基于极大极小算法和alpha-beta剪枝实现AI井字棋
关于极大极小算法和alpha-beta剪枝可以参考文章的参考资料，这里仅对其进行代码实现。
其实这个算法单纯的理解并不容易，下面用代码进行实现。
说一下实现这个AI井字棋的思路：
简单的来说就是计算机希望估值函数值最大，而下棋人希望这个估值最小，因此在计算机决策是就用递归的向前看，这里的递归其实蛮不好理解的，但是可以宏观的去理解。
下面是代码的构成：
- 一个ChessBoard的类用来处理一些关于棋盘的事件。
- 一个TicTacToe的类用来实现游戏开始和计算机，玩家决策的事件
ChessBoard.h
//Author : yqtao
//Date :2016.10.30
#ifndef CHESS_BOARD_H
#define CHESS_BOARD_H
#includeChessBoard.cpp
#include"ChessBoard.h"
ChessBoard::ChessBoard() { //默认构造函数
boardInOneDimens.resize(GridNuber);
for (int i = 0; i < GridNuber; i++)
boardInOneDimens[i] = ' ';
vector<string>tmp = {
"- - - - - - - - - - - - -",
"| | | |",
"| | | |",
"| | | |",
"- - - - - - - - - - - - -",
"| | | |",
"| | | |",
"| | | |",
"- - - - - - - - - - - - -",
"| | | |",
"| | | |",
"| | | |",
"- - - - - - - - - - - - -"
};
board = tmp;
}
//check the posion is empty or not
bool ChessBoard::isEmpty(int pos) {
return boardInOneDimens[pos] == ' ';
}
//
bool ChessBoard::isFull() {
for (int i = 0; i < GridNuber; i++) {
if (boardInOneDimens[i] == ' ')
return false;
}
return true;
}
//
bool ChessBoard::canWin(char c) {
//check every row
for (int i = 0; i <= 6; i+=3) {
if (boardInOneDimens[i] == c&&boardInOneDimens[i] == boardInOneDimens[i + 1] &&
boardInOneDimens[i] == boardInOneDimens[i + 2])
return true;
}
//check every col
for (int i = 0; i < 3; i++) {
if (boardInOneDimens[i] == c&&boardInOneDimens[i] == boardInOneDimens[i + 3] &&
boardInOneDimens[i] == boardInOneDimens[i + 6])
return true;
}
//check every diagonals
if (boardInOneDimens[0] == c&&boardInOneDimens[4] == c&&boardInOneDimens[8] == c)
return true;
if (boardInOneDimens[2] == c&&boardInOneDimens[4] == c&&boardInOneDimens[6] == c)
return true;
return false;
}
//
bool ChessBoard::immediateComWin(int& bestMove) {
for (int i = 0; i < GridNuber; i++) {
if (isEmpty(i)) {
boardInOneDimens[i] =Comp_Char;
bool Win = canWin(Comp_Char);
boardInOneDimens[i] = Blank_Char; //backtraceing
if (Win) {
bestMove = i;
return true;
}
}
}
return false;
}
//
bool ChessBoard::immediateHumanWin(int& bestMove) {
for (int i = 0; i < GridNuber; i++) {
if (isEmpty(i)) {
boardInOneDimens[i] = Human_Char;
bool Win = canWin(Human_Char);
boardInOneDimens[i] = Blank_Char; //backtraceing
if (Win) {
bestMove = i;
return true;
}
}
}
return false;
}
//
void ChessBoard::placeComp(int pos) {
boardInOneDimens[pos] = 'X';
}
//
void ChessBoard::placeHuman(int pos) {
boardInOneDimens[pos] = 'O';
}
//
void ChessBoard::unPlace(int pos) {
boardInOneDimens[pos] = ' ';
}
//
void ChessBoard::print() {
int cnt = 0;
for (int i = 2; i <= 10; i += 4) {
for (int j = 4; j <= 20; j += 8) {
board[i][j] = boardInOneDimens[cnt++];
}
}
for (int i = 0; i < ChessBoard_Row; ++i) {
cout << board[i] << endl;
}
}TicTacToe.h
#ifndef TICTACTOE_H
#define TICTACTOE_H
#include "ChessBoard.h"
enum Role { Human, Comp };
static const int CompWin = 2;
static const int Draw = 1;
static const int HumanWin = 0;
class TicTacToe {
public:
void start();
bool handleGameOver();
bool gameIsOver(bool &draw, Role &win);
Role chooseFirstPlace();
void compPlace();
int getBestMove();
void humanPlace();
int getPlacePosition();
void findCompMove(int& bestMove,int& value, int alpha, int beta);
void findHumanMove(int& bestMove,int& value, int alpha, int beta);
private:
ChessBoard board;
};
#endif // !TicTacToeTicTacToe.cpp
#include "TicTacToe.h"
void TicTacToe::start() {
Role firstPlace = chooseFirstPlace();
if (firstPlace == Comp) { // Choose computer to be the first
compPlace();
}
while (1) {
board.print();
if (handleGameOver()) break;
humanPlace();
board.print();
if (handleGameOver()) break;
compPlace();
}
}
Role TicTacToe::chooseFirstPlace() {
int choose;
while (1) {
cout << "who will place first" << endl;
cout << "0 : human first " << endl;
cout << "1 : computer first" << endl;
cin >> choose;
if (choose == 0 || choose == 1) {
cout << endl;
break;
}
else
cout << "error,please enter again" << endl;
}
return (Role)choose;
}
//
void TicTacToe::humanPlace() {
int pos = getPlacePosition();
board.placeHuman(pos);
cout << "Your choice:" << endl;
}
//
int TicTacToe::getPlacePosition() {
int m, n,pos;
while (1) {
cout << "It is your turn, please input where you want :" << endl;
cout << "for example: 2 2 mean you want to add position 2 row,2 col:" << endl;
cin >> m >> n;
if (m < 0 || m>3 || n < 0 || n>3)
cout << "error,please input again:" << endl;
else {
pos = (m - 1) * 3 + n - 1;
if (board.isEmpty(pos))
break;
else
cout << "error,please input again:" << endl;
}
}
return pos;
}
//
void TicTacToe::compPlace() {
int bestMove = getBestMove();
board.placeComp(bestMove);
cout << "the computer choice is: " << endl;
}
int TicTacToe::getBestMove() {
int bestMove = 0, value = 0;
findCompMove(bestMove,value,HumanWin, CompWin);
return bestMove;
}
void TicTacToe::findCompMove(int& bestMove,int &value, int alpha, int beta) {
if (board.isFull())
value = Draw;
else if (board.immediateComWin(bestMove))
value = CompWin;
else {
value = alpha;
for (int i = 0; i < GridNuber&&value < beta; i++) {
if (board.isEmpty(i)) {
board.placeComp(i);
int tmp = -1, response = -1; // Tmp is useless
findHumanMove(tmp, response, value, beta);
board.unPlace(i);
if (response > value) {
value = response;
bestMove = i;
}
}
}
}
}
void TicTacToe::findHumanMove(int& bestMove,int & value, int alpha, int beta) {
if (board.isFull())
value=Draw;
else if (board.immediateHumanWin(bestMove)){
value=HumanWin;
}
else {
value = beta;
for (int i = 0; i < GridNuber&&value>alpha; i++) {
if (board.isEmpty(i)) {
board.placeHuman(i);
int tmp = -1, response = -1; // Tmp is useless
findCompMove(tmp, response, alpha, value);
board.unPlace(i);
if (response <value) {
value = response;
bestMove = i;
}
}
}
}
}
//
bool TicTacToe::gameIsOver(bool &draw, Role &win) {
if (board.canWin(Comp_Char)) {
win =Comp ;
draw = false;
return true;
}
else if (board.canWin(Human_Char)) {
win = Human;
draw = false;
return true;
}
else if (board.isFull()) {
draw = true;
return true;
}
else {
return false;
}
}
bool TicTacToe::handleGameOver() {
bool draw = false;
Role whoWin = Human;
if (gameIsOver(draw, whoWin)) {
if (draw) {
cout << "Draw!" << endl;
}
else {
if (whoWin == Comp) {
cout << "You lose!" << endl;
}
else if (whoWin == Human) {
cout << "Congratulations! You defeat the computer." << endl;
}
}
return true;
}
else {
return false;
}
}实现的结果如下图所示：
完成的代码和运行：
在Linux的环境下：
git clone https://github.com/yqtaowhu/DataStructureAndAlgorithm
cd DataStructureAndAlgorithm/Algorithm/TicTacToe/
make run在window的环境下可用VS进行编译。
完整的代码在我的Github:
https://github.com/yqtaowhu/DataStructureAndAlgorithm/tree/master/Algorithm/TicTacToe
参考资料：
1. 数据结构与算法分析——C++语言描述
2. http://blog.csdn.net/tianranhe/article/details/8301756
3. http://blog.csdn.net/qq_22885773/article/details/50967021