tic-tac-toe.cpp

// Tic-Tac-Toe
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

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

void instructions();
char askYesNo(string question);
int askNumber(string question, int high, int low = 0);
char humanPiece();
char opponent(char piece);
void displayBoard(const vector<char>& board);
char winner(const vector<char>& board);
bool isLegal(int move, const vector<char>& board);
int humanMove(const vector<char>& board, char human);
int computerMove(vector<char> board, char computer);
void announceWinner(char winner, char computer, char human);

int main()
{
  int move;
  const int NUM_SQUARES = 9;
  vector<char> board(NUM_SQUARES, EMPTY);
  instructions();
  char human = humanPiece();
  char computer = opponent(human);
  char turn = X;
  displayBoard(board);

  while (winner(board) == NO_ONE)
  {
    if (turn == human)
    {
      move = humanMove(board, human);
      board[move] = human;
    }
    else {
      move = computerMove(board, computer);
      board[move] = computer;
    }
    displayBoard(board);
    turn = opponent(turn);
  }
  announceWinner(winner(board), computer, human);

  return 0;
}

void instructions()
{
  cout << "Welcome to the ultimate man-machine showdown: Tic-Tac-Toe.\n";
  cout << "--where human brain is pit against silicon processor\n\n";
  cout << "Make your move known by entering a number, 0 - 8. The number\n";
  cout << "corresponds to the desired board position, as illustrated:\n\n";
  cout << " 0 | 1 | 2\n";
  cout << " ---------\n";
  cout << " 3 | 4 | 5\n";
  cout << " ---------\n";
  cout << " 6 | 7 | 8\n\n";
  cout << "Prepare yourself, human. The battle is about to begin.\n\n";
}

char askYesNo(string question)
{
  char response;
  do {
    cout << question << " (y/n): ";
    cin >> response;
  } while (response != 'y' && response != 'n');
  return response;
}

int askNumber(string question, int high, int low) {
  int number;
  do {
    cout << question << " (" << low << " - " << high << "): ";
    cin >> number;
  } while (number > high || number < low);
  return number;
}

char humanPiece()
{
  char go_first = askYesNo("Do you require the first move?");
  if (go_first == 'y')
  {
    cout << "\nThen take the first move. You will need it.\n";
    return X;
  }
  else
  {
    cout << "\nYour bravery will be your undoing... I will go first.\n";
    return O;
  }
}

char opponent(char piece)
{
  return (piece == X) ? O : X;
}

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_ROWS[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} };

  const int TOTAL_ROWS = 8;
  for (int row = 0; row < TOTAL_ROWS; ++row)
  {
    if ( (board[WINNING_ROWS[row][0]] != EMPTY) && 
      (board[WINNING_ROWS[row][0]] == board[WINNING_ROWS[row][1]]) &&
      (board[WINNING_ROWS[row][1]] == board[WINNING_ROWS[row][2]]) )
    {
      return board[WINNING_ROWS[row][0]];
    }
  }

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

  return NO_ONE;
}

inline bool isLegal(int move, const vector<char>& board)
{
  return (board[move] == EMPTY);
}

int humanMove(const vector<char>& board, char human)
{
  int move = askNumber("Where will you move?", (board.size() - 1));
  while (!isLegal(move, board))
  {
    cout << "\nThat square is already occupied, dear human.\n";
    move = askNumber("Where will you move?", (board.size() - 1));
  }
  cout << "Fine...\n";
  return move;
}

int computerMove(vector<char> board, char computer)
{
  unsigned int move = 0;
  bool found = false;

  while (!found && move < board.size())
  {
    if (isLegal(move, board))
    {
      board[move] = computer;
      found = winner(board) == computer;
      board[move] = EMPTY;
    }
    if (!found)
    {
      ++move;
    }
  }

  if (!found)
  {
    move = 0;
    char human = opponent(computer);
    while (!found && move < board.size())
    {
      if (isLegal(move, board))
      {
        board[move] = human;
        found = winner(board) == human;
        board[move] = EMPTY;
      }
      if (!found)
      {
        ++move;
      }
    }
  }

  if (!found)
  {
    move = 0;
    unsigned int i = 0;
    const int BEST_MOVES[] = { 4, 0, 2, 6, 8, 1, 3, 5, 7 };
    while (!found && board.size())
    {
      move = BEST_MOVES[i];
      if (isLegal(move, board))
        found = true;
      ++i;
    }
  }

  cout << "I shall take square number " << move << endl;
  return move;
}

void announceWinner(char winner, char computer, char human)
{
  if (winner == computer)
  {
    cout << winner << "'s 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 << winner << "'s 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";
  }
}