Connect4 MiniMax algorithm

This is a minimax algorithm designed to play the game of Connect Four. The algorithm searches to a depth of 9 and uses alpha-beta pruning for better performance. This ensures a very high probability of winning, as the algorithm is able to look ahead several moves and choose the best possible move. The minimax algorithm is a classic game-playing algorithm that can be used to play a variety of games, including chess, checkers, and tic-tac-toe. 1 It works by recursively exploring the game tree and choosing the move that leads to the best possible outcome for the player. Alpha-beta pruning is a technique that can be used to improve the performance of the minimax algorithm by eliminating branches of the game tree that are not worth exploring.

Code fragments

max algorithm

if(maxPlayer){
            int value = -9999999;
            int column = validSteps.get(new Random().nextInt(validSteps.size()));
            for(int i: validSteps){
                Board boardCopy = new Board(board);
                boardCopy.step(this.playerIndex, i);
                int newScore = miniMax(boardCopy, depth-1, alpha, beta, false)[1];
                if(newScore > value){
                    value = newScore;
                    column = i;
                }
                alpha = max(alpha, value);
                if(alpha >= beta){
                    break;
                }
            }
            return new int[]{column, value};
        }

note

The mentioned alpha/beta pruning can be seen in the marked section.

Score calculation

private int scoreOfFour(int[] four, int playerIndex){
        int score = 0;
        if(outOfFour(four, playerIndex) == 4) score += 100;
        else if(outOfFour(four, playerIndex) == 3 && outOfFour(four, 0) == 1) score += 5;
        else if(outOfFour(four, playerIndex) == 2 && outOfFour(four, 0) == 2) score += 2;
        if(outOfFour(four, 1) == 3 && outOfFour(four, 0) == 1) score -= 100;
        else if(outOfFour(four, 1) == 2 && outOfFour(four, 0) == 2) score -= 2;
        return score;
    }