main()
+{
+    using boost::mp11::mp_list;
+    using boost::mp11::mp_intersperse;
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<mp_list<>, void>, mp_list<>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<mp_list<X1>, void>, mp_list<X1>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<mp_list<X1, X2>, void>, mp_list<X1, void, X2>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<mp_list<X1, X2, X3>, void>, mp_list<X1, void, X2, void, X3>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<mp_list<X1, X2, X3, X4>, void>, mp_list<X1, void, X2, void, X3, void, X4>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<mp_list<X1, X2, X3, X4, X5>, void>, mp_list<X1, void, X2, void, X3, void, X4, void, X5>>));
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<std::tuple<>, void>, std::tuple<>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<std::tuple<X1>, void>, std::tuple<X1>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<std::tuple<X1, X2>, void>, std::tuple<X1, void, X2>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<std::tuple<X1, X2, X3>, void>, std::tuple<X1, void, X2, void, X3>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<std::tuple<X1, X2, X3, X4>, void>, std::tuple<X1, void, X2, void, X3, void, X4>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_intersperse<std::tuple<X1, X2, X3, X4, X5>, void>, std::tuple<X1, void, X2, void, X3, void, X4, void, X5>>));
+
+    return boost::report_errors();
+}