Simpler magic_enum which only implements enum_to_string()

magic_enum_reduced.h

// SPDX-License-Identifier: MIT
// Copyright (c) 2023 Rink Springer <rink@rink.nu>
//
// Based on https://github.com/Neargye/magic_enum/blob/master/include/magic_enum.hpp
// Copyright (c) 2019 - 2022 Daniil Goncharov <neargye@gmail.com>.
//
// Permission is hereby  granted, free of charge, to any  person obtaining a copy
// of this software and associated  documentation files (the "Software"), to deal
// in the Software  without restriction, including without  limitation the rights
// to  use, copy,  modify, merge,  publish, distribute,  sublicense, and/or  sell
// copies  of  the Software,  and  to  permit persons  to  whom  the Software  is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE  IS PROVIDED "AS  IS", WITHOUT WARRANTY  OF ANY KIND,  EXPRESS OR
// IMPLIED,  INCLUDING BUT  NOT  LIMITED TO  THE  WARRANTIES OF  MERCHANTABILITY,
// FITNESS FOR  A PARTICULAR PURPOSE AND  NONINFRINGEMENT. IN NO EVENT  SHALL THE
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#include <array>
#include <iostream>
#include <string_view>
#include <cstdio>
#include <algorithm>

constexpr auto ENUM_MIN_VALUE = -128;
constexpr auto ENUM_MAX_VALUE = 128;

template<std::size_t N>
struct static_string
{
    constexpr static_string(std::string_view sv) noexcept
    {
        // std::copy() is not constexpr in C++17, hence...
        for(std::size_t n = 0; n < N; ++n)
            content[n] = sv[n];
    }
    constexpr operator std::string_view() const noexcept { return { content.data(), N }; }
    
private:
    std::array<char, N + 1> content{};
};

constexpr auto is_pretty(char ch) noexcept
{
    return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z');
}

constexpr auto pretty_name(std::string_view sv) noexcept
{
    for(std::size_t n = sv.size() - 1; n > 0; --n) {
        if (!is_pretty(sv[n])) {
            sv.remove_prefix(n + 1);
            break;
        }
    }
    return sv;
}

template<typename E, E V>
constexpr auto n() noexcept
{
#if defined(__GNUC__) || defined(__clang__)
    return pretty_name({ __PRETTY_FUNCTION__, sizeof(__PRETTY_FUNCTION__) - 2 });
#elif defined(_MSC_VER)
    return pretty_name({ __FUNCSIG__, sizeof(__FUNCSIG__) - 17 });
#endif
}

template<typename E, E V>
constexpr auto is_valid()
{
    constexpr E v = static_cast<E>(V);
    return !n<E, V>().empty();
}

template<typename E>
constexpr auto ualue(std::size_t v)
{
    return static_cast<E>(ENUM_MIN_VALUE + v);
}

template<std::size_t N>
constexpr auto count_values(const bool (&valid)[N])
{
    std::size_t count = 0;
    for(std::size_t n = 0; n < N; ++n)
        if (valid[n]) ++count;
    return count;
}

template<typename E, std::size_t... I>
constexpr auto values(std::index_sequence<I...>) noexcept
{
    constexpr bool valid[sizeof...(I)] = { is_valid<E, ualue<E>(I)>()... };
    constexpr auto num_valid = count_values(valid);
    static_assert(num_valid > 0, "no support for empty enums");

    std::array<E, num_valid> values = {};
    for(std::size_t offset = 0, n = 0; n < num_valid; ++offset) {
        if (valid[offset]) {
            values[n] = ualue<E>(offset);
            ++n;
        }
    }

    return values;
}

template<typename E>
constexpr auto values() noexcept
{
    constexpr auto enum_size = ENUM_MAX_VALUE - ENUM_MIN_VALUE + 1;
    return values<E>(std::make_index_sequence<enum_size>({}));
}

template<typename E>
inline constexpr auto values_v = values<E>();

template<typename E, E V>
constexpr auto enum_name()
{
    constexpr auto name = n<E, V>();
    return static_string<name.size()>(name);
}

template<typename E, E V>
inline constexpr auto enum_name_v = enum_name<E, V>();

template<typename E, std::size_t... I>
constexpr auto entries(std::index_sequence<I...>) noexcept
{
    return std::array<std::pair<E, std::string_view>, sizeof...(I)>{
        {{ values_v<E>[I], enum_name_v<E, values_v<E>[I]>}...}
    };
}

template<typename E>
inline constexpr auto entries_v = entries<E>(std::make_index_sequence< values_v<E>.size() >());

template<typename E>
constexpr std::string_view enum_to_string(E value)
{
    for (const auto& [ key, name ]: entries_v<E>) {
        if (value == key) return name;
    }
    return {};
}

enum class Colour {
    Red = 27,
    Green,
    Blue = 40,
};

int main()
{
    auto f = values_v<Colour>;
    for(const auto& i: f)
        std::cout << static_cast<int>(i) << '\n';

    auto q = entries_v<Colour>;
    for(const auto [a, b]: q)
        std::cout << static_cast<int>(a) << ' ' << b << '\n';

    const std::string_view s = enum_to_string(Colour::Green);
    puts(s.data()); // Green
}