libcruft-util/types/description.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2018 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "../std.hpp"
#include "../types.hpp"
#include "../debug.hpp"

#include <cstddef>
#include <type_traits>
#include <functional>
#include <utility>


namespace cruft::types {
    ///////////////////////////////////////////////////////////////////////////
    /// Computes the number of elements of a scalar (ie, 1), or of  statically
    /// sized vector type
    template <typename T, typename = void>
    struct arity_trait { };


    //-------------------------------------------------------------------------
    // void arity is explicitly zero.
    template <>
    struct arity_trait<void, void> : public std::integer_sequence<std::size_t, 0> { };


    //-------------------------------------------------------------------------
    // All basic types are unit arity (except for void which is zero).
    template <typename T>
    struct arity_trait<
        T,
        std::enable_if_t<std::is_fundamental_v<T>>
    > : public std::integral_constant<
        std::size_t, 1u
    > { };


    //-------------------------------------------------------------------------
    template <typename T>
    constexpr auto arity_v = arity_trait<T>::value;


    ///////////////////////////////////////////////////////////////////////////
    enum class category {
        NONE,
        UNSIGNED,
        SIGNED,
        REAL,
        BOOL,
    };


    ///------------------------------------------------------------------------
    /// A description of the type that makes up a scalar or vector type.
    ///
    /// A vector3f would be { REAL, sizeof(float), 3 };
    struct description {
        /// The signed, realness, or voidness of the type.
        enum category category;

        /// The number of bytes for an individual instance of this type.
        std::size_t width;

        /// The number of variables that make up an instance.
        std::size_t arity;

        std::size_t alignment;

        /// Returns the number of bytes required to store this type.
        constexpr std::size_t size (void) const noexcept { return width * arity; }
    };


    //-------------------------------------------------------------------------
    constexpr bool
    operator== (description const &a, description const &b) noexcept
    {
        return a.category  == b.category &&
               a.width     == b.width    &&
               a.arity     == b.arity    &&
               a.alignment == b.alignment;
    }


    //-------------------------------------------------------------------------
    constexpr bool
    operator!= (description const &a, description const &b) noexcept
    {
        return !(a == b);
    }


    //-------------------------------------------------------------------------
    template <typename T>
    struct category_traits;

    template <> struct category_traits<u08> : public std::integral_constant<category,category::UNSIGNED> {};
    template <> struct category_traits<u16> : public std::integral_constant<category,category::UNSIGNED> {};
    template <> struct category_traits<u32> : public std::integral_constant<category,category::UNSIGNED> {};
    template <> struct category_traits<u64> : public std::integral_constant<category,category::UNSIGNED> {};

    template <> struct category_traits<i08> : public std::integral_constant<category,category::SIGNED> {};
    template <> struct category_traits<i16> : public std::integral_constant<category,category::SIGNED> {};
    template <> struct category_traits<i32> : public std::integral_constant<category,category::SIGNED> {};
    template <> struct category_traits<i64> : public std::integral_constant<category,category::SIGNED> {};

    template <> struct category_traits<f32> : public std::integral_constant<category,category::REAL> {};
    template <> struct category_traits<f64> : public std::integral_constant<category,category::REAL> {};

    template <> struct category_traits<bool> : public std::integral_constant<category,category::BOOL> {};

    template <typename T>
    constexpr auto category_traits_v = category_traits<T>::value;


    //-------------------------------------------------------------------------
    template <typename T>
    constexpr description
    make_description (void) noexcept
    {
        if constexpr (std::is_void_v<T>) {
            return {
                .category  = category::NONE,
                .width     = 0,
                .arity     = 1,
                .alignment = 0,
            };
        } else {
            return {
                .category = category_traits_v<T>,
                .width = sizeof (T),
                .arity = arity_v<T>,
                .alignment = alignof (T)
            };
        }
    }


    /// Call a functor with the supplied arguments and a type_tag for the
    /// described native type.
    ///
    /// If the type does not exist then throw and invalid_argument exception.
    template <typename FunctionT, typename ...Args>
    decltype(auto)
    visit (description const &descriminator, FunctionT &&func, Args&&...args)
    {
        switch (descriminator.category) {
        case category ::NONE:
            return std::invoke (
                std::forward<FunctionT> (func),
                std::forward<Args> (args)...,
                type_tag<void> {}
            );

        case category::REAL:
            switch (descriminator.width) {
                case 4: return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<f32> {}
                );
                case 8: return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<f64> {}
                );
            default:
                throw std::invalid_argument ("Unsupported floating point width");
            }

        case category::SIGNED:
            switch (descriminator.width) {
            case 1:  return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<i08> {}
                );
            case 2:  return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<i16> {}
                );
            case 4:  return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<i32> {}
                );
            case 8:  return std::invoke (
                std::forward<FunctionT> (func),
                std::forward<Args> (args)...,
                type_tag<i64> {}
            );
            default:
                throw std::invalid_argument ("Unsupported unsigned width");
            }

        case category::UNSIGNED:
            switch (descriminator.width) {
            case 1:  return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<u08> {}
                );
            case 2:  return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<u16> {}
                );
            case 4:  return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<u32> {}
                );
            case 8:  return std::invoke (
                    std::forward<FunctionT> (func),
                    std::forward<Args> (args)...,
                    type_tag<u64> {}
                );
            }

        case category::BOOL:
            return std::invoke (
                std::forward<FunctionT> (func),
                std::forward<Args> (args)...,
                type_tag<bool> {}
            );
        }

        unhandled (descriminator.category);
    }
}

#include <iosfwd>
namespace cruft::types {
    std::ostream& operator<< (std::ostream&, category);
    std::ostream& operator<< (std::ostream&, description);
}