game.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>

enum { NOUGHTS, CROSSES, BORDER, EMPTY };
enum { HUMANWIN, COMPWIN, DRAW };

const int Directions[4] = { 1, 5, 4, 6 };

const int ConvertTo25[9] = {
    6, 7, 8,
    11,12,13,
    16,17,18
};

const int InMiddle = 4;
const int Corners[4] = { 0, 2, 6, 8 };

int GetNumForDir(int startSq, const int dir, const int *board, const int us) {
    int found = 0;
    while (board[startSq] != BORDER) {
        if(board[startSq] != us) {
            break;
        }
        found++;
        startSq += dir;
    }
    return found;
}

int FindThreeInARow(const int *board, const int ourindex, const int us) {

    int DirIndex = 0;
    int Dir = 0;
    int threeCount = 1;

    for(DirIndex = 0; DirIndex < 4; ++DirIndex) {
        Dir = Directions[DirIndex];
        threeCount += GetNumForDir(ourindex + Dir, Dir, board, us);
        threeCount += GetNumForDir(ourindex + Dir * -1, Dir * -1, board, us);
        if(threeCount == 3) {
            break;
        }
        threeCount = 1;
    }
    return threeCount;
}

void InitialiseBoard(int *board) {
    int index = 0;

    for(index = 0; index < 25; ++index) {
        board[index] = BORDER;
    }

    // converts from iteration index to actual play squares 0>6,1>7,2>8 etc.
    for(index = 0; index < 9; ++index) {
        board[ConvertTo25[index]] = EMPTY;
    }
}

int HasEmpty(const int *board) {
    int index = 0;
    
    for(index = 0; index < 9; ++index) {
        if( board[ConvertTo25[index]] == EMPTY ) return 1;
    }
    return 0; 
}

void MakeMove(int *board, const int sq, const int side) {
    board[sq] = side;
}

int GetNextBest(const int *board) {

    int ourMove = ConvertTo25[InMiddle];
    if (board[ourMove] == EMPTY) {
        return ourMove;
    }

    int index = 0;
    ourMove   = -1;

    for(index = 0; index < 4; index++) {
        ourMove = ConvertTo25[Corners[index]];
        if (board[ourMove] == EMPTY) {
            break;
        }   
        ourMove = -1;
    }
    return ourMove;
}

int GetWinningMove(int *board, const int side) {
    
    int ourMove = -1;
    int index = 0;
    int winFound = 0;

    for(index = 0; index < 9; ++index) {
        if( board[ConvertTo25[index]] == EMPTY) {
            ourMove = ConvertTo25[index];
            board[ourMove] = side;

            if(FindThreeInARow(board, ourMove, side) == 3) {
                winFound = 1;
            }
            board[ourMove] = EMPTY;
            if(winFound == 1) {
                break;
            }
            ourMove = -1;
        }
    }
    return ourMove;
}

int GetComputerMove(int *board, const int side) {
    int index = 0;
    int numFree = 0;
    int availableMoves[9];
    int randMove = 0;

    randMove = GetWinningMove(board, side);
    if(randMove != -1) {
        return randMove;
    }

    randMove = GetWinningMove(board, side ^ 1);
    if(randMove != -1) {
        return randMove;
    }

    randMove = GetNextBest(board);
    if(randMove != -1) {
        return randMove;
    }

    randMove = 0;
    for(index = 0; index < 9; ++index) {
        if( board[ConvertTo25[index]] == EMPTY) {
            availableMoves[numFree++] = ConvertTo25[index];
        }
    }
    
    randMove = (rand() % numFree);
    return availableMoves[randMove];

}

int GetHumanMove(const int *board) {
    char userInput[4];

    int moveOk = 0;
    int move   = -1;

    while(moveOk == 0) {
        printf("Please enter a move from 1 to 9:");
        fgets(userInput, 3, stdin);
        fflush(stdin); 

        if(strlen(userInput) != 2) {
            printf("Invalid strlen()\n");
            continue;
        }
        if ( sscanf(userInput, "%d", &move) != 1 ) {
            move = -1;
            printf("Invalid sscanf()\n");
            continue;
        }
        if ( move < 1 || move > 9 ) {
            move = -1;
            printf("Invalid range\n");
            continue;
        }

        move--; // zero indexing

        if ( board[ConvertTo25[move]] != EMPTY ) {
            move=-1;
            printf("Square not available\n");
            continue;
        }

        moveOk = 1;
    }
    printf("Making move...%d\n", (move+1));
    return ConvertTo25[move];
}

void PrintBoard(const int *board) {
    int index = 0;
    char pceChars[] = "OX|-";
    printf("\n\nBoard:\n\n");
    for(index = 0; index < 9; ++index) {
        if (index!=0 && index%3==0) {
            printf("\n\n");
        }
        printf("%4c",pceChars[board[ConvertTo25[index]]]);
    }
    printf("\n");
}

void RunGame() {
    int GameOver = 0;
    int Side = NOUGHTS;
    int LastMoveMade = 0;
    int board[25];

    InitialiseBoard(&board[0]);
    PrintBoard(&board[0]);

    while(!GameOver) {
        if(Side==NOUGHTS) {
            /// get move from human, make move on board, change side
            LastMoveMade = GetHumanMove(&board[0]);
            MakeMove(&board[0], LastMoveMade, Side);
            Side=CROSSES;
        } else {
            LastMoveMade = GetComputerMove(&board[0], Side);
            MakeMove(&board[0], LastMoveMade, Side);
            Side=NOUGHTS;
            PrintBoard(&board[0]);
        }

        // if three in a row exists, Game is over
        if( FindThreeInARow(board, LastMoveMade, Side ^ 1) == 3) {
            printf("Game over!\n");
            GameOver = 1;
            if(Side==NOUGHTS){
                printf("Computer wins\n");
            } else {
                printf("Human wins\n");
            }
        }

        // if no more moves, game is drawn
        if(!HasEmpty(board)) {
            printf("Game over!\n");
            GameOver = 1;
            printf("It's a draw\n");
        }
    }
    PrintBoard(&board[0]);
}

int main() {
    
    srand(time(NULL));   
    RunGame();
    
    return 0;

}