Add mp_rotate_left and mp_rotate_right

include/boost/mp11/algorithm.hpp

@@ -1101,6 +1101,59 @@ struct mp_starts_with_impl<L1<T1...>, L2<T2...> > {
 template<class L1, class L2>
 using mp_starts_with = typename detail::mp_starts_with_impl<L1, L2>::type;
 
+// mp_rotate_left(_c)<L, N>
+namespace detail
+{
+
+template<class L, std::size_t N> struct canonical_rotation
+{
+    // provide a default rotation so that "no type named 'type'" errors appear in mp_rotate_impl instead of here
+    using type = mp_size_t<0>;
+};
+
+template<template<class...> class L, class... T, std::size_t N> struct canonical_rotation<L<T...>, N>
+{
+    // limit divisor to 1 to avoid division by 0 and give a rotation of 0 for lists containing 0 or 1 elements
+    using type = mp_size_t<N % (sizeof...(T) == 0 ? 1 : sizeof...(T))>;
+};
+
+template<class L, std::size_t N> using canonical_rotation_t = typename canonical_rotation<L, N>::type;
+
+struct left_rotation;
+struct right_rotation;
+
+template<class L, class N, class D> struct mp_rotate_impl
+{
+// An error "no type named 'type'" here means that the first argument to mp_rotate_left/mp_rotate_right is not a list
+};
+
+template<template<class...> class L, class... T> struct mp_rotate_impl<L<T...>, mp_size_t<0>, left_rotation>
+{
+    // fast-track the case of an identity rotation
+    using type = L<T...>;
+};
+
+template<template<class...> class L, class... T, std::size_t N> struct mp_rotate_impl<L<T...>, mp_size_t<N>, left_rotation>
+{
+    // avoid errors when rotating fixed-sized lists by using mp_list for the transformation
+    using type = mp_rename<mp_append<mp_drop_c<mp_list<T...>, N>, mp_take_c<mp_list<T...>, N>>, L>;
+};
+
+template<template<class...> class L, class... T, std::size_t N> struct mp_rotate_impl<L<T...>, mp_size_t<N>, right_rotation>
+{
+    // perform right rotation as a left rotation by inverting the number of elements to rotate
+    using type = typename mp_rotate_impl<L<T...>, canonical_rotation_t<L<T...>, sizeof...(T) - N>, left_rotation>::type;
+};
+
+} // namespace detail
+
+template<class L, std::size_t N = 1> using mp_rotate_left_c = typename detail::mp_rotate_impl<L, detail::canonical_rotation_t<L, N>, detail::left_rotation>::type;
+template<class L, class N = mp_size_t<1>> using mp_rotate_left = mp_rotate_left_c<L, std::size_t{ N::value }>;
+
+// mp_rotate_right(_c)<L, N>
+template<class L, std::size_t N = 1> using mp_rotate_right_c = typename detail::mp_rotate_impl<L, detail::canonical_rotation_t<L, N>, detail::right_rotation>::type;
+template<class L, class N = mp_size_t<1>> using mp_rotate_right = mp_rotate_right_c<L, std::size_t{ N::value }>;
+
 // mp_min_element<L, P>
 // mp_max_element<L, P>
 //   in detail/mp_min_element.hpp