memcpy() with a bitshift specified

memshift.c

// positive shifts are left shifts
// left shifts are shifts higher into the word, which in a little-endian system
// is higher in memory
// XXX make endian-agnostic
void *memshift(void *dest, const void *src, size_t size, signed shift)
{
    int shiftleft  = shift > 0;
    int shiftright = shift < 0;
    int destbyteshift = (shiftleft  * (+shift / CHAR_BIT));
    int  srcbyteshift = (shiftright * (-shift / CHAR_BIT));
    int safetop = size - MAX(srcbyteshift, destbyteshift);
    int posshift = abs(shift);
    int  leftshift = posshift % CHAR_BIT;
    int rightshift = CHAR_BIT - leftshift;

    if ((shift % CHAR_BIT) == 0) {
        // fast case
        return memcpy(((char*)dest) + destbyteshift,
                      ((char*)src ) +  srcbyteshift,
                      safetop);
    } else {
        0[(char*)dest + destbyteshift] = 0;
        for (int i = 0; i < safetop; i++) {
            i[(char*)dest + destbyteshift] |=
                i[(unsigned char*)src + srcbyteshift] << leftshift;
            i[(char*)dest + destbyteshift + 1] =
                i[(unsigned char*)src + srcbyteshift] >> rightshift;
        }
    }

    return dest;
}