From f387448581ce29d923ebd42efb60b94834de74ca Mon Sep 17 00:00:00 2001
From: Glen Fernandes <glen.fernandes@gmail.com>
Date: Thu, 30 Aug 2018 22:35:23 -0400
Subject: [PATCH] Implement mp_starts_with

---
 doc/mp11/algorithm.adoc          |  7 ++++
 include/boost/mp11/algorithm.hpp | 24 ++++++++++++
 test/Jamfile                     |  2 +
 test/mp_starts_with.cpp          | 64 ++++++++++++++++++++++++++++++++
 test/mp_starts_with_sf.cpp       | 45 ++++++++++++++++++++++
 5 files changed, 142 insertions(+)
 create mode 100644 test/mp_starts_with.cpp
 create mode 100644 test/mp_starts_with_sf.cpp

diff --git a/doc/mp11/algorithm.adoc b/doc/mp11/algorithm.adoc
index aad27c4..8f53163 100644
--- a/doc/mp11/algorithm.adoc
+++ b/doc/mp11/algorithm.adoc
@@ -199,6 +199,13 @@ As `mp_count_if`, but takes a quoted metafunction.
 
 `mp_contains<L, V>` is `mp_true` when `L` contains an element `V`, `mp_false` otherwise.
 
+## mp_starts_with<L1, L2>
+
+    template<class L1, class L2> using mp_starts_with = /*...*/;
+
+`mp_starts_with<L1, L2>` is `mp_true` when `L1` starts with `L2`, `mp_false`
+otherwise.
+
 ## mp_repeat_c<L, N>
 
     template<class L, std::size_t N> using mp_repeat_c = /*...*/;
diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index 5a6a200..581a213 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -987,6 +987,30 @@ template<class L, class I, class J> using mp_erase = mp_append<mp_take<L, I>, mp
 // mp_erase_c<L, I, J>
 template<class L, std::size_t I, std::size_t J> using mp_erase_c = mp_append<mp_take_c<L, I>, mp_drop_c<L, J>>;
 
+// mp_starts_with<L1, L2>
+// contributed by Glen Joseph Fernandes (glenjofe@gmail.com)
+namespace detail {
+
+template<class L1, class L2>
+struct mp_starts_with_impl { };
+
+template<template<class...> class L1, class... T1, template<class...> class L2,
+    class... T2>
+struct mp_starts_with_impl<L1<T1...>, L2<T2...> > {
+    template<class L>
+    static mp_false check(L);
+
+    template<class... T>
+    static mp_true check(mp_list<T2..., T...>);
+
+    using type = decltype(check(mp_list<T1...>()));
+};
+
+} // namespace detail
+
+template<class L1, class L2>
+using mp_starts_with = typename detail::mp_starts_with_impl<L1, L2>::type;
+
 // mp_min_element<L, P>
 // mp_max_element<L, P>
 //   in detail/mp_min_element.hpp
diff --git a/test/Jamfile b/test/Jamfile
index 85ac992..168e0fd 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -46,6 +46,8 @@ run mp_count.cpp ;
 run mp_count_if.cpp ;
 run mp_count_if_q.cpp ;
 run mp_contains.cpp ;
+run mp_starts_with.cpp ;
+run mp_starts_with_sf.cpp ;
 run mp_repeat.cpp ;
 run mp_product.cpp ;
 run mp_drop.cpp ;
diff --git a/test/mp_starts_with.cpp b/test/mp_starts_with.cpp
new file mode 100644
index 0000000..8801220
--- /dev/null
+++ b/test/mp_starts_with.cpp
@@ -0,0 +1,64 @@
+// Copyright 2018 Glen Joseph Fernandes
+// (glenjofe@gmail.com)
+//
+// 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>
+#include <type_traits>
+#include <tuple>
+#include <utility>
+
+int main()
+{
+    using boost::mp11::mp_list;
+    using boost::mp11::mp_starts_with;
+    using boost::mp11::mp_true;
+    using boost::mp11::mp_false;
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<>,
+        mp_list<> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<>,
+        mp_list<int> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int>,
+        mp_list<> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<>,
+        std::tuple<int> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<std::tuple<int>,
+        mp_list<> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int>,
+        mp_list<int> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int>,
+        mp_list<char> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<std::tuple<int>,
+        mp_list<int> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int>,
+        std::tuple<char> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int, void>,
+        mp_list<int> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int, void>,
+        mp_list<char> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int>,
+        mp_list<int, void> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<std::pair<int, char>,
+        mp_list<int> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int>,
+        std::pair<int, char> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int, void>,
+        mp_list<int, void> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int, void>,
+        mp_list<int, int> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<std::tuple<int, char>,
+        mp_list<int, char> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int, char>,
+        std::tuple<int, int> >, mp_false>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int, void, int>,
+        mp_list<int, void> >, mp_true>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_starts_with<mp_list<int, void, int>,
+        mp_list<int, int> >, mp_false>));
+    return boost::report_errors();
+}
diff --git a/test/mp_starts_with_sf.cpp b/test/mp_starts_with_sf.cpp
new file mode 100644
index 0000000..40d757e
--- /dev/null
+++ b/test/mp_starts_with_sf.cpp
@@ -0,0 +1,45 @@
+// Copyright 2018 Glen Joseph Fernandes
+// (glenjofe@gmail.com)
+//
+// 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/utility.hpp>
+#include <boost/mp11/algorithm.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+
+int main()
+{
+    using boost::mp11::mp_valid;
+    using boost::mp11::mp_starts_with;
+    using boost::mp11::mp_list;
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        void>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        mp_list<> >));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        mp_list<int> >));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        void, void>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        mp_list<>, void>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        mp_list<int>, void>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        void, mp_list<> >));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_starts_with,
+        void, mp_list<int> >));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_starts_with,
+        mp_list<>, mp_list<> >));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_starts_with,
+        mp_list<>, mp_list<int> >));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_starts_with,
+        mp_list<int>, mp_list<> >));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_starts_with,
+        mp_list<int>, mp_list<char> >));
+    return boost::report_errors();
+}