Align raw string literals to a common baseline at compiletime

cpp_raw_string_align_udl.cpp

#include <cstdlib>
#include <array>
#include <boost/mp11/list.hpp>
#include <boost/mp11/algorithm.hpp>

using namespace boost::mp11;

template <class C, C... Cs>
struct nttp_string {};

template <class String, class Acc, class Splits>
struct nttp_string_splitlines;

template <class C, C... String, C... Acc, template <class...> class SplitsC, class... Splits>
struct nttp_string_splitlines<nttp_string<C, C('\n'), String...>, nttp_string<C, Acc...>, SplitsC<Splits...>> {
    using type = typename nttp_string_splitlines<
        nttp_string<C, String...>, nttp_string<C>, SplitsC<Splits..., nttp_string<C, Acc...>>
    >::type;
};

template <class C, C Front, C... String, C... Acc, class Splits>
struct nttp_string_splitlines<nttp_string<C, Front, String...>, nttp_string<C, Acc...>, Splits> {
    using type = typename nttp_string_splitlines<
        nttp_string<C, String...>, nttp_string<C, Acc..., Front>, Splits
    >::type;
};

template <class C, C... Acc, template <class...> class SplitsC, class... Splits>
struct nttp_string_splitlines<nttp_string<C>, nttp_string<C, Acc...>, SplitsC<Splits...>> {
    using type = SplitsC<Splits..., nttp_string<C, Acc...>>;
};


template <class Splits, class Acc>
struct nttp_string_joinlines {
    using type = Acc;
};

template <template <class...> class SplitsC, class C, C... Cs, class... Splits, C... Acc>
struct nttp_string_joinlines<SplitsC<nttp_string<C, Cs...>, Splits...>, nttp_string<C, Acc...>> {
    using type = typename nttp_string_joinlines<SplitsC<Splits...>, nttp_string<C, Acc..., C('\n'), Cs...>>::type;
};


template <std::size_t Count, class C, C Front, C... Rest>
struct nttp_string_drop_impl {
    using type = typename nttp_string_drop_impl<Count - 1, C, Rest...>::type;
}; 

template <class C, C Front, C... Cs>
struct nttp_string_drop_impl<0, C, Front, Cs...> {
    using type = nttp_string<C, Front, Cs...>;
};

template <std::size_t Count, class From>
struct nttp_string_drop;

template <std::size_t Count, class C, C... Cs>
struct nttp_string_drop<Count, nttp_string<C, Cs...>> {
    using type = typename nttp_string_drop_impl<Count, C, Cs...>::type;
};


template <class From, std::size_t Acc = 0>
struct get_indent;

template <class C, C... From, std::size_t Acc>
struct get_indent<nttp_string<C, C(' '), From...>, Acc> {
    constexpr static std::size_t value = get_indent<nttp_string<C, From...>, Acc + 1>::value;
};

template <class C, C... FromCs, std::size_t Acc>
struct get_indent<nttp_string<C, FromCs...>, Acc> {
    constexpr static std::size_t value = Acc;
};


template <class S>
struct nttp_string_to_cstring;

template <class C, C... Cs>
struct nttp_string_to_cstring<nttp_string<C, Cs...>> {
    constexpr static std::array<C, sizeof...(Cs)> data {Cs...};
    constexpr static C const* value = data.data();
};


template <class C, C... Cs>
struct align_impl {
    using split = typename nttp_string_splitlines<nttp_string<C, Cs...>, nttp_string<C>, mp_list<>>::type;
    using front = mp_front<split>;
    using rest = mp_pop_front<split>;

    constexpr static std::size_t indent = get_indent<mp_front<rest>>::value;

    template <class From>
    using strip_indent = typename nttp_string_drop<indent, From>::type;
    using stripped_rest = mp_transform<strip_indent, rest>;

    using nttp_value = typename nttp_string_joinlines<stripped_rest, front>::type;
    constexpr static C const* value = nttp_string_to_cstring<nttp_value>::value;
};

template <class C, C... Cs>
constexpr auto operator""_align() noexcept {
    return align_impl<C, Cs...>::value;
}

// let's try it out
auto source = R"(for x in [1, 2, 3]:
                    squared = x * x
                    for y in range(squared):
                        print(y))"_align;

auto expected_output = R"(0
                          01
                          012
                          0123
                          01234
                          012345
                          0123456
                          01234567
                          012345678
                          0123456789)"_align;