4.2.4 Tile Re-Entry

The majority of the rules in the ruleset control behavior on subsequent visits to an already-recognized tile. The algorithm that is implemented may be described as follows:

• If any of the exits from the intersection have not been explored, leave the intersection through one of the unexplored exits.
• If all of the exits from the intersection have been explored, leave through the exit that is adjacent to the last one, going clockwise. If it entered through the exit that this rule would have selected, leave the tile by the next clockwise exit.

When the robot is in the process of traversing an intersection for the second time, the tile is placed in a state that only denotes the portion of the tile which has been explored while the robot state is set to traverse the intersection appropriately. When the robot leaves the intersection, it is left in a state that expresses both the amount of the intersection which has been explored to date, as well as the exit that was taken.

Since this algorithm iterates through all exits of all intersection tiles, it is certain to explore the entire course.