Constant Expression Generation of Roman Numerals. Every roman numeral generated with this is a string literal, and can be used as such.

roman_numerals.cpp

/**
 * MIT License
 * 
 * Copyright (c) 2022 Sam Belliveau
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
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 * 
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 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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 * SOFTWARE.
 */

#ifndef ROMAN_NUMERALS_HPP
#define ROMAN_NUMERALS_HPP 1

#include <cstdint>
#include <array>

namespace roman
{
    template<std::size_t N>
    struct base : public base<N-1> {};

    #define ROMAN_NUMERALS_MAKE_BASE(N, T) \
    template<> \
    struct base<N> \
    { \
        static constexpr std::size_t value{N}; \
        static constexpr std::size_t size{sizeof(T)}; \
        static constexpr std::array<char, size> letters{T}; \
        static constexpr char const* text{T}; \
    };

    ROMAN_NUMERALS_MAKE_BASE(1, "I")
    ROMAN_NUMERALS_MAKE_BASE(4, "IV")
    ROMAN_NUMERALS_MAKE_BASE(5, "V")
    ROMAN_NUMERALS_MAKE_BASE(9, "IX")
    ROMAN_NUMERALS_MAKE_BASE(10, "X")
    ROMAN_NUMERALS_MAKE_BASE(40, "XL")
    ROMAN_NUMERALS_MAKE_BASE(50, "L")
    ROMAN_NUMERALS_MAKE_BASE(90, "XC")
    ROMAN_NUMERALS_MAKE_BASE(100, "C")
    ROMAN_NUMERALS_MAKE_BASE(400, "CD")
    ROMAN_NUMERALS_MAKE_BASE(500, "D")
    ROMAN_NUMERALS_MAKE_BASE(900, "CM")
    ROMAN_NUMERALS_MAKE_BASE(1000, "M")
    ROMAN_NUMERALS_MAKE_BASE(2000, "MM")
    ROMAN_NUMERALS_MAKE_BASE(3000, "MMM")
    
    #undef ROMAN_NUMERALS_MAKE_BASE

    template <std::size_t N>
    struct numeral
    {
        using MSD = base<N>;
        using SUB = numeral<N - MSD::value>;

        static constexpr std::size_t sizea = MSD::size - 1;
        static constexpr std::size_t sizeb = SUB::size;

        static constexpr std::size_t value = N;
        static constexpr std::size_t size = sizea + sizeb;

        using letters_t = std::array<char, size>;
        static constexpr letters_t concat() {
            letters_t c{};
            for(std::size_t i = 0; i < sizea; ++i) c[00000 + i] = MSD::letters[i];
            for(std::size_t i = 0; i < sizeb; ++i) c[sizea + i] = SUB::letters[i];
            return c;
        }

        static constexpr letters_t letters{concat()};
        static constexpr char const* text{letters.data()};
    };
    
    template<>
    struct numeral<0>
    {
        static constexpr std::size_t value = 0;
        static constexpr std::size_t size = 1;
        static constexpr std::array<char, 1> letters{'\0'};
        static constexpr char const* text{letters.data()};
    };

    template<std::size_t N>
    constexpr char const* numeral_t = numeral<N>::text;
}

#endif