TicTacToe.ino

int difficulty=8;

void setup(){
  Serial.begin(9600);
}



char displayChar(int c) {
    switch(c) {
        case -1:
            return 'X';
        case 0:
            return ' ';
        case 1:
            return 'O';
    }
}

void draw(int board[9]) {
    Serial.print(" ");Serial.print(displayChar(board[0])); Serial.print(" | ");Serial.print(displayChar(board[1])); Serial.print(" | ");Serial.print(displayChar(board[2])); Serial.println(" ");
    Serial.println("---+---+---");
    Serial.print(" ");Serial.print(displayChar(board[3])); Serial.print(" | ");Serial.print(displayChar(board[4])); Serial.print(" | ");Serial.print(displayChar(board[5])); Serial.println(" ");
    Serial.println("---+---+---");
    Serial.print(" ");Serial.print(displayChar(board[6])); Serial.print(" | ");Serial.print(displayChar(board[7])); Serial.print(" | ");Serial.print(displayChar(board[8])); Serial.println(" ");
}

int win(const int board[9]) {
    //list of possible winning positions
    unsigned wins[8][3] = {{0,1,2},{3,4,5},{6,7,8},{0,3,6},{1,4,7},{2,5,8},{0,4,8},{2,4,6}};
    int winPos;
    for(winPos = 0; winPos < 8; ++winPos) {
        if(board[wins[winPos][0]] != 0 && board[wins[winPos][0]] == board[wins[winPos][1]] && board[wins[winPos][0]] == board[wins[winPos][2]])
            return board[wins[winPos][2]];
    }
    return 0;
}

int minimax(int board[9], int player, int depth) {
    //check the positions for players
    int winner = win(board);
    if(winner != 0) return winner*player;

    int move = -1;
    int score = -2;
    int i;
    for(i = 0; i < 9; ++i) {
     
        if(board[i] == 0) {
            board[i] = player;
            int thisScore=0;
            if (depth<difficulty){
              thisScore = -minimax(board, player*-1,depth+1);
            }
            
            if(thisScore > score) {
                score = thisScore;
                move = i;
            }
            //choose the worst move for opponent
            board[i] = 0;
        }
    }
    if(move == -1) return 0;
    return score;
}

void computerMove(int board[9]) {
    int move = -1;
    int score = -2;
    int i;
    for(i = 0; i < 9; ++i) {
        if(board[i] == 0) {
            board[i] = 1;
            int tempScore = -minimax(board, -1, 0);
            board[i] = 0;
            if(tempScore > score) {
                score = tempScore;
                move = i;
            }
        }
    }
    //returns a score based on minimax tree at a given node.
    board[move] = 1;
}

void playerMove(int board[9]) {
    int move = 0;
    do {
        Serial.println("\nInput move ([0..8]): ");
        
          while(Serial.available()==0){
              delay(100);
            }
          move = Serial.parseInt();
        
        Serial.println("\n");
    } while ((move >= 9 || move < 0) || board[move] != 0);
    board[move] = -1;
}

void loop() {
    int board[9] = {0,0,0,0,0,0,0,0,0};

    Serial.println("Would you like to play X(1) or O(2)?");

    while(Serial.available()==0){
      delay(100);
    }
    
    int player=Serial.parseInt();

 
    unsigned turn;
    for(turn = 0; turn < 9 && win(board) == 0; ++turn) {
        if((turn+player) % 2 == 0)
            computerMove(board);
        else {
            draw(board);
            playerMove(board);
        }
    }
    switch(win(board)) {
        case 0:
            Serial.println("It's a draw.\n");
            break;
        case 1:
            draw(board);
            Serial.println("You lose.\n");
            break;
        case -1:
            Serial.println("You win!\n");
            break;
    }
}