litemerafrukt / gomokuai

function nextBestMove(board, boardSize) {

    var m,

        l,

        position,

        type,

        line,

        totalCells,

        consecutiveCells,

        emptySides,

        bestScore,

        cellScore,

        directionScore,

        score;

    // iterate through all the board's cells

    for (var i = boardSize * boardSize; i--; ) {

        // skip to next cell if this cell is owned by the computer

        // if (board[i] == 1) continue;

        // by default, the best move is the first free cell

        // (position, attack, defense)

            bestScore = [i, 0, 0];

        }

        // we will give a "score" to the position,

        // based on its surroundings horizontally, vertically and

        // on both diagonals; for example: for a row like 000XXPXX000 (where "0" means empty,

        // "X" represents the opponent's pieces and "P" is the position for which we are

        // determining the "score", we would check

        // 0|00XXP|XX000, 00|0XXPX|X000, 000|XXPXX|000, 000X|XPXX0|00, 000XX|PXX00|0,

        // and then we would do the same on the vertical, and on both diagonals)

        // cell's default score (attack and defense)

        cellScore = [0, 0];

        // the 4 directions to check: vertical, horizontal, diagonal /, diagonal \ (in this order)

        for (var j = 4; j--; ) {

            // the default score for the direction we're checking

            directionScore = [0, 0];

            // check the 5 possible outcomes, as described above

            // (if we're checking whether the player won,

            // we'll do this iteration only once, checking for 5 in a row)

            for (var k = !board[i] ? 5 : 1; k--; ) {

                // initialize the type of cells we're looking for,

                // and the array with the cells on the current direction

                type = board[i] || undefined;

                line = [];

                // check the 5 pieces for each possible outcome, plus the 2 sides

                for (l = 7; l--; ) {

                    // used to compute position

                    m = -5 + k + l;

                    var n = i % boardSize;

                    if (

                        // vertical

                        ((j === 0 &&

                            (position = i + boardSize * m) !== false &&

                            n == position % boardSize) ||

                            // horizontal

                            (j == 1 &&

                                (position = i + m) !== false &&

                                ~~(position / boardSize) == ~~(i / boardSize)) ||

                            // diagonal /

                            (j == 2 &&

                                (position = i - boardSize * m + m) !== false &&

                                ((position > i && position % boardSize < n) ||

                                    (position < i && position % boardSize > n) ||

                                    position == i)) ||

                            // diagonal \

                            ((j == 3 &&

                                (position = i + boardSize * m + m) !== false &&

                                ((position < i && position % boardSize < n) ||

                                    (position > i && position % boardSize > n))) ||

                        // the position is not off-board

                        position >= 0 &&

                        position < boardSize * boardSize &&

                        // the cell is of the same type as the ones we are looking for

                        // or, we are checking the score for an empty cell,

                        // the current position is empty,

                        // or is "undefined" (meaning we didn't yet find any non-empty cells)

                        (board[position] == type ||

                            (!board[i] && (!board[position] || undefined === type)) ||

                            // or we're just checking the sides

                            !l ||

                            l == 6)

                    ) {

                        // add position to the line

                        line.push(position);

                        // if we're not just checking the sides,

                        // this is not an empty cell, and is of the same type as

                        // the ones we're looking for,

                        // update the type of cells we're looking for

                        // (we use ^ instead of !=)

                        if (l && l ^ 6 && undefined === type && board[position]) {

                            type = board[position];

                        }

                        // if the computed position is off-board,

                        // but this is a side - cell, save it as "undefined"

                    } else if (!l || l == 6) {

                        line.push(undefined);

                    } else {

                        // skip the rest of the test if we found this row to be "non-compliant"

                        // (a different type of cell than the ones we're looking for,

                        // one of the 5 cells is off - board)

                        break;

                    }

                }

                // if we added exactly 7 position to our line, and

                // the line is not containing * only * empty cells

                if (line.length == 7 && undefined !== type) {

                    // if we are checking whether the player won,

                    // set this flag so that later on we do not

                    // overwrite the value of the cell

                    m = board[i] ? true : false;

                    // calculate the score when setting the current cell to

                    // the same type as the other ones we found

                    board[i] = type;

                    // calculate the number of consecutive cells we get like this

                    // (we'll do this by looking in our "line" array)

                    consecutiveCells = 0;

                    totalCells = 0;

                    emptySides = 0;

                    // the total number of cells of the same type

                    for (l = 5; l--; ) {

                        if (board[line[l + 1]] == type) {

                            totalCells++;

                        }

                    }

                    // look to the left of the current cell

                    for (

                        l = line.indexOf(i) - 1;

                        l >= 0;

                        // if the cell is of the same type,

                        //increment the number of consecutive cells

                        l--

                    ) {

                        if (board[line[l]] == type) {

                            consecutiveCells++;

                        } else {

                            // otherwise

                            // if the adjacent cell is empty, increment the number of empty sides

                            // we have to use === 0 (instead of !) because it

                            // can also be "undefined"

                            if (board[line[l]] === 0) {

                                emptySides++;

                            }

                            // don't look further

                            break;

                        }

                    }

                    // look to the right of the current cell

                    for (

                        l = line.indexOf(i);

                        l < line.length;

                        // if the cell is of the same type,

                        // increment the number of consecutive cells

                        l++

                    ) {

                        if (board[line[l]] == type) {

                            consecutiveCells++;

                        } else {

                            // otherwise

                            // if the adjacent cell is empty, increment the number of empty sides

                            // we have to use === 0 (instead of !)

                            // because it can also be "undefined"

                            if (board[line[l]] === 0) {

                                emptySides++;

                            }

                            // don't look further

                            break;

                        }

                    }

                    // give a score to the row based on the

                    // array below(number of cells / empty sides)

                    score = [[0, 1], [2, 3], [4, 12], [10, 64], [256, 256]][

                        consecutiveCells >= totalCells

                            ? Math.min(consecutiveCells, 5) - 1

                            : totalCells - 1

                    ][consecutiveCells >= totalCells ? (emptySides ? emptySides - 1 : 0) : 0];

                    // reset the cell's value (unless we are looking to see if the player won)

                    if (!m) {

                        board[i] = 0;

                    }

                    // } else if (score >= 256) {

                    //     // if the player won, update the score

                    //     score = 1024;

                    // }

                    // if, so far, this is the best

                    // attack / defense score(depending on the value of "type")

                    // for this direction, update it

                    if (score > directionScore[type - 1]) {

                        directionScore[type - 1] = score;

                    }

                }

            }

            // update the cell's attack and defense score

            // (we simply sum the best scores of all 4 directions)

            for (k = 2; k--; ) {

                cellScore[k] += directionScore[k];

            }

        }

        // used below

        j = cellScore[0] + cellScore[1];

        k = bestScore[1] + bestScore[2];

        // if cell's attack + defense score is better than

        // the current best attack and defense score

        // or, cell's score is equal to the best score,

        // but computer's move is better or equal to the player's,

        // and the current best move is not *exactly* the same

        if (

            (j > k || (j == k && cellScore[0] >= bestScore[1])) &&

            // we're checking the score of an empty cell,

            // or we're checking to see if the player won and he won

            // (we don't update the score when checking if the

            // player won * unless * the player actually won)

            (!board[i] || cellScore[1] >= 1024)

            // update best score (position, attack, defense)

        ) {

            bestScore = [i, cellScore[0], cellScore[1]];

        }

    }

    // Calculation done return x, y

    var x, y;

    y = Math.floor(bestScore[0] / boardSize);

    x = bestScore[0] % boardSize;

    return { x: x, y: y };

}