Convert endianness of integral types.

endianness.cpp

// Original: http://stackoverflow.com/a/12867287/1091536
// Usage:
// To convert from given endian to host, use:
//     host = endian(source, endian_of_source)
// To convert from host endian to given endian, use:
//     output = endian(hostsource, endian_you_want_to_output)

enum class Endian: int {
    Big = 1, Little = 0
};

template <typename T>
T endian(T w, Endian endian)
{
    // this gets optimized out into if (endian == host_endian) return w;
    union { uint64_t quad; uint32_t islittle; } t;
    t.quad = 1;
    if (t.islittle ^ static_cast<int>(endian)) return w;
    T r = 0;

    // decent compilers will unroll this (gcc)
    // or even convert straight into single bswap (clang)
    for (int i = 0; i < sizeof(r); i++) {
        r <<= 8;
        r |= w & 0xff;
        w >>= 8;
    }
    return r;
};