General questions on Pathfinding

Feedback & Bugs
#1

I am trying to understand pathfinding to finish the medium puzzles and arm myself for the hard ones. I use C# and have a question to the pathfinding tutorial provided by CodinGame (url: http://codingame.com/direct-puzzle/path-finding ).

Question 1: Cell as datatype?
In the pseudo code, they refer to “cell” as a datatye:

    Add start cell to the queue
    While the queue is not empty:
        Get a cell E from the queue
        [...]

What is th best way to represent this cell. I have been struggling with this a little (for example in the bender challenge or in the codingame optimisation super secret stuff) where I wanted in both case to pair integers. I ended up working with 2 or 3 or 4 integers as scalar instead of a single value representing them (for example an array of 4 values).

I prefer having:

    int x;
    int y;
    int z;
    int t;
    string[,,,] Map;
    string location;
    (...)
    location = Map[x, y, z, t];

than

    int[] coordinates = new int[4];
    string[,,,] Map;
    string location;
    (...)
    location = Map(coordinates[0], coordinates[1], coordinates[2], coordinates[3]);

Is that the right approach? Of should I create a coordinate class with public X, Y, Z and T?

Coordinate coord = new Coordinate(...);
location = Map(coord.X, coord.Y, coord.Z, coord.T);

Question 2: How to test that a cell have not been tested yet?
The tutorial mentions:

For each neighbor V of E (UP, DOWN, RIGHT, LEFT):
        If V not yet tested and is valid
        (...)

Do I keep a list of all existing cells?

List<Coordinate> coordList = new List<Coordinate>();
(...)
Coordinate coord = new Coordinate(...);
if (!coordList.Exists(x => x == coord))
{
    coordList.Add(coord);
    //todo enqueue
}

If this is the right approach, this means my class has to contain:

public override bool Equals(Object obj) 
{...}

public override int GetHashCode()
{...}

public static bool operator == (Coordinate a, Coordinate b)
{...}

public static bool operator != (Coordinate a, Coordinate b)
{...}

How would I define my Hashcode?

Am I generally thinking in the right direction?

Thanks in advance :slight_smile:

1 Like
#2

For BFS, you can use and adapt this pseudo-code:

Create the list of cells that are to be visited (containing the first cell): todo.
Create the list of cells that are already visited: visited.

While todo is not empty
{
  Create temp (the list of visited cells in this loop)
  For each cell c in todo
  {
    For each neighbour n of c that is not in todo nor in temp nor in visited
    {
      add n in temp
    }
  }
  add todo in visited
  replace todo by temp
}
1 Like
#3

Hi Nicolas,

Thanks for your input.

Just to make sure I understood the more concrete implementation:

  1. You create cell as a class in your program. Correct?

  2. You advise to create Lists instead of Queues (as datatypes) since it is possible to have a FIFO macanism in both but Lists have more advanced search functions. Correct?

  3. My question regarding the way to search weither a cell has been tested or not is still open. The most “natural” implementation would be to, for your pseudo code in C# (sorry, I still need this “link” to a language to assimlate '^^)

    List visited = new List();
    List todo = new List();
    […]
    foreach (Cell n in neighbour)
    {
    if(!todo.Contains(n) && !temp.Contains(n) && !visited.Contains(n))
    {
    temp.Add(n)
    }
    }
    […]

But this requires me to override Equals() et GetHashCode() .


This starts to scratch at the limit of my programming understanding. I tried to do something which seems to work but I do not know if I am overriding those methods as intended as I am not sure I understand those articles compltely. My approach would be (C# again):

Constraints:
2D Grid
1<'X<100
1<'Y<100

class Cell
{
    private int X = -1;
    private int Y = -1;

    public Cell (int coordX, int coordY)
    {
        X = coordX;
        Y = coordY;
    }
        
    public override bool Equals(Object obj) 
    {
        return obj is Cell && this == (Cell)obj; // '==' defined below
    }
        
    public override int GetHashCode()
    {
        return X*100+Y; //X and Y <100 so return #XXYY
    }
        
    public static bool operator == (Cell a, Cell b)
    {
        // If both are null, or both are same instance, return true.
        if (System.Object.ReferenceEquals(a, b))
        {
            return true;
        }
        // If one is null, but not both, return false.
        if (((object)a == null) || ((object)b == null))
        {
            return false;
        }
        // Return true if the fields match:
        return a.X == b.X && a.Y == b.Y &&;
    }
        
    public static bool operator !=(Cell a, Cell b)
    {
        return !(a == b);
    }
}

Is this approach correct?

#4

I don’t know C# but Python, where this kind of pseudo-code can be pythonized quite easily.
In fact, I use sets, it’s easier to manage the operations.

#5

Pythonized… ^^ I learned a new word :wink:

Well, if a Java programmer stops by, it might also be helpful, as it seems to me that both languages are relatively similar.

Else I will open a thread in the c# catebory explicitely for this Equals() and GetHashCode() question :slight_smile:

P.S.: I love your Cacodemon avatar!

#6

equals et hashcode are the same in C# and Java so i think i can respond to that.

When you use an “array” class (a list, a queue, a set, a map, anything you want), it use internally equals or hashcode to know if the object is already in the list. For example in Java, ArraySet use equals, and HashSet use hashcode.

Why using an hashcode instead of equals ? Because it is faster most of the time. hashcode should return an unique integer for a specific object in a specific state.

I use C++ for multiplayers puzzles. I always implement Cell and Grid as class. So if i have to do a BFS or a A*, i use a std::queue<Cell*>.

#7

Thanks Magus,

C# forces you to override GetHashCode when you override equals and vice versa. Is the base idea of getting a hashcode based on x and y as showned in my previous post (for example, new Call(13, 52) would have an hashcode of 1352)?

Beside, nicolas, I implemented your pseudocode for the Teads challenge. Thanks for the help. It worked well (until the dataset becomes too big, in which case it seems to show its limits)

1 Like
#8

Hi Naity.
Why can’t I find this puzzle from my puzzles page?https://www.codingame.com/games/puzzles

#9

Because it is not a “real” puzzle. From what I understood they are more exercises that codingame ppl are working on, as a way to train a certain kind of algorithm. They do not reward coding points, thus are not puzzles :wink:

#10

Cool!!.:grinning:
Are they available for all users, if so how can I find them?

#11

For now there arnt much. You can find them on this toppic:

1 Like
#12

Thanks Naity :relaxed:

#13

Yes, you need a heavily modified version of this pseudocode.

#14

Yeah, I searchd for pathfindinng in wikipedia, if i’m right your pseudocode is BFS algorithm. I might need something efficienter to 100% the Teads challenge (A* sounds appealing on paper).

Your pseudocode also helped me solve the dwarfs sitting on the shoulders of the giant. Thanks :slightly_smiling:

A translated working C# implementation looks like

            toTest.Add(allN); //starting node
            int j = 0; //iterator count
            while (toTest.Count > 0)
            {
                List<Node> tempNodes = new List<Node>();
                foreach (Node todo in toTest)
                {
                    foreach (int neighbour in todo.GetNeighbours())
                    {
                        if(!toTest.Exists(x => x.Person == neighbour) && !tempNodes.Exists(x => x.Person == neighbour) && !checkedNodes.Exists(x => x.Person == neighbour))
                        {
                            tempNodes.Add(allNodes.Find(x => x.Person == neighbour));
                        }
                    }
                }
                foreach (Node test in toTest)
                {
                    checkedNodes.Add(test);
                }
                toTest = tempNodes;
                j++;
            }