Question

I’m using the wikipedia pseudocode to base against –

function alphabeta(node, depth, α, β, Player)         
    if  depth = 0 or node is a terminal node
        return the heuristic value of node
    if  Player = MaxPlayer
        for each child of node
            α := max(α, alphabeta(child, depth-1, α, β, not(Player) ))     
            if β ≤ α
                break                             (* Beta cut-off *)
        return α
    else
        for each child of node
            β := min(β, alphabeta(child, depth-1, α, β, not(Player) ))     
            if β ≤ α
                break                             (* Alpha cut-off *)
        return β

and here is my java implementation –

private int alphabeta(Node newNode, int depth, int alpha, int beta, boolean Player) {
    Integer[] children;
    if(depth == 0 || newNode.allNodesFull()){
        return (newNode.blacknodes() - newNode.whitenodes());
    }
    if(Player == false){
        children = newNode.findMovesBlack();
        Arrays.sort(children);
        for(Integer child: children){
            nodesGenerated ++;
            alpha = Math.max(alpha, alphabeta(new Node(newNode.move(child), true),
                            depth - 1, alpha, beta, !Player));
            if(beta <= alpha)
                break;
        }return alpha;
    }else{
        children = newNode.findMovesWhite();
        Arrays.sort(children);
        for(Integer child: children){
            nodesGenerated ++;
            beta  = Math.min(beta, alphabeta(new Node(newNode.move(child), false), 
                            depth - 1, alpha, beta, !Player));
            if(beta <= alpha)
                break;
        }return beta;
    }
} 

after some revisions to my code there is no longer a problem with it returning early, but I do have a problem with alpha and beta never changing

I’ll give an explanation of what happens, assume they work

findMovesBlack() and findMovesWhite() both return Integer[] arrays that have the possible positions that either player can move regardless of whos turn it is.
For the initial position of Reversi, findMovesBlack() would return [19, 26, 37, 44]

allNodesFull() returns a boolean if findMovesBlack() and findMovesWhite() both have lengths of 0.

blacknodes() and whitenodes() return the amount of black or white nodes respectively.

Node.move(int coordinate) returns a String[] array which contains the new position that has been flipped and placed. trust me, it works correctly.

a Node(String[] gameboard, boolean player-to-move) just sets up a new position with the parameters we’ve found.

I believe that’s all you need to see. I’ve ironed all the kinks out of the back-end.

Answer 1

The answer was in the implementation of the beta and alpha values. I had to mess with the positions relative to the = sign a lot.