Question

I’m trying to implement a “6 Degrees”-style algorithm, in that I’m trying to find the shortest path between two links. In this scenario, it is a video game, which has area maps, which have portals/warps that link to one another.

Each map has a map ID, int id, and can have several portals, implemented as List<Portal>.
Each portal leads to a map, int ToMapID and has the map that contains it, Map HomeMap.

Anyway, I’ve been successful in finding the least number of warps between two maps, which is good because that is what I am going for. The problem is, I’m having a hard time wrapping my head around how to record the path taken from point A to point B.

Here is what I have implemented:

Map start = allMaps[0];
Map end = allMaps[1];

int distance = 0;

List<Portal> alreadyChecked = new List<Portal>();
List<Portal> queue = start.Portals;

while (queue.Count > 0) {
    distance++;
    List<Portal> newQueue = new List<Portal>();
    foreach (Portal p in queue) {
        foreach (Portal tm in theMap.Portals) {
            if (!alreadyChecked.Contains(tm)) {
                alreadyChecked.Add(tm);
                if (tm.ToMap == end.ID) {
                    Console.WriteLine("Found a path, it is " + distance + " map(s) away;");

                    Console.ReadKey();
                    Environment.Exit(0);
                }
                newQueue.Add(tm);
            }
        }

    }
    queue = newQueue;
}

Ideally, I would like to have an array of Map[], which has the order of steps to go from point A to point B.

I have absolutely no idea where to start, though. How do I unravel the path?

Answer 1

When you do newQueue.Add(tm); you should also record where you’re coming from. An easy solution is adding the relation to a dictionary: dict.Add(tm,p). In the end you can use this dictionary to walk the path backward from the goal by simply asking for the parent of the current portal.