TicTacToe.cpp

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

const char X = 'X', O = 'O', EMPTY = ' ', TIE = 'T', NO_ONE = 'N';


using namespace std;

void displayInstructions();
bool getConfirm(string question);
int getMove(string question, int high, int low);
char humanPiece();
void displayBoard(const vector<char> &board);
char winner(const vector<char>& board);
bool isLegal(const vector<char> &board, int move);
int humansMove(const vector<char> &board, char human);
int computersMove(vector<char> board, char computer);
void announceWinner(char winner, char computer, char human);


int main()
{
    int move;
    vector<char> board (9, EMPTY);

    displayInstructions();
    char human = humanPiece();
    char computer;

    char turn = X;

    if (human == X)
        computer = O;
    else computer = X;

    displayBoard(board);

    while (winner(board) == NO_ONE){
        if (turn == computer){
            move = computersMove(board, computer);
            board[move] = computer;
        }
        else{
            move = humansMove(board, human);
            board[move] = human;
        }
        displayBoard(board);
        if (turn == computer) turn = human; else turn = computer;
    }

    announceWinner(winner(board), computer, human);

    return 0;
}

void displayInstructions(){
    cout << "Welcome to Tic-Tac-Toe.\n";
    cout << "Make your move known by entering a number, 0 - 8. The number\n";
    cout << " 0 | 1 | 2\n";
    cout << " ----------\n";
    cout << " 3 | 4 | 5\n";
    cout << " ----------\n";
    cout << " 6 | 7 | 8\n\n";
}

bool getConfirm(string question){
    string response;
    do{
        cout << question << " [y/n] ";
        cin >> response;
    } while (response != "y" && response != "n");

    if (response == "y")
        return true;
    return false;
}

int getMove(string question, int high, int low){
    int number;
    cout << question << " ( " << low << " - " << high << " ): ";
    cin >> number;
    while (number > high || number < low){
        cout << "Sorry, out of range. Retry: ";
        cin >> number;
    }

    return number;
}

char humanPiece(){
    if (getConfirm("Do you want to go first? \n"))
        return X;
    return O;
}

void displayBoard(const vector<char> &board){
    cout << "\n\t" << board[0] << " | " << board[1] << " | " << board[2];
    cout << "\n\t" << "---------";
    cout << "\n\t" << board[3] << " | " << board[4] << " | " << board[5];
    cout << "\n\t" << "---------";
    cout << "\n\t" << board[6] << " | " << board[7] << " | " << board[8];
    cout << "\n\n";
}

char winner(const vector<char> &board){
    const int WINNING_MOVES[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}};

    for (int i = 0; i < 8; ++i)
        if (board[WINNING_MOVES[i][0]] != EMPTY)
            if ((board[WINNING_MOVES[i][0]] == board[WINNING_MOVES[i][1]]) && (board[WINNING_MOVES[i][1]] == board[WINNING_MOVES[i][2]]))
                return board[WINNING_MOVES[i][0]];

    if (count(board.begin(), board.end(), EMPTY) == 0)
        return TIE;

    return NO_ONE;
}

int humansMove(const vector<char> &board, char human){
    int move = getMove("\nWhat's your next move?", board.size()-1, 0);

    while (board[move] != EMPTY)
        move = getMove("\nThat's already occupied, choose an another cell.", board.size()-1, 0);
    return move;
}

int computersMove(vector<char> board, char computer){

    cout << "\nMy Turn\n";
    //Check for a winning move
    for (int i = 0; i < board.size(); ++i){
        if (board[i] == EMPTY){
            board[i] = computer;
            if (winner(board) == computer)
                return i;
            board[i] = EMPTY;
        }
    }

   char human;
   if (computer == X) human = O; else human = X;

   //Check for an opponent's possibility of win
   for (int i = 0; i < board.size(); ++i){
        if (board[i] == EMPTY){
            board[i] = human;
            if (winner(board) == human)
                return i;
            board[i] = EMPTY;
        }
    }

   const int BEST_MOVES[] = {4, 0, 2, 6, 8, 1, 3, 5, 7};


   //Else take the best available cell
   for (int i = 0; i < 9; ++i)
        if (board[BEST_MOVES[i]] == EMPTY)
            return BEST_MOVES[i];
}

void announceWinner(char winner, char computer, char human)
{
    if (winner == computer){
        cout << "I won!\n";
        cout << "As I predicted, human, I am triumphant once more - proof\n";
        cout << "that computers are superior to humans in all regards.\n";
    }

    else if (winner == human){
        cout << "You won...\n";
        cout << "No, no! It cannot be! Somehow you tricked me, human.\n";
        cout << "But never again! I, the computer, so swear it!\n";
    }

    else{
        cout << "It's a tie.\n";
        cout << "You were most lucky, human, and somehow managed to tie me.\n";
        cout << "Celebrate... for this is the best you will ever achieve.\n";
    }
}