Chapter 8

let canvas = document.getElementById("canvas");
let ctx    = canvas.getContext("2d");
 
const width       = Number(canvas.width);
const height      = Number(canvas.height);
const tileWidth   = 50;
const tileHeight  = 50;
const tilesX      = Math.round(width / tileWidth);
const tilesY      = Math.round(height / tileHeight);
const totalTiles  = tilesX * tilesY;
const borderWidth = 2;
 
let gameOver = false;
let eaten    = false;
 
let player = [];
let apple;
let highscore = 0;
 
// Keyboard controls
let lastKey;
let bufferedKey;
let deniedKey = {
  "ArrowLeft": "ArrowRight",
  "ArrowRight": "ArrowLeft",
  "ArrowUp": "ArrowDown",
  "ArrowDown": "ArrowUp",
  "undefined": "ArrowLeft"
}
 
document.addEventListener("keydown", function (e) {
  bufferedKey = e.key;
});
 
function getKey() {
  if(bufferedKey == "ArrowLeft" || bufferedKey == "ArrowRight" || bufferedKey == "ArrowUp" || bufferedKey == "ArrowDown") {
    if(!(deniedKey[lastKey] == bufferedKey)) {
      lastKey = bufferedKey;
    }
  }
  else if(gameOver && bufferedKey == " ") {
    lastKey = bufferedKey;
  }
}
 
// Setup the game - called once each time we init the game.
function setup() {
  if(player.length > highscore)
    highscore = player.length;
  
  gameOver = false;
  lastKey = undefined;
  player = [
    {
      x: 4,
      y: 5
    },
    {
      x: 3,
      y: 5
    },
    {
      x: 2,
      y: 5
    },
    {
      x: 1,
      y: 5
    }
  ];
 
  createApple();
}
 
// Update - called every frame
function update() {
  getKey();
 
  if(lastKey == undefined)
    return;
 
  if(gameOver) {
    if(lastKey == " ") {
      setup();
    }
    return;
  }
 
  // Remove the last element in the array
  if(!eaten)
    player.pop();
  else
    eaten = false;
 
  // This is a confusing line of code. We are copying the head of the snake at player[0]
  // into an empty object {} and returning it to the variable `head`.
  // If we use `head = player[0]`, we would be referencing `player[0]` instead of copying it.
  let head = Object.assign({}, player[0]);
 
  if(lastKey == "ArrowLeft")
    --head.x;
  else if(lastKey == "ArrowRight")
    ++head.x;
  else if(lastKey == "ArrowUp")
    --head.y;
  else if(lastKey == "ArrowDown")
    ++head.y;
 
  // Add the head of the snake to the beginning of the array
  player.unshift(head);
 
  for(let i = 0; i < player.length; ++i) {
    // Check if the snake has hit itself
    if(i > 0 && head.x == player[i].x && head.y == player[i].y) {
      gameOver = true;
    }
    
    // Check if the snake is out of bounds
    if(player[i].x == tilesX)
      player[i].x = 0;
    else if(player[i].x == -1)
      player[i].x = tilesX -1;
    
    if(player[i].y == tilesY) 
      player[i].y = 0;
    else if(player[i].y == -1)
      player[i].y = tilesY -1;
  }
 
  // Check if the snake hit the apple
  if(head.x == apple.x && head.y == apple.y) {
    eaten = true;
    createApple();
  }
}
 
// Render - called every frame
function render() {
  // Draw background
  ctx.fillStyle = "black";
  ctx.fillRect(0, 0, width, height);
 
  if(!gameOver) {
    // Draw grid
    for(var i = 0; i < tilesX; ++i) {
      for(var j = 0; j < tilesY; ++j) {
 
        // Set the default tile color
        ctx.fillStyle = "blue";
 
        // Detect if the player is in this tile
        for(var k = 0; k < player.length; ++k) {
          if(player[k].x == i && player[k].y == j) {
            ctx.fillStyle = "green";
            break;
          }
        }
        
        drawTile(i,j);
      }
    }
 
    // Draw apple
    ctx.fillStyle = "red";
    drawTile(apple.x, apple.y);
  }
  else {
    // Draw GameOver screen
    ctx.font = "50px serif";
    ctx.fillStyle = "red";
    ctx.textAlign = "center";
    ctx.fillText("GAME OVER", width / 2, height / 2);
    ctx.font = "25px serif";
    ctx.fillText("Press Space to Restart", width / 2, height / 2 + 50);
  }
 
  // Draw the score
  ctx.font = "20px sans serif";
  ctx.fillStyle = "yellow";
  ctx.textAlign = "left";
  ctx.fillText(`Score: ${player.length}`, 10, 20);
  ctx.textAlign = "right";
  ctx.fillText(`Highscore: ${highscore}`, width - 10, 20);
}
 
// The entry point for the game
function main() {
  update();
  render();
}
 
// Utility functions
function randomTile(max) {
  // Generate a random number between 0 and max
  let randomFloat = Math.random() * max;
 
  // Convert float (decimal) to integer
  return Math.floor(randomFloat);
}
 
function drawTile(x, y) {
  // Calculate the x, y, w, h for each tile
  x = x * tileWidth + borderWidth;
  y = y * tileHeight + borderWidth;
  let w = tileWidth - borderWidth * 2;
  let h = tileHeight - borderWidth * 2;
 
  // Draw each tile
  ctx.fillRect(x, y, w, h);
}
 
function createApple() {
  let collision;
 
  do {
    collision = false;
 
    // Choose a random tile for the Apple
    apple = {
      x: randomTile(tilesX),
      y: randomTile(tilesY)
    }
 
    // Check to see if the apple was generated on a tile the player occupies
    for(let i = 0; i < player.length; ++i) {
      if(player[i].x == apple.x && player[i].y == apple.y) {
        collision = true;
        break;
      }
    }
  } while(collision);
}
 
// Setup the game before starting the game loop
setup();
 
// This will call the function main at 10fps and start the game
setInterval(main, 1000 / 10);