Algorithm for Determining Tic Tac Toe Game Over

You know a winning move can only happen after X or O has made their most recent move, so you can only search row/column with optional diag that are contained in that move to limit your search space when trying to determine a winning board. Also since there are a fixed number of moves in a draw tic-tac-toe game once the last move is made if it wasn't a winning move it's by default a draw game.

This code is for an n by n board with n in a row to win (3x3 board requires 3 in a row, etc)

public class TripleT {
    
    enum State{Blank, X, O};
    
    int n = 3;
    State[][] board = new State[n][n];
    int moveCount;
    
    void Move(int x, int y, State s){
        if(board[x][y] == State.Blank){
            board[x][y] = s;
        }
        moveCount++;
        
        //check end conditions
        
        //check col
        for(int i = 0; i < n; i++){
            if(board[x][i] != s)
                break;
            if(i == n-1){
                //report win for s
            }
        }
        
        //check row
        for(int i = 0; i < n; i++){
            if(board[i][y] != s)
                break;
            if(i == n-1){
                //report win for s
            }
        }
        
        //check diag
        if(x == y){
            //we're on a diagonal
            for(int i = 0; i < n; i++){
                if(board[i][i] != s)
                    break;
                if(i == n-1){
                    //report win for s
                }
            }
        }
            
        //check anti diag (thanks rampion)
        if(x + y == n - 1){
            for(int i = 0; i < n; i++){
                if(board[i][(n-1)-i] != s)
                    break;
                if(i == n-1){
                    //report win for s
                }
            }
        }

        //check draw
        if(moveCount == (Math.pow(n, 2) - 1)){
            //report draw
        }
    }
}

you can use a magic square http://mathworld.wolfram.com/MagicSquare.html if any row, column, or diag adds up to 15 then a player has won.

How about this pseudocode:

After a player puts down a piece at position (x,y):

col=row=diag=rdiag=0
winner=false
for i=1 to n
  if cell[x,i]=player then col++
  if cell[i,y]=player then row++
  if cell[i,i]=player then diag++
  if cell[i,n-i+1]=player then rdiag++
if row=n or col=n or diag=n or rdiag=n then winner=true

I'd use an array of char [n,n], with O,X and space for empty.

1. simple.
2. One loop.
3. Five simple variables: 4 integers and one boolean.
4. Scales to any size of n.
5. Only checks current piece.
6. No magic. :)

Ultra-efficient Bit-boarding

Let's store the game in a binary integer, and evaluate everything using just one step!

• We know that X's moves occupy 9 bits: xxx xxx xxx
• We know that O's moves occupy 9 bits: ooo ooo ooo

So, a board position could be represented in just 18 bits: xoxoxo xoxoxo xoxoxo

But, whilst this might look efficient, it doesn't help us with determining a win. We need a more useful bit pattern... one that not only encodes the moves, but also encodes the rows, columns and diagonals in a reasonable way.

I would do this by using a clever integer value for each board position.

Choosing a more useful representation

First, we need a board notation, just so that we can discuss this. So, similar to Chess, lets number the rows with letters and the columns with numbers - so we know which square we're talking about

And let's give each a binary value.

a1 = 100 000 000 100 000 000 100 000 ; Row A Col 1 (top left corner)
a2 = 010 000 000 000 100 000 000 000 ; Row A Col 2 (top edge)
a3 = 001 000 000 000 000 100 000 100 ; Row A Col 3 (top right corner)
b1 = 000 100 000 010 000 000 000 000 ; Row B Col 1 (left edge)
b2 = 000 010 000 000 010 000 010 010 ; Row B Col 2 (middle square)
b3 = 000 001 000 000 000 010 000 000 ; Row B Col 4 (right edge)
c1 = 000 000 100 001 000 000 000 001 ; Row C Col 1 (bottom left corner)
c2 = 000 000 010 000 001 000 000 000 ; Row C Col 2 (bottom edge)
c3 = 000 000 001 000 000 001 001 000 ; Row C Col 3 (bottom right corner)

... where, the binary values encode which rows, columns and diagonals the position appears in. (we'll look at how this works this later)

We will use these values to build two representations of the game, one for X and one for O

• X starts with an empty board : 000 000 000 000 000 000 000 000
• O starts with an empty board : 000 000 000 000 000 000 000 000

Let's follow X's moves (O would be the same principle)

• X plays A1... so we OR (the X board) with value A1
• X plays A2... so we OR with value A2
• X plays A3... so we OR with value A3

What does that do to X's board value :

1. a1 = 100 000 000 100 000 000 100 000 ... ORed with
2. a2 = 010 000 000 000 100 000 000 000 ... ORed with
3. a3 = 001 000 000 000 000 100 000 100 ... equals :

XB = 111 000 000 100 100 100 100 100

Reading from left to right we see that X has :

• 111 (All positions) in Row 1 (\o/ A win, Yay!)
• 000 (No positions) in Row 2
• 000 (No positions) in Row 3
• 100 (One position) Only the first position of Column 1
• 100 (One position) Only the first position of Column 1
• 100 (One position) Only the first position of Column 1
• 100 (One position) Only the first position of Diagonal 1
• 100 (One position) Only the first position of Diagonal 2

You'll notice that whenever X (or O) has a winning line, then there will also be three consecutive bits in his board value. Precisely Where those three bits are, dictates which row/column/diagonal he won on.

So, the trick now is to find a way to check for this (three consecutive bits set) condition in a single operation.

Modifying the values to make detection easier

To assist with this, let's change our bit representation so that there are always ZEROs between the groups of three (Because 001 110 is also three consecutive bits - but they are NOT a valid win ... so, a fixed zero spacer would break these up: 0 001 0 110)

So, after adding some spacing ZEROes, we can be confident that ANY three consecutive set bits in X's or O's board value indicates a win!

So, our new binary values (with zero-padding) look like this :

• a1 = 100 0 000 0 000 0 100 0 000 0 000 0 100 0 000 0 ; 0x80080080 (hex)
• a2 = 010 0 000 0 000 0 000 0 100 0 000 0 000 0 000 0 ; 0x40008000
• a3 = 001 0 000 0 000 0 000 0 000 0 100 0 000 0 100 0 ; 0x20000808
• b1 = 000 0 100 0 000 0 010 0 000 0 000 0 000 0 000 0 ; 0x08040000
• b2 = 000 0 010 0 000 0 000 0 010 0 000 0 010 0 010 0 ; 0x04004044
• b3 = 000 0 001 0 000 0 000 0 000 0 010 0 000 0 000 0 ; 0x02000400
• c1 = 000 0 000 0 100 0 001 0 000 0 000 0 000 0 001 0 ; 0x00820002
• c2 = 000 0 000 0 010 0 000 0 001 0 000 0 000 0 000 0 ; 0x00402000
• c3 = 000 0 000 0 001 0 000 0 000 0 001 0 001 0 000 0 ; 0x00200220

You'll notice that each "winline" of the board now requires 4 bits.

8 winlines x 4 bits each = 32 bits! Isn't that convenient : )))))

Parsing

We could shift through all the bits looking for three consecutive bits, but that will take 32 shifts x 2 players... and a counter to keep track. It's slow!

We could AND with 0xF, looking for the value 8+4+2=14. And this would allow us to check 4 bits at a time. Cutting the number of shifts by a quarter. But again, this is slow!

So, instead, let's check ALL of the possibilities at once...

Ultra-efficient win detection

Imagine we wanted to evaluate the A3+A1+B2+C3 case (a win on the diagonal)

a1 = 100 0 000 0 000 0 100 0 000 0 000 0 100 0 000 0, OR
a3 = 001 0 000 0 000 0 000 0 000 0 100 0 000 0 100 0, OR
b2 = 000 0 010 0 000 0 000 0 010 0 000 0 010 0 010 0, OR
c3 = 000 0 000 0 001 0 000 0 000 0 001 0 001 0 000 0, =

XB = 101 0 010 0 001 0 100 0 010 0 101 0 111 0 110 0  (See the win, on Diagonal 1?)

Now, let's check it for a win, by efficiently merging three bits into one...

Simply use : XB AND (XB << 1) AND (XB >> 1) in other words: XB ANDed with (XB shifted left) AND (XB shiftted right)

Let's try an example...

10100100001010000100101011101100 ; whitespaces removed for easy shifting
(AND)
01001000010100001001010111011000 ; XB shifted left
(AND)
01010010000101000010010101110110 ; XB shifted left
(Equals)
00000000000000000000000001000000

See that? Any non-zero result means a win!

But, where did they win

Want to know where they won? Well, you could just use a second table :

0x40000000 = RowA
0x04000000 = RowB
0x00400000 = RowC
0x00040000 = Col1
0x00004000 = Col2
0x00000400 = Col3
0x00000040 = Diag1
0x00000004 = Diag2

However, we can be smarter than that, as the pattern is VERY regular!

For example, in assembly you can use BSF (Bit Scan Forward) to find the number of leading zeros. Then subtract 2 and then /4 (Shift Right 2) - to get a number between 0 and 8... which you can use as an index to look up into an array of win strings :

{"wins the top row", "takes the middle row!", ... "steals the diagonal!" }

This makes the whole game logic... from move checking, to board updating and right through to win/loss detection and an appropriate success message, all fit in a handful of ASM instructions.

... it's tiny, efficient and ultrafast!

Checking whether a move is playable

Obviously, ORing "X's board" with "O's board" = ALL POSITIONS

So, you can check if a move is valid quite easily. If user chooses UpperLeft, this position has an integer value. Just check the 'AND' of this Value with (XB OR OB)...

... if the result is nonzero, then the position is already in use.

Conclusion

If you're looking for efficient ways to process a board, don't start with a board object. Try to discover some useful abstraction.

See if the states fit within an integer, and think about what an 'easy' bitmask to process would look like. With some clever choice of integers to represent moves, positions or boards... you might find that the entire game can be played, evaluated and scored VERY efficiently - using simply bitwise logic.

Closing apologies

BTW I'm not a regular here on StackOverflow, so I hope this post wasn't too chaotic to follow. Also, please be kind... "Human" is my second language and I'm not quite fluent yet ;)

Anyway, I hope this helps someone.

I just wrote this for my C programming class.

I am posting it because none of the other examples here will work with any size rectangular grid, and any number N-in-a-row consecutive marks to win.

You'll find my algorithm, such as it is, in the checkWinner() function. It doesn't use magic numbers or anything fancy to check for a winner, it simply uses four for loops - The code is well commented so I'll let it speak for itself I guess.

// This program will work with any whole number sized rectangular gameBoard.
// It checks for N marks in straight lines (rows, columns, and diagonals).
// It is prettiest when ROWS and COLS are single digit numbers.
// Try altering the constants for ROWS, COLS, and N for great fun!    

// PPDs come first

    #include <stdio.h>
    #define ROWS 9              // The number of rows our gameBoard array will have
    #define COLS 9              // The number of columns of the same - Single digit numbers will be prettier!
    #define N 3                 // This is the number of contiguous marks a player must have to win
    #define INITCHAR ' '        // This changes the character displayed (a ' ' here probably looks the best)
    #define PLAYER1CHAR 'X'     // Some marks are more aesthetically pleasing than others
    #define PLAYER2CHAR 'O'     // Change these lines if you care to experiment with them


// Function prototypes are next

    int playGame    (char gameBoard[ROWS][COLS]);               // This function allows the game to be replayed easily, as desired
    void initBoard  (char gameBoard[ROWS][COLS]);               // Fills the ROWSxCOLS character array with the INITCHAR character
    void printBoard (char gameBoard[ROWS][COLS]);               // Prints out the current board, now with pretty formatting and #s!
    void makeMove   (char gameBoard[ROWS][COLS], int player);   // Prompts for (and validates!) a move and stores it into the array
    int checkWinner (char gameBoard[ROWS][COLS], int player);   // Checks the current state of the board to see if anyone has won

// The starting line
int main (void)
{
    // Inits
    char gameBoard[ROWS][COLS];     // Our gameBoard is declared as a character array, ROWS x COLS in size
    int winner = 0;
    char replay;

    //Code
    do                              // This loop plays through the game until the user elects not to
    {
        winner = playGame(gameBoard);
        printf("\nWould you like to play again? Y for yes, anything else exits: ");

        scanf("%c",&replay);        // I have to use both a scanf() and a getchar() in
        replay = getchar();         // order to clear the input buffer of a newline char
                                    // (http://cboard.cprogramming.com/c-programming/121190-problem-do-while-loop-char.html)

    } while ( replay == 'y' || replay == 'Y' );

    // Housekeeping
    printf("\n");
    return winner;
}


int playGame(char gameBoard[ROWS][COLS])
{
    int turn = 0, player = 0, winner = 0, i = 0;

    initBoard(gameBoard);

    do
    {
        turn++;                                 // Every time this loop executes, a unique turn is about to be made
        player = (turn+1)%2+1;                  // This mod function alternates the player variable between 1 & 2 each turn
        makeMove(gameBoard,player);
        printBoard(gameBoard);
        winner = checkWinner(gameBoard,player);

        if (winner != 0)
        {
            printBoard(gameBoard);

            for (i=0;i<19-2*ROWS;i++)           // Formatting - works with the default shell height on my machine
                printf("\n");                   // Hopefully I can replace these with something that clears the screen for me

            printf("\n\nCongratulations Player %i, you've won with %i in a row!\n\n",winner,N);
            return winner;
        }

    } while ( turn < ROWS*COLS );                           // Once ROWS*COLS turns have elapsed

    printf("\n\nGame Over!\n\nThere was no Winner :-(\n");  // The board is full and the game is over
    return winner;
}


void initBoard (char gameBoard[ROWS][COLS])
{
    int row = 0, col = 0;

    for (row=0;row<ROWS;row++)
    {
        for (col=0;col<COLS;col++)
        {
            gameBoard[row][col] = INITCHAR;     // Fill the gameBoard with INITCHAR characters
        }
    }

    printBoard(gameBoard);                      // Having this here prints out the board before
    return;                             // the playGame function asks for the first move
}


void printBoard (char gameBoard[ROWS][COLS])    // There is a ton of formatting in here
{                                               // That I don't feel like commenting :P
    int row = 0, col = 0, i=0;                  // It took a while to fine tune
                                                // But now the output is something like:
    printf("\n");                               // 
                                                //    1   2   3
    for (row=0;row<ROWS;row++)                  // 1    |   |
    {                                           //   -----------
        if (row == 0)                           // 2    |   |
        {                                       //   -----------
            printf("  ");                       // 3    |   |

            for (i=0;i<COLS;i++)
            {
                printf(" %i  ",i+1);
            }

            printf("\n\n");
        }

        for (col=0;col<COLS;col++)
        {
            if (col==0)
                printf("%i ",row+1);

            printf(" %c ",gameBoard[row][col]);

            if (col<COLS-1)
                printf("|");
        }

        printf("\n");

        if (row < ROWS-1)
        {
            for(i=0;i<COLS-1;i++)
            {
                if(i==0)
                    printf("  ----");
                else
                    printf("----");
            }

            printf("---\n");
        }
    }

    return;
}


void makeMove (char gameBoard[ROWS][COLS],int player)
{
    int row = 0, col = 0, i=0;
    char currentChar;

    if (player == 1)                    // This gets the correct player's mark
        currentChar = PLAYER1CHAR;
    else
        currentChar = PLAYER2CHAR;

    for (i=0;i<21-2*ROWS;i++)           // Newline formatting again :-(
        printf("\n");

    printf("\nPlayer %i, please enter the column of your move: ",player);
    scanf("%i",&col);
    printf("Please enter the row of your move: ");
    scanf("%i",&row);

    row--;                              // These lines translate the user's rows and columns numbering
    col--;                              // (starting with 1) to the computer's (starting with 0)

    while(gameBoard[row][col] != INITCHAR || row > ROWS-1 || col > COLS-1)  // We are not using a do... while because
    {                                                                       // I wanted the prompt to change
        printBoard(gameBoard);
        for (i=0;i<20-2*ROWS;i++)
            printf("\n");
        printf("\nPlayer %i, please enter a valid move! Column first, then row.\n",player);
        scanf("%i %i",&col,&row);

        row--;                          // See above ^^^
        col--;
    }

    gameBoard[row][col] = currentChar;  // Finally, we store the correct mark into the given location
    return;                             // And pop back out of this function
}


int checkWinner(char gameBoard[ROWS][COLS], int player)     // I've commented the last (and the hardest, for me anyway)
{                                                           // check, which checks for backwards diagonal runs below >>>
    int row = 0, col = 0, i = 0;
    char currentChar;

    if (player == 1)
        currentChar = PLAYER1CHAR;
    else
        currentChar = PLAYER2CHAR;

    for ( row = 0; row < ROWS; row++)                       // This first for loop checks every row
    {
        for ( col = 0; col < (COLS-(N-1)); col++)           // And all columns until N away from the end
        {
            while (gameBoard[row][col] == currentChar)      // For consecutive rows of the current player's mark
            {
                col++;
                i++;
                if (i == N)
                {
                    return player;
                }
            }
            i = 0;
        }
    }

    for ( col = 0; col < COLS; col++)                       // This one checks for columns of consecutive marks
    {
        for ( row = 0; row < (ROWS-(N-1)); row++)
        {
            while (gameBoard[row][col] == currentChar)
            {
                row++;
                i++;
                if (i == N)
                {
                    return player;
                }
            }
            i = 0;
        }
    }

    for ( col = 0; col < (COLS - (N-1)); col++)             // This one checks for "forwards" diagonal runs
    {
        for ( row = 0; row < (ROWS-(N-1)); row++)
        {
            while (gameBoard[row][col] == currentChar)
            {
                row++;
                col++;
                i++;
                if (i == N)
                {
                    return player;
                }
            }
            i = 0;
        }
    }
                                                        // Finally, the backwards diagonals:
    for ( col = COLS-1; col > 0+(N-2); col--)           // Start from the last column and go until N columns from the first
    {                                                   // The math seems strange here but the numbers work out when you trace them
        for ( row = 0; row < (ROWS-(N-1)); row++)       // Start from the first row and go until N rows from the last
        {
            while (gameBoard[row][col] == currentChar)  // If the current player's character is there
            {
                row++;                                  // Go down a row
                col--;                                  // And back a column
                i++;                                    // The i variable tracks how many consecutive marks have been found
                if (i == N)                             // Once i == N
                {
                    return player;                      // Return the current player number to the
                }                                       // winnner variable in the playGame function
            }                                           // If it breaks out of the while loop, there weren't N consecutive marks
            i = 0;                                      // So make i = 0 again
        }                                               // And go back into the for loop, incrementing the row to check from
    }

    return 0;                                           // If we got to here, no winner has been detected,
}                                                       // so we pop back up into the playGame function

// The end!

// Well, almost.

// Eventually I hope to get this thing going
// with a dynamically sized array. I'll make
// the CONSTANTS into variables in an initGame
// function and allow the user to define them.

If the board is n × n then there are n rows, n columns, and 2 diagonals. Check each of those for all-X's or all-O's to find a winner.

If it only takes x < n consecutive squares to win, then it's a little more complicated. The most obvious solution is to check each x × x square for a winner. Here's some code that demonstrates that.

(I didn't actually test this *cough*, but it did compile on the first try, yay me!)

public class TicTacToe
{
    public enum Square { X, O, NONE }

    /**
     * Returns the winning player, or NONE if the game has
     * finished without a winner, or null if the game is unfinished.
     */
    public Square findWinner(Square[][] board, int lengthToWin) {
        // Check each lengthToWin x lengthToWin board for a winner.    
        for (int top = 0; top <= board.length - lengthToWin; ++top) {
            int bottom = top + lengthToWin - 1;

            for (int left = 0; left <= board.length - lengthToWin; ++left) {
                int right = left + lengthToWin - 1;

                // Check each row.
                nextRow: for (int row = top; row <= bottom; ++row) {
                    if (board[row][left] == Square.NONE) {
                        continue;
                    }

                    for (int col = left; col <= right; ++col) {
                        if (board[row][col] != board[row][left]) {
                            continue nextRow;
                        }
                    }

                    return board[row][left];
                }

                // Check each column.
                nextCol: for (int col = left; col <= right; ++col) {
                    if (board[top][col] == Square.NONE) {
                        continue;
                    }

                    for (int row = top; row <= bottom; ++row) {
                        if (board[row][col] != board[top][col]) {
                            continue nextCol;
                        }
                    }

                    return board[top][col];
                }

                // Check top-left to bottom-right diagonal.
                diag1: if (board[top][left] != Square.NONE) {
                    for (int i = 1; i < lengthToWin; ++i) {
                        if (board[top+i][left+i] != board[top][left]) {
                            break diag1;
                        }
                    }

                    return board[top][left];
                }

                // Check top-right to bottom-left diagonal.
                diag2: if (board[top][right] != Square.NONE) {
                    for (int i = 1; i < lengthToWin; ++i) {
                        if (board[top+i][right-i] != board[top][right]) {
                            break diag2;
                        }
                    }

                    return board[top][right];
                }
            }
        }

        // Check for a completely full board.
        boolean isFull = true;

        full: for (int row = 0; row < board.length; ++row) {
            for (int col = 0; col < board.length; ++col) {
                if (board[row][col] == Square.NONE) {
                    isFull = false;
                    break full;
                }
            }
        }

        // The board is full.
        if (isFull) {
            return Square.NONE;
        }
        // The board is not full and we didn't find a solution.
        else {
            return null;
        }
    }
}

I don't know Java that well, but I do know C, so I tried adk's magic square idea (along with Hardwareguy's search restriction).

// tic-tac-toe.c
// to compile:
//  % gcc -o tic-tac-toe tic-tac-toe.c
// to run:
//  % ./tic-tac-toe
#include <stdio.h>

// the two types of marks available
typedef enum { Empty=2, X=0, O=1, NumMarks=2 } Mark;
char const MarkToChar[] = "XO ";

// a structure to hold the sums of each kind of mark
typedef struct { unsigned char of[NumMarks]; } Sum;

// a cell in the board, which has a particular value
#define MAGIC_NUMBER 15
typedef struct {
  Mark mark;
  unsigned char const value;
  size_t const num_sums;
  Sum * const sums[4];
} Cell;

#define NUM_ROWS 3
#define NUM_COLS 3

// create a sum for each possible tic-tac-toe
Sum row[NUM_ROWS] = {0};
Sum col[NUM_COLS] = {0};
Sum nw_diag = {0};
Sum ne_diag = {0};

// initialize the board values so any row, column, or diagonal adds to
// MAGIC_NUMBER, and so they each record their sums in the proper rows, columns,
// and diagonals
Cell board[NUM_ROWS][NUM_COLS] = { 
  { 
    { Empty, 8, 3, { &row[0], &col[0], &nw_diag } },
    { Empty, 1, 2, { &row[0], &col[1] } },
    { Empty, 6, 3, { &row[0], &col[2], &ne_diag } },
  },
  { 
    { Empty, 3, 2, { &row[1], &col[0] } },
    { Empty, 5, 4, { &row[1], &col[1], &nw_diag, &ne_diag } },
    { Empty, 7, 2, { &row[1], &col[2] } },
  },
  { 
    { Empty, 4, 3, { &row[2], &col[0], &ne_diag } },
    { Empty, 9, 2, { &row[2], &col[1] } },
    { Empty, 2, 3, { &row[2], &col[2], &nw_diag } },
  }
};

// print the board
void show_board(void)
{
  size_t r, c;
  for (r = 0; r < NUM_ROWS; r++) 
  {
    if (r > 0) { printf("---+---+---\n"); }
    for (c = 0; c < NUM_COLS; c++) 
    {
      if (c > 0) { printf("|"); }
      printf(" %c ", MarkToChar[board[r][c].mark]);
    }
    printf("\n");
  }
}


// run the game, asking the player for inputs for each side
int main(int argc, char * argv[])
{
  size_t m;
  show_board();
  printf("Enter moves as \"<row> <col>\" (no quotes, zero indexed)\n");
  for( m = 0; m < NUM_ROWS * NUM_COLS; m++ )
  {
    Mark const mark = (Mark) (m % NumMarks);
    size_t c, r;

    // read the player's move
    do
    {
      printf("%c's move: ", MarkToChar[mark]);
      fflush(stdout);
      scanf("%d %d", &r, &c);
      if (r >= NUM_ROWS || c >= NUM_COLS)
      {
        printf("illegal move (off the board), try again\n");
      }
      else if (board[r][c].mark != Empty)
      {
        printf("illegal move (already taken), try again\n");
      }
      else
      {
        break;
      }
    }
    while (1);

    {
      Cell * const cell = &(board[r][c]);
      size_t s;

      // update the board state
      cell->mark = mark;
      show_board();

      // check for tic-tac-toe
      for (s = 0; s < cell->num_sums; s++)
      {
        cell->sums[s]->of[mark] += cell->value;
        if (cell->sums[s]->of[mark] == MAGIC_NUMBER)
        {
          printf("tic-tac-toe! %c wins!\n", MarkToChar[mark]);
          goto done;
        }
      }
    }
  }
  printf("stalemate... nobody wins :(\n");
done:
  return 0;
}

It compiles and tests well.

% gcc -o tic-tac-toe tic-tac-toe.c
% ./tic-tac-toe
     |   |
  ---+---+---
     |   |
  ---+---+---
     |   |
  Enter moves as " " (no quotes, zero indexed)
  X's move: 1 2
     |   |
  ---+---+---
     |   | X
  ---+---+---
     |   |
  O's move: 1 2
  illegal move (already taken), try again
  O's move: 3 3
  illegal move (off the board), try again
  O's move: 2 2
     |   |
  ---+---+---
     |   | X
  ---+---+---
     |   | O
  X's move: 1 0
     |   |
  ---+---+---
   X |   | X
  ---+---+---
     |   | O
  O's move: 1 1
     |   |
  ---+---+---
   X | O | X
  ---+---+---
     |   | O
  X's move: 0 0
   X |   |
  ---+---+---
   X | O | X
  ---+---+---
     |   | O
  O's move: 2 0
   X |   |
  ---+---+---
   X | O | X
  ---+---+---
   O |   | O
  X's move: 2 1
   X |   |
  ---+---+---
   X | O | X
  ---+---+---
   O | X | O
  O's move: 0 2
   X |   | O
  ---+---+---
   X | O | X
  ---+---+---
   O | X | O
  tic-tac-toe! O wins!
% ./tic-tac-toe
     |   |
  ---+---+---
     |   |
  ---+---+---
     |   |
  Enter moves as " " (no quotes, zero indexed)
  X's move: 0 0
   X |   |
  ---+---+---
     |   |
  ---+---+---
     |   |
  O's move: 0 1
   X | O |
  ---+---+---
     |   |
  ---+---+---
     |   |
  X's move: 0 2
   X | O | X
  ---+---+---
     |   |
  ---+---+---
     |   |
  O's move: 1 0
   X | O | X
  ---+---+---
   O |   |
  ---+---+---
     |   |
  X's move: 1 1
   X | O | X
  ---+---+---
   O | X |
  ---+---+---
     |   |
  O's move: 2 0
   X | O | X
  ---+---+---
   O | X |
  ---+---+---
   O |   |
  X's move: 2 1
   X | O | X
  ---+---+---
   O | X |
  ---+---+---
   O | X |
  O's move: 2 2
   X | O | X
  ---+---+---
   O | X |
  ---+---+---
   O | X | O
  X's move: 1 2
   X | O | X
  ---+---+---
   O | X | X
  ---+---+---
   O | X | O
  stalemate... nobody wins :(
%

That was fun, thanks!

Actually, thinking about it, you don't need a magic square, just a count for each row/column/diagonal. This is a little easier than generalizing a magic square to n × n matrices, since you just need to count to n.

a non-loop way to determine if the point was on the anti diag:

`if (x + y == n - 1)`

Here's an example implementation in React: CodeSandbox Demo.

The algorithm is quite straightforward:

For every move:
   checkDiagonals()
   checkVerticals()
   checkHorizontals()

Assigning 0 for an "O" input and 1 for an "X" input. The result of the check functions can be either 0, or 1, or 2 (draw).

Here's the implementation:

    const checkDiagonals = () => {
        if ((state[0][0] === val && state[1][1] === val && state[2][2] === val) ||
            (state[0][2] === val && state[1][1] === val && state[2][0] === val)) {
            return val;
        }
        return -1;
    }

    const checkVerticals = () => {
        for (let i = 0; i <= 2; i++) {
            if (state[0][i] === val && state[1][i] === val && state[2][i] === val) {
                return val;
            }
        }
        return -1;
    }

    const checkHorizontals = () => {
        for (let i = 0; i <= 2; i++) {
            if (state[i][0] === val && state[i][1] === val && state[i][2] === val) {
                return val;
            }
        }
        return -1;
    }

Then all is left, is to have a function that will trigger on every single user input:

const updateWinningPlayer = () => {

    const diagonals = checkDiagonals();
    const verticals = checkVerticals();
    const horizontals = checkHorizontals();

    if (diagonals !== -1) {
        setWinner(diagonals)
        return;
    }

    if (verticals !== -1) {
        setWinner(verticals);
        return;
    }

    if (horizontals !== -1) {
        setWinner(horizontals);
        return;
    }

    if (isDraw()) {
        setWinner(2);
    }
}

And there you have it!

GitHub repo link: https://github.com/mkotsollaris/tic-tac-toe

Here is my solution using an 2-dimensional array:

private static final int dimension = 3;
private static final int[][] board = new int[dimension][dimension];
private static final int xwins = dimension * 1;
private static final int owins = dimension * -1;

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    int count = 0;
    boolean keepPlaying = true;
    boolean xsTurn = true;
    while (keepPlaying) {
        xsTurn = (count % 2 == 0);
        System.out.print("Enter i-j in the format:");
        if (xsTurn) {
            System.out.println(" X plays: ");
        } else {
            System.out.println(" O plays: ");
        }
        String result = null;
        while (result == null) {
            result = parseInput(scanner, xsTurn);
        }
        String[] xy = result.split(",");
        int x = Integer.parseInt(xy[0]);
        int y = Integer.parseInt(xy[1]);
        keepPlaying = makeMove(xsTurn, x, y);
        count++;
    }
    if (xsTurn) {
        System.out.print("X");
    } else {
        System.out.print("O");
    }
    System.out.println(" WON");
    printArrayBoard(board);
}

private static String parseInput(Scanner scanner, boolean xsTurn) {
    String line = scanner.nextLine();
    String[] values = line.split("-");
    int x = Integer.parseInt(values[0]);
    int y = Integer.parseInt(values[1]);
    boolean alreadyPlayed = alreadyPlayed(x, y);
    String result = null;
    if (alreadyPlayed) {
        System.out.println("Already played in this x-y. Retry");
    } else {
        result = "" + x + "," + y;
    }
    return result;
}

private static boolean alreadyPlayed(int x, int y) {
    System.out.println("x-y: " + x + "-" + y + " board[x][y]: " + board[x][y]);
    if (board[x][y] != 0) {
        return true;
    }
    return false;
}

private static void printArrayBoard(int[][] board) {
    for (int i = 0; i < dimension; i++) {
        int[] height = board[i];
        for (int j = 0; j < dimension; j++) {
            System.out.print(height[j] + " ");
        }
        System.out.println();
    }
}

private static boolean makeMove(boolean xo, int x, int y) {
    if (xo) {
        board[x][y] = 1;
    } else {
        board[x][y] = -1;
    }
    boolean didWin = checkBoard();
    if (didWin) {
        System.out.println("keep playing");
    }
    return didWin;
}

private static boolean checkBoard() {
    //check horizontal
    int[] horizontalTotal = new int[dimension];
    for (int i = 0; i < dimension; i++) {
        int[] height = board[i];
        int total = 0;
        for (int j = 0; j < dimension; j++) {
            total += height[j];
        }
        horizontalTotal[i] = total;
    }
    for (int a = 0; a < horizontalTotal.length; a++) {
        if (horizontalTotal[a] == xwins || horizontalTotal[a] == owins) {
            System.out.println("horizontal");
            return false;
        }
    }
    //check vertical
    int[] verticalTotal = new int[dimension];

    for (int j = 0; j < dimension; j++) {
        int total = 0;
        for (int i = 0; i < dimension; i++) {
            total += board[i][j];
        }
        verticalTotal[j] = total;
    }
    for (int a = 0; a < verticalTotal.length; a++) {
        if (verticalTotal[a] == xwins || verticalTotal[a] == owins) {
            System.out.println("vertical");
            return false;
        }
    }
    //check diagonal
    int total1 = 0;
    int total2 = 0;
    for (int i = 0; i < dimension; i++) {
        for (int j = 0; j < dimension; j++) {
            if (i == j) {
                total1 += board[i][j];
            }
            if (i == (dimension - 1 - j)) {
                total2 += board[i][j];
            }
        }
    }
    if (total1 == xwins || total1 == owins) {
        System.out.println("diagonal 1");
        return false;
    }
    if (total2 == xwins || total2 == owins) {
        System.out.println("diagonal 2");
        return false;
    }
    return true;
}

Not sure if this approach is published yet. This should work for any m*n board and a player is supposed to fill "winnerPos" consecutive position. The idea is based on running window.

private boolean validateWinner(int x, int y, int player) {
    //same col
    int low = x-winnerPos-1;
    int high = low;
    while(high <= x+winnerPos-1) {
        if(isValidPos(high, y) && isFilledPos(high, y, player)) {
            high++;
            if(high - low == winnerPos) {
                return true;
            }
        } else {
            low = high + 1;
            high = low;
        }
    }

    //same row
    low = y-winnerPos-1;
    high = low;
    while(high <= y+winnerPos-1) {
        if(isValidPos(x, high) && isFilledPos(x, high, player)) {
            high++;
            if(high - low == winnerPos) {
                return true;
            }
        } else {
            low = high + 1;
            high = low;
        }
    }
    if(high - low == winnerPos) {
        return true;
    }

    //diagonal 1
    int lowY = y-winnerPos-1;
    int highY = lowY;
    int lowX = x-winnerPos-1;
    int highX = lowX;
    while(highX <= x+winnerPos-1 && highY <= y+winnerPos-1) {
        if(isValidPos(highX, highY) && isFilledPos(highX, highY, player)) {
            highX++;
            highY++;
            if(highX - lowX == winnerPos) {
                return true;
            }
        } else {
            lowX = highX + 1;
            lowY = highY + 1;
            highX = lowX;
            highY = lowY;
        }
    }

    //diagonal 2
    lowY = y+winnerPos-1;
    highY = lowY;
    lowX = x-winnerPos+1;
    highX = lowX;
    while(highX <= x+winnerPos-1 && highY <= y+winnerPos-1) {
        if(isValidPos(highX, highY) && isFilledPos(highX, highY, player)) {
            highX++;
            highY--;
            if(highX - lowX == winnerPos) {
                return true;
            }
        } else {
            lowX = highX + 1;
            lowY = highY + 1;
            highX = lowX;
            highY = lowY;
        }
    }
    if(highX - lowX == winnerPos) {
        return true;
    }
    return false;
}

private boolean isValidPos(int x, int y) {
    return x >= 0 && x < row && y >= 0 && y< col;
}
public boolean isFilledPos(int x, int y, int p) throws IndexOutOfBoundsException {
    return arena[x][y] == p;
}

I just want to share what I did in Javascript. My idea is to have search directions; in grid it could be 8 directions, but search should be bi-directional so 8 / 2 = 4 directions. When a player does its move, the search starts from the location. It searches 4 different bi-directions until its value is different from the player's stone(O or X).

Per a bi-direction search, two values can be added but need to subtract one because starting point was duplicated.

getWin(x,y,value,searchvector) {
if (arguments.length==2) {
  var checkTurn = this.state.squares[y][x];
  var searchdirections = [[-1,-1],[0,-1],[1,-1],[-1,0]];
  return searchdirections.reduce((maxinrow,searchdirection)=>Math.max(this.getWin(x,y,checkTurn,searchdirection)+this.getWin(x,y,checkTurn,[-searchdirection[0],-searchdirection[1]]),maxinrow),0);
} else {
  if (this.state.squares[y][x]===value) {
    var result = 1;
    if (
      x+searchvector[0] >= 0 && x+searchvector[0] < 3 && 
      y+searchvector[1] >= 0 && y+searchvector[1] < 3
      ) result += this.getWin(x+searchvector[0],y+searchvector[1],value,searchvector);
    return result;
  } else {
    return 0;
  }
}

}

This function can be used with two parameters (x,y), which are coordinates of the last move. In initial execution, it calls four bi-direction searches recursively with 4 parameters. All results are returned as lengths and the function finally picks the maximum length among 4 search bi-directions.

class Square extends React.Component {
  constructor(props) {
    super(props);
    this.state = {value:null};
  }
  render() {
    return (
      <button className="square" onClick={() => this.props.onClick()}>
        {this.props.value}
      </button>
    );
  }
}

class Board extends React.Component {
  renderSquare(x,y) {
    return <Square value={this.state.squares[y][x]} onClick={() => this.handleClick(x,y)} />;
  }
  handleClick(x,y) {
    const squares = JSON.parse(JSON.stringify(this.state.squares));
    if (!squares[y][x] && !this.state.winner) {
      squares[y][x] = this.setTurn();
      this.setState({squares: squares},()=>{
        console.log(`Max in a row made by last move(${squares[y][x]}): ${this.getWin(x,y)-1}`);
        if (this.getWin(x,y)==4) this.setState({winner:squares[y][x]});
      });
    }
  }
  setTurn() {
    var prevTurn = this.state.turn;
    this.setState({turn:prevTurn == 'X' ? 'O':'X'});
    return prevTurn;
  }
  
  getWin(x,y,value,searchvector) {
    if (arguments.length==2) {
      var checkTurn = this.state.squares[y][x];
      var searchdirections = [[-1,-1],[0,-1],[1,-1],[-1,0]];
      return searchdirections.reduce((maxinrow,searchdirection)=>Math.max(this.getWin(x,y,checkTurn,searchdirection)+this.getWin(x,y,checkTurn,[-searchdirection[0],-searchdirection[1]]),maxinrow),0);
    } else {
      if (this.state.squares[y][x]===value) {
        var result = 1;
        if (
          x+searchvector[0] >= 0 && x+searchvector[0] < 3 && 
          y+searchvector[1] >= 0 && y+searchvector[1] < 3
          ) result += this.getWin(x+searchvector[0],y+searchvector[1],value,searchvector);
        return result;
      } else {
        return 0;
      }
    }
  }
  
  constructor(props) {
    super(props);
    this.state = {
      squares: Array(3).fill(Array(3).fill(null)),
      turn: 'X',
      winner: null
    };
  }
  render() {
    const status = !this.state.winner?`Next player: ${this.state.turn}`:`${this.state.winner} won!`;

    return (
      <div>
        <div className="status">{status}</div>
        <div className="board-row">
          {this.renderSquare(0,0)}
          {this.renderSquare(0,1)}
          {this.renderSquare(0,2)}
        </div>
        <div className="board-row">
          {this.renderSquare(1,0)}
          {this.renderSquare(1,1)}
          {this.renderSquare(1,2)}
        </div>
        <div className="board-row">
          {this.renderSquare(2,0)}
          {this.renderSquare(2,1)}
          {this.renderSquare(2,2)}
        </div>
      </div>
    );
  }
}

class Game extends React.Component {
  render() {
    return (
      <div className="game">
        <div className="game-board">
          <Board />
        </div>
        <div className="game-info">
          <div>{/* status */}</div>
          <ol>{/* TODO */}</ol>
        </div>
      </div>
    );
  }
}

// ========================================

ReactDOM.render(
  <Game />,
  document.getElementById('root')
);

body {
  font: 14px "Century Gothic", Futura, sans-serif;
  margin: 20px;
}

ol, ul {
  padding-left: 30px;
}

.board-row:after {
  clear: both;
  content: "";
  display: table;
}

.status {
  margin-bottom: 10px;
}

.square {
  background: #fff;
  border: 1px solid #999;
  float: left;
  font-size: 24px;
  font-weight: bold;
  line-height: 34px;
  height: 34px;
  margin-right: -1px;
  margin-top: -1px;
  padding: 0;
  text-align: center;
  width: 34px;
}

.square:focus {
  outline: none;
}

.kbd-navigation .square:focus {
  background: #ddd;
}

.game {
  display: flex;
  flex-direction: row;
}

.game-info {
  margin-left: 20px;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
<div id="errors" style="
  background: #c00;
  color: #fff;
  display: none;
  margin: -20px -20px 20px;
  padding: 20px;
  white-space: pre-wrap;
"></div>
<div id="root"></div>
<script>
window.addEventListener('mousedown', function(e) {
  document.body.classList.add('mouse-navigation');
  document.body.classList.remove('kbd-navigation');
});
window.addEventListener('keydown', function(e) {
  if (e.keyCode === 9) {
    document.body.classList.add('kbd-navigation');
    document.body.classList.remove('mouse-navigation');
  }
});
window.addEventListener('click', function(e) {
  if (e.target.tagName === 'A' && e.target.getAttribute('href') === '#') {
    e.preventDefault();
  }
});
window.onerror = function(message, source, line, col, error) {
  var text = error ? error.stack || error : message + ' (at ' + source + ':' + line + ':' + col + ')';
  errors.textContent += text + '\n';
  errors.style.display = '';
};
console.error = (function(old) {
  return function error() {
    errors.textContent += Array.prototype.slice.call(arguments).join(' ') + '\n';
    errors.style.display = '';
    old.apply(this, arguments);
  }
})(console.error);
</script>

I developed an algorithm for this as part of a science project once.

You basically recursively divide the board into a bunch of overlapping 2x2 rects, testing the different possible combinations for winning on a 2x2 square.

It is slow, but it has the advantage of working on any sized board, with fairly linear memory requirements.

I wish I could find my implementation