// An old fashion tennis game based on Japanese comic "こんにちわマイコン".
// Testted on IE and Firefox.
// Select all, and do it.
// Set focus to the game screen, and use left and right cursor keys.

// run() // Start the game.
// stop() // Stop the game.

lastCode = 1;
timer = undefined;

run = function () {
  display = new Screen(40, 20);
  ballX = 0;
  ballY = 0;
  ballDX = 1;
  ballDY = 1;
  playerX = 20;
  playerY = display.height - 1;

  display.node.onkeydown = onGameKey;
  if (timer) clearInterval(timer);
  timer = setInterval('tick(display)',100);
}

stop = function () {
    clearInterval(timer);
}

tick = function (display) {
  display.print(ballX, ballY, " ");
  display.print(playerX, display.height - 1, "   ");
  ballX += ballDX;
  ballY += ballDY;
  if (ballX <= 0 || display.width <= ballX) ballDX = 0 - ballDX;
  if (ballY <= 0) ballDY = 0 - ballDY;
  if (ballY == (playerY - 1) &&
    (playerX - 1) <= ballX &&
    ballX <= (playerX + 3)) ballDY = -1;

  if (lastCode) {
    if (lastCode== 37) playerX = (display.width + playerX - 1) % display.width;
    if (lastCode== 39) playerX = (playerX + 1) % display.width;
    lastCode= 0;
  }
  display.print(ballX, ballY, "O");
  display.print(playerX, display.height - 1, "===");
  if (playerY <= ballY) {
    stop();
    display.print(10, 10, "[ G A M E   O V E R ]");
  }
  display.show();
}

onGameKey = function(evt) {
  evt = (evt) ? evt : window.event;
  var charCode = (evt.charCode) ? evt.charCode : evt.keyCode;
  lastCode = charCode ;
  return false;
}

/* Screen library */
Screen = function (width, height) {
  this.node = initScreen(width, height);
  this.width = width;
  this.height = height;
  this.buffer ="";
  for (var i = 0; i < width * height; i++) this.buffer += " ";
}

/* Initialize the screen. */
initScreen = function (width, height) {
  if (typeof textarea != "undefined") {
    textarea.parentNode.removeChild(textarea);
    textarea = undefined;
  }
  textarea = document.createElement("textarea");
  workspace.parentNode.insertBefore(textarea, workspace);
  textarea.cols = width + 1;
  textarea.rows = height + 1;
  with (textarea.style) {
    background = "black"; color= "green";
    border = "4px solid #cccccc"
    position = "absolute"; right = "10px"; top = "10px";
  }
  return textarea;
}

Screen.prototype.show = function () {
  var screenBuffer = "";
  for (var y = 0; y < this.height; y++) {
    screenBuffer +=
      this.buffer.slice(y * this.width, y * this.width + this.width) + "\n";
  }
  this.node.value = screenBuffer;
}

Screen.prototype.print = function (x, y, aString) {
  var position = x + y * this.width;
  this.buffer = this.buffer.slice(0, position) +
    aString +
    this.buffer.slice(position + aString.length);
}

run();

// workspace.rows = 120; /* I recommend larger textarea during hacking. */