descr.h

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/*
    pybind11/detail/descr.h: Helper type for concatenating type signatures at compile time

    Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>

    All rights reserved. Use of this source code is governed by a
    BSD-style license that can be found in the LICENSE file.
*/

#pragma once

#include "common.h"

PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
PYBIND11_NAMESPACE_BEGIN(detail)

#if !defined(_MSC_VER)
#  define PYBIND11_DESCR_CONSTEXPR static constexpr
#else
#  define PYBIND11_DESCR_CONSTEXPR const
#endif

/* Concatenate type signatures at compile time */
template <size_t N, typename... Ts>
struct descr {
    char text[N + 1];

    constexpr descr() : text{'\0'} { }
    constexpr descr(char const (&s)[N+1]) : descr(s, make_index_sequence<N>()) { }

    template <size_t... Is>
    constexpr descr(char const (&s)[N+1], index_sequence<Is...>) : text{s[Is]..., '\0'} { }

    template <typename... Chars>
    constexpr descr(char c, Chars... cs) : text{c, static_cast<char>(cs)..., '\0'} { }

    static constexpr std::array<const std::type_info *, sizeof...(Ts) + 1> types() {
        return {{&typeid(Ts)..., nullptr}};
    }
};

template <size_t N1, size_t N2, typename... Ts1, typename... Ts2, size_t... Is1, size_t... Is2>
constexpr descr<N1 + N2, Ts1..., Ts2...> plus_impl(const descr<N1, Ts1...> &a, const descr<N2, Ts2...> &b,
                                                   index_sequence<Is1...>, index_sequence<Is2...>) {
    return {a.text[Is1]..., b.text[Is2]...};
}

template <size_t N1, size_t N2, typename... Ts1, typename... Ts2>
constexpr descr<N1 + N2, Ts1..., Ts2...> operator+(const descr<N1, Ts1...> &a, const descr<N2, Ts2...> &b) {
    return plus_impl(a, b, make_index_sequence<N1>(), make_index_sequence<N2>());
}

template <size_t N>
constexpr descr<N - 1> _(char const(&text)[N]) { return descr<N - 1>(text); }
constexpr descr<0> _(char const(&)[1]) { return {}; }

template <size_t Rem, size_t... Digits> struct int_to_str : int_to_str<Rem/10, Rem%10, Digits...> { };
template <size_t...Digits> struct int_to_str<0, Digits...> {
    static constexpr auto digits = descr<sizeof...(Digits)>(('0' + Digits)...);
};

// Ternary description (like std::conditional)
template <bool B, size_t N1, size_t N2>
constexpr enable_if_t<B, descr<N1 - 1>> _(char const(&text1)[N1], char const(&)[N2]) {
    return _(text1);
}
template <bool B, size_t N1, size_t N2>
constexpr enable_if_t<!B, descr<N2 - 1>> _(char const(&)[N1], char const(&text2)[N2]) {
    return _(text2);
}

template <bool B, typename T1, typename T2>
constexpr enable_if_t<B, T1> _(const T1 &d, const T2 &) { return d; }
template <bool B, typename T1, typename T2>
constexpr enable_if_t<!B, T2> _(const T1 &, const T2 &d) { return d; }

template <size_t Size> auto constexpr _() -> decltype(int_to_str<Size / 10, Size % 10>::digits) {
    return int_to_str<Size / 10, Size % 10>::digits;
}

template <typename Type> constexpr descr<1, Type> _() { return {'%'}; }

constexpr descr<0> concat() { return {}; }

template <size_t N, typename... Ts>
constexpr descr<N, Ts...> concat(const descr<N, Ts...> &descr) { return descr; }

template <size_t N, typename... Ts, typename... Args>
constexpr auto concat(const descr<N, Ts...> &d, const Args &...args)
    -> decltype(std::declval<descr<N + 2, Ts...>>() + concat(args...)) {
    return d + _(", ") + concat(args...);
}

template <size_t N, typename... Ts>
constexpr descr<N + 2, Ts...> type_descr(const descr<N, Ts...> &descr) {
    return _("{") + descr + _("}");
}

PYBIND11_NAMESPACE_END(detail)
PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)