libcruft-util/endian.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 2010-2014 Danny Robson <danny@nerdcruft.net>
 */

#ifndef __UTIL_ENDIAN_HPP
#define __UTIL_ENDIAN_HPP

#include "types/bits.hpp"

#include <cstring>
#include <cstdint>
#include <type_traits>

namespace cruft {
    //-------------------------------------------------------------------------
    template <typename T>
    constexpr T
    bswap (T) noexcept;

    // CXX: Update using "if constexpr" when available. It doesn't seem to be
    // worth the dicking about to use enable_if_t to differentiate the
    // signed/unsigned cases.
    #define SIGNED_BSWAP(S)                 \
    template <>                             \
    constexpr                               \
    int##S##_t                              \
    bswap (int##S##_t v) noexcept {         \
        const union {                       \
             int##S##_t s;                  \
            uint##S##_t u;                  \
        } vu = { v };                       \
                                            \
        return __builtin_bswap##S (vu.u);   \
    }

    SIGNED_BSWAP(16)
    SIGNED_BSWAP(32)
    SIGNED_BSWAP(64)

    template <> constexpr  int8_t bswap ( int8_t v) noexcept { return v; }
    template <> constexpr uint8_t bswap (uint8_t v) noexcept { return v; }

    template <> constexpr uint16_t bswap (uint16_t v) noexcept { return __builtin_bswap16 (v); }
    template <> constexpr uint32_t bswap (uint32_t v) noexcept { return __builtin_bswap32 (v); }
    template <> constexpr uint64_t bswap (uint64_t v) noexcept { return __builtin_bswap64 (v); }


    // use a type-punning union to punt the byte swapping operation to the
    // unsigned integer code.
    //
    // this is debatably safe under production compilers like gcc + clang
    // despite what the standard may say about unions.
    template <>
    constexpr float
    bswap (float v) noexcept
    {
        union {
            float f;
            uint32_t u;
        } temp { .f = v };

        temp.u = bswap (temp.u);
        return temp.f;
    }


    //-------------------------------------------------------------------------
    template <typename T>
    constexpr T
    identity (T &&v) {
        return v;
    }


    template <
        typename T,
        typename = std::enable_if_t<
            std::is_integral_v<T>
        >
    >
    T readle (const void *_data)
    {
        auto data = reinterpret_cast<const uint8_t*> (_data);

        T value = 0;

        for (size_t i = 0; i < sizeof (value); ++i)
            value |= T{data[i]} << (i * 8);

        return value;
    }


    /// read bytes from a supplied data pointer and return an integer using
    /// host endian layout.
    template <
        typename T,
        typename = std::enable_if_t<
            std::is_integral_v<T>
        >
    >
    T readhe (const std::uint8_t *data)
    {
        std::aligned_union_t <sizeof (T), T> buffer;
        memcpy (reinterpret_cast<char *> (&buffer), data, sizeof (T));
        return *reinterpret_cast<const T*> (&buffer);
    }


    //-------------------------------------------------------------------------
#if defined(WORDS_BIGENDIAN)
    template <typename T> constexpr T hton (T v) { return v; }
    template <typename T> constexpr T ntoh (T v) { return v; }

    template <typename T> constexpr T htob (T v) { return v; }
    template <typename T> constexpr T htol (T v) { return bswap (v); }

    template <typename T> constexpr T btoh (T v) { return v; }
    template <typename T> constexpr T ltoh (T v) { return bswap (v); }
#else
    template <typename T> constexpr T hton (T v) { return bswap (v); }
    template <typename T> constexpr T ntoh (T v) { return bswap (v); }

    template <typename T> constexpr T htob (T v) { return bswap (v); }
    template <typename T> constexpr T htol (T v) { return v; }

    template <typename T> constexpr T btoh (T v) { return bswap (v); }
    template <typename T> constexpr T ltoh (T v) { return v; }
#endif

    template <typename T> T btol (T t) { return bswap (t); }
    template <typename T> T ltob (T t) { return bswap (t); }

    namespace endian {
        //---------------------------------------------------------------------
        // Uses the TIFF header values. Just because. Don't rely on this.
        enum class scheme : uint16_t {
            BIG    = 0x4D4D,
            LITTLE = 0x4949,

            big = BIG,
            little = LITTLE,

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
            native = little,
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
            native = big
#else
#error Unhandled endianness
#endif
        };


        template <scheme DstV, scheme SrcV>
        struct converter;

        template <> struct converter<scheme::little,scheme::big> {
            template <typename T>
            static T convert (T t) { return btol (t); }
        };

        template <> struct converter<scheme::big,scheme::little> {
             template <typename T>
             static T convert (T t) { return ltob (t); }
        };

        template <scheme DstV, scheme SrcV, typename T>
        T convert (T t)
        {
            return converter<DstV,SrcV>::convert (t);
        }
    }

    //-------------------------------------------------------------------------
    struct from_endian {
        explicit from_endian (endian::scheme _endian):
            src (_endian)
        { ; }

        template <typename T>
        T operator() (const T v) const {
            static_assert (std::is_integral<T>::value || std::is_enum<T>::value,
                           "endian conversion is only defined for integrals currently");

            union {
                typename sized_type<T>::sint sint;
                typename sized_type<T>::uint uint;
            };

            if (std::is_signed<T>::value)
                sint = v;
            else
                uint = v;

            uint = (src == endian::scheme::LITTLE) ? ltoh (uint) : btoh (uint);

            if (std::is_signed<T>::value)
                return T (sint);
            else
                return T (uint);
        }

        endian::scheme src;
    };
}
#endif