const EMPTY = 0;                    // constants and variables
const QUEEN = 1;
var board = [];                     // chessboard
var countQueens;                    // number of queens
var excludedRow;                    // row to be skipped

function loop(row)                  // recursive function
{               
    if (row == excludedRow)         // skip the excluded row
        row++;                      // this is for the fixed queen

    for (var x = 0; x < 8; x++)     // go through all columns
    {
        if (isPossible(row, x))     // check all directions
        {
            board[row][x] = QUEEN;  // place a queen on the board
            countQueens++;          // increase the number of queens

            if (countQueens >= 8)   // check the number of queens
                return true;        // return true when solved
                                    // otherwise 
            if (loop(row + 1))      // start a new loop in the next row
                return true;        // return true when solved
                                    // otherwise if it does not work
            countQueens--;          // decrease the number of queens
            board[row][x] = EMPTY;  // delete the queen 
        }
    }

    return false;
}

function isPossible(y1, x1)     // check all directions
{                           
    if (board[y1][x1] == EMPTY) 
    {
        for (let x = 0; x < 8; x++) // check row
        {
            if (board[y1][x] == QUEEN)
                return false;
        }

        for (let y = 0; y < 8; y++) // check column
        {
            if (board[y][x1] == QUEEN)
                return false;
        }

        for (let y = y1, x = x1; y < 8 && x < 8; y++, x++)
        {                           // check bottom right
            if (board[y][x] == QUEEN)
                return false;
        }

        for (let y = y1, x = x1; y >= 0 && x >= 0; y--, x--)
        {                           // check top left
            if (board[y][x] == QUEEN)
                return false;
        }

        for (let y = y1, x = x1; y >= 0 && x < 8; y--, x++)
        {                           // check top right
            if (board[y][x] == QUEEN)
                return false;
        }

        for (let y = y1, x = x1; y < 8 && x >= 0; y++, x--)
        {                           // check bottom left
            if (board[y][x] == QUEEN)
                return false;
        }
    }
    else 
        return false;

    return true;
}

function clickBoard(ev) // mouse click
{
    if (ev.target.id[1] >= 0 && ev.target.id[1] < 8 && ev.target.id[2] >= 0 
        && ev.target.id[1] < 8) // ensures that the values are valid
    {
        resetBoard(); // reset the chessboard

        // this position is set with a mouse click
        // the excludedRow is skipped in the algorithm
        // this position is fixed
        excludedRow = ev.target.id[1];  
        board[excludedRow][ev.target.id[2]] = QUEEN;
        countQueens = 1;

        loop(0);  // start the recursive loop in row zero

        updateBoard();
    }
}       

function load() // onLoad event
{
    // reset the chessboard at first
    resetBoard();                                                                   

    // place the first queen at random
    // the excludedRow is skipped in the algorithm
    // this position is fixed
    excludedRow = Math.floor(Math.random() * 8);
    board[excludedRow][Math.floor(Math.random() * 8)] = QUEEN;  
    countQueens = 1;

    // start the recursive loop in row zero
    loop(0);                

    updateBoard();
}

function resetBoard()
{
    board = [];                             // overwrite the board
    for (let i = 0; i < 8; i++)
        board[i] = new Array(8).fill(EMPTY);// fill the board with zeros
}

function updateBoard() // show the result
{
    for (var y = 0; y < 8; y++)
    {
        for (var x = 0; x < 8; x++)
        {
            var el = document.getElementById("I" + y.toString() + x.toString());
            
            if (board[y][x] == QUEEN)       // draw a queen
            {
                if (y == excludedRow)
                    el.innerHTML = "<span style='color: red;'>&#x2655</span>";
                else
                    el.innerHTML = "&#x2655";
            }
            else // empty square
                el.innerHTML = "";
        }
    }
}           