/// <summary>
/// Perform a Breadth First Search on the graph, given a station to start at, and a destination. This function will always find a path
/// (and currently prints it out to the console and returns true) if the graph is connected, however, the path is not (guaranteed to be)
/// optimal.
///
/// Because Breadth First Search is primarily a search algorithm, and not a path finding algorithm, some extra functionality has been added
/// to the graph, namely the wrapping of GraphNode objects in PathNode objects, which contain a linked-list style reference from a node to
/// it's previous node, to enable a backtrace once the destination is found. This backtrace returns the path taken to the node that is found
/// and is the desired output of this function. The return value needs to adjusted a JSON object that contains this path.
/// </summary>
/// <param name="start">The station to start at.</param>
/// <param name="end">The station to end at.</param>
/// <returns>At this stage, true = found, false = not found (and generally means an error has occurred, probably in the creation
/// of the graph structure, and probably with the format of the input data).</returns>
public bool BFS(Station start, Station end)
{
// this dictionary contains PathNode pairs, a node and it's previous node, to determine the path
// a PathNode contains a GraphNode and the name of the line that was used when passing the node.
Dictionary<PathNode, PathNode> path = new Dictionary<PathNode, PathNode>();
// FIRST CHECK IF THE start NODE EXISTS
GraphNode root = null;
bool found = false;
foreach (GraphNode n in graph) // first check if the station is in the graph
{
if (n.name == start.Name)
{
root = n; // store this node as the root node to begin BFS with
found = true; // the station does exist
}
}
if (found == false) // the station does NOT exist, return
return false;
// END CHECK IF start NODE EXISTS
// BFS PROPER
Queue<PathNode> queue = new Queue<PathNode>(); // pathnode stores the line as well as the node
List<string> visited = new List<string>(); // this list contains the name of ALL nodes that have been visited.
queue.Enqueue(new PathNode(root.name, "", root)); // enqueue the root node (starting point)
visited.Add(root.name); // and mark it as visited (this list keeps track of visited nodes so they don't get visited again)
path.Add(new PathNode(root.name, "", root), null); // no line given to PathNode just yet
while (queue.Count > 0) // while the queue is not empty, we still have nodes to check
{
PathNode check = queue.Dequeue(); // dequeue the next node
if (check.node.name == end.Name) // if this is the end node, we are done
{
//Console.WriteLine("FOUND IT!! Start {0} End {1}\n", start.Name, end.Name); // print a message
PathNode curr = check, prev; // set curr to the node we found and init prev to null
List<string> list_path = new List<string>(); // this list contains the path
list_path.Add(curr.line + " - " + curr.station); // add the node we just found as the end of the path
while (curr.node != root) // while we are not back at the first node
{
if (path.TryGetValue(curr, out prev) == true) // found a value matching the key
{
list_path.Add(curr.line + " - " + prev.station); // add the previous node to the path
curr = prev; // set current node to previous node and repeat
}
else
{
Console.WriteLine("Couldn't find the prev pathnode, path broken."); // uh oh
//return false; // we should return if this happens, there's no way to show the path, it shouldn't happen though
break; // finished, we're doomed
}
}
//print the path to console
//Console.WriteLine();
list_path.Reverse(); // the nodes are back to front, reverse them
foreach (string s in list_path)
{
Console.WriteLine("Path: {0}", s); // print each 'line - station' in the path
}
return true;
}
// This part is the main BFS loop, it just keeps going until we find the destination
// FIND ALL NODES THAT THIS NODE CONNECTS TO AND ADD THEM TO THE QUEUE
string[] lines = check.node.adjacency_list.Keys.ToArray<string>(); // get all the lines that this node is part of
foreach (string line in lines)
{
Dictionary<string, Station> adj = check.node.adjacency_list[line]; // grab the next/prev dict
foreach (KeyValuePair<string, Station> pair in adj) // for next, and prev
{
if (pair.Value != null) // either next or prev exists
{
foreach (GraphNode n in graph) // find the station in the graph (this sucks, I have to search the whole graph to find the node again)
{
if (n.name == pair.Value.Name && visited.Contains(n.name) == false) // if the node name matches the next/prev name we are checking AND we haven't visited it before
{
PathNode to_queue = new PathNode(n.name, line, n); // create a new PathNode with the node name, line and a copy of the GraphNode
queue.Enqueue(to_queue); // enqueue it
visited.Add(n.name); // mark it as visited
found = true; // flag the station does exist
path.Add(to_queue, check); // add the node, and it's previous node to the path so we can retrace steps at the end
}
}
if (found == false) // the station does NOT exist, return false (this is bad and shouldn't happen, means we have bad references)
return false;
}
}
}
}
return false; // not found, again, shouldn't happen if we are searching for a station that exists
}
public List<string> lines; // what lines is this station part of (ie a train line and a bus route)
#endregion Fields
#region Constructors
public GraphNode(Station station, string line, Station next, Station prev)
{
name = station.Name; // set the name of the node to the name of the station
lines = new List<string>(); // this list is kinda redundant, but you can see what lines exist in the graph if you access it
lines.Add(line); // add this line/route to the list of lines/routes that this station is part of
adjacency_list = new Dictionary<string, Dictionary<string, Station>>(); // <line, <next/prev, station>>
Dictionary<string, Station> adj = new Dictionary<string, Station>(); // <next = Station, prev = Station>
adj.Add("next", next); // "next" = Station next
adj.Add("prev", prev); // "prev" = Station prev
adjacency_list.Add(line, adj);
}
