C move memory parts inplace

Question

I am implementing several datastructures and one primitive I want to use is the following: I have a memory chunk A[N] (it has a variable length, but I take 100 for my examples)

Answer 1

Here's an O(n²) algorithm is pretty straightforward - just rotate the entire buffer a single byte, and then repeat that as many times as steps you want to:

void rotateBuffer(char *buffer, int size, int steps)
{
    char tmp;
    int i;

    for (i = 0; i < steps; i++)
    {
        tmp = buffer[size - 1];
        memmove(buffer + 1, buffer, size - 1);
        buffer[0] = tmp;
    }
}

It won't be fast, but its get the job done, and with only constant temporary storage.

Edit:

If you need to rotate just a sub-part of the buffer relative to a static underlying 'background', as discussed below in the comments, you can do something like this:

void rotateBuffer(int count, int start, int length)
{
    int i;
    int j;
    int index;

    // rotate 'count' bytes
    for (i = 0; i < count; i++)
    {
        // rotate by a single byte
        for (j = length - 1; j >= 0; j--)
        {
            index = start + i + j;
            buf[(index + 1) % SIZE] = buf[index % SIZE];
        }
    }
}

I think it might have a problem if you need to rotate the entire buffer, but in that case you could just fall back to the code above.