The sliding window algorithm is an algorithm which sequentially fills a predetermined buffer size called a window with the corresponding amount of data rows. As each row is added to the window it's compared to each item already contained in the window. If a match with an item is determined then both the driver record (the new item to add to the window) and the candidates (items already in the window) is given the same group ID. This comparison is continued until the driver record has been compared to all items contained within the window.
As new drivers are added the window will eventually reach its predetermined capacity. At this point the window will slide, hence the term Sliding Window. Sliding simply means that the window buffer will remove and write the oldest item in the window as it adds the newest driver record to the window.