(function() {

    var S = window.Snake = {};

    var ROWS = S.ROWS = 50;
    var COLS = S.COLS = 50;

    var board;
    var body = new Array();
    var headCur = coord2idx(Math.floor((ROWS - 1) / 2), Math.floor((COLS - 1) / 2));
    var direction = 1;
    var g = S.graphicsContext = null;
    var canvas = S.canvas = null;
    var cellWidth, cellHeight;
    var foodEaten = false;
    var dead = true;
    var skipTicks;
    var nextGameTick;
    var cmdQueue = new Array();

    function coord2idx(row, col) { return (COLS * row) + col; }
    function idx2row(idx) { return Math.floor(idx/COLS); }
    function idx2col(idx) { return idx % COLS; }

    function initialize(options) { //givenCanvas, keyContext, fps) {

        if (!options.canvas) {
            alert("Missing the canvas element.");
            return false }

        canvas = options.canvas;
        g = S.graphicsContext = canvas.getContext("2d");

        if (options.rows) S.ROWS = ROWS = options.rows;
        if (options.cols) S.COLS = COLS = options.cols;

        board = new Array(ROWS * COLS);

        cellWidth = canvas.width / COLS;
        cellHeight = canvas.height / ROWS;
        
        if (options.keyContext) options.keyContext.onkeydown = handleKey;
        else canvas.onkeydown = handleKey;
        
        g.fillStyle = "white";
        g.fillRect(0, 0, canvas.width, canvas.height);
        
        g.font = "24px sans-serif";
        g.fillStyle = "black";
        g.fillText("Press Enter to begin.", 50, Math.floor(canvas.height/2), canvas.width - 100);

        if (options.fps) skipTicks = Math.ceil(1000 / options.fps);
        else skipTicks = 150; }

    function startGame(canvas, keyContext) {
        while (body.length > 0) body.pop();
        while (cmdQueue.length > 0) cmdQueue.pop();

        headCur = coord2idx(Math.floor((ROWS - 1) / 2), Math.floor((COLS - 1) / 2));
        direction = 1;
        dead = false;
        nextGameTick = (new Date).getTime() + skipTicks;

        body.push(headCur);

        // Wipe the board and draw the walls.
        var i;
        for (i = 0; i < board.length; i++ ) {
            if (Math.floor(i / COLS) == 0 || (i % COLS) == 0) board[i] = 'W';
            else if (Math.floor(i / COLS) == (ROWS - 1) ||
                     (i % COLS) == (COLS - 1)) board[i] = 'W';
            else board[i] = ''; }

        // Write the initial body segment onto the board.
        board[headCur] = 'S';

        // Create a new food item and add it to the board.
        board[newFoodLocation()] = 'F';

        // Draw the board
        for (i = 0; i < board.length; i++) { paintIdx(i); }
        
        gameLoop(); }

    function gameLoop() {
        if (dead) return // break the callback loop

        if ((new Date).getTime() > nextGameTick) {
            updateAndDraw();
            nextGameTick += skipTicks; }

        requestAnimationFrame(gameLoop); }

    // For this game I want the game logic and the refresh rate to be tied
    // together. The snake moves on step per animation frame. We do limit the
    // animation frame to 1fps. This works because there are no other elements
    // being animated other than the snake.
    function updateAndDraw() {
        if (!g) return false;

        foodEaten = false;

        // 1. Read up to one command from the user.
        if (cmdQueue.length > 0) {
            var cmd = cmdQueue.shift();
            if (cmd == 37) direction = -1;          // Left
            else if (cmd == 38) direction = -COLS;  // Up
            else if (cmd == 39) direction = 1;      // Right
            else if (cmd == 40) direction = COLS;   /* Down */ }

        // 2. Find the head's next spot.
        headCur = headCur + direction;

        // 3. First check to see if we've eaten food (since it will affect
        //    collision detection for the body). This is also where growth
        //    happens via not moving the tail.
        if (board[headCur] == 'F') foodEaten = true;

        // 4. Remove the tail.
        if (!foodEaten) {
            var tailIdx = body.shift();
            // Value on the board doesn't matter, just not W,S, or F
            board[tailIdx] = '';
            paintIdx(tailIdx); }
            

        // 5. Detect wall and snake collisions
        if (board[headCur] == 'W' || board[headCur] == 'S') {
            die(); return }

        // 6. Move the head
        body.push(headCur);
        board[headCur] = 'S';
        paintIdx(headCur);
        
        // 7. Create more food if needed.
        if (foodEaten) {
            var foodLoc = newFoodLocation();
            board[foodLoc] = 'F';
            paintIdx(foodLoc); } }

    function newFoodLocation() {
        var emptySpaces = new Array();
        for (var i = 0; i < board.length; i++)
            if (board[i] != 'W' && board[i] != 'S')
                emptySpaces.push(i);

        return emptySpaces[Math.floor(Math.random() * emptySpaces.length)] }

    function die() {
        paintIdxColor(headCur, "red");
        dead = true;
        //g.fillStyle = "white";
        //g.fillRect(0, 0, canvas.width, canvas.height);
        
        g.font = "24px sans-serif";
        g.fillStyle = "red";
        g.fillText("Press Enter to retry.", 50, Math.floor(canvas.height/2), canvas.width - 100); }

    function handleKey(ke) {
        var key = ke.keyCode ? ke.keyCode : ke.which;

        // Enter, start a new game.
        if (key == 13) startGame();

        // Queue directions in the command queue
        else if ((key > 36) && (key < 41)) cmdQueue.push(key); }

    function paintCoord(row, col) {
        var idx = coord2idx(row, col);
        if (board[idx] == 'W') g.fillStyle = "black";
        else if (board[idx] == 'F') g.fillStyle = "yellow";
        else if (board[idx] == 'S') g.fillStyle = "blue";
        else g.fillStyle = "white";

        g.fillRect(col, row, cellWidth, cellHeight); }

    function paintIdxColor(idx, color) {
        g.fillStyle = color;

        g.fillRect(
            (idx % COLS) * cellWidth,
            Math.floor(idx / COLS) * cellHeight,
            cellWidth, cellHeight); }
    
    function paintIdx(idx) {
        if (board[idx] == 'W') g.fillStyle = "black";
        else if (board[idx] == 'F') g.fillStyle = "yellow";
        else if (board[idx] == 'S') g.fillStyle = "blue";
        else g.fillStyle = "white";

        g.fillRect(
            (idx % COLS) * cellWidth,
            Math.floor(idx / COLS) * cellHeight,
            cellWidth, cellHeight); }

    S.initialize = initialize;

    // Exports for debugging
    S.board = board;
    S.body = body;
    S.updateAndDraw = updateAndDraw;
})();