rotate.cpp

// positive rotations go left, negative go right
void rotate(int arr[], const ptrdiff_t len, const ptrdiff_t m) {
	if(0 == m || m >= len) {
		return;
	}

	ptrdiff_t dst = 0;
	ptrdiff_t first_in_cycle = 0;
	int tmp = arr[first_in_cycle];
	for(ptrdiff_t i = 0; i < len; ++i) {
		ptrdiff_t src = dst + m;
		if(src < 0) {
			src += len;
		}
		if(src >= len) {
			src -= len;
		}
		if(src != first_in_cycle) {
			arr[dst] = arr[src];
			dst = src;
		} else {
			arr[dst] = tmp;
			dst = ++first_in_cycle;
			tmp = arr[first_in_cycle];
		}
	}
}