in-place interleave

void deinterleave_split(int *a, int len)
{
    if (2 == len)
    {
        // not needed
    }
    else if (4 == len)
    {
        int t = a[1];
        a[1] = a[2];
        a[2] = t;
    }
    else if (0 == (len % 4))
    {
        deinterleave_split(&a[0], len/2);
        deinterleave_split(&a[len/2], len/2);
        for (int i = len/4; i < len/2; i++)
        {
            int t = a[i];
            a[i] = a[i+len/4];
            a[i+len/4] = t;
        }
    }
    else
    {
        int t;
        deinterleave_split(&a[0], len-2);
        t = a[len-2];
        memmove(&a[len/2],&a[len/2-1],sizeof(int)*(len/2-1));
        a[len/2-1] = t;
    }
}

void interleave_split(int *a, int len)
{
    if (2 == len)
    {
        // no need
    }
    else if (4 == len)
    {
        int t = a[1];
        a[1] = a[2];
        a[2] = t;
    }
    else if (0 == (len % 4))
    {
        for (int i = len/4; i < len/2; i++)
        {
            int t = a[i];
            a[i] = a[i+len/4];
            a[i+len/4] = t;
        }
        interleave_split(&a[0], len/2);
        interleave_split(&a[len/2], len/2);
    }
    else
    {
        int t;
        t = a[len/2-1];
        memmove(&a[len/2-1],&a[len/2],sizeof(int)*(len/2-1));
        a[len-2] = t;
        interleave_split(&a[0], len-2);
    }
}