Question

The dramatic highlight of our world, if you watch for a bit, is when two critters bounce off each other. Can you think of another interesting form of behavior?

The one I came up with is a critter that moves along walls. Conceptually, the critter keeps its left hand (paw, tentacle, whatever) to the wall and follows along. This turns out to be not entirely trivial to implement.

We need to be able to “compute” with compass directions. Since directions are modeled by a set of strings, we need to define our own operation ( dirPlus ) to calculate relative directions. So dirPlus("n", 1) means one 45-degree turn clockwise from north, giving "ne" . Similarly, dirPlus("s", -2) means 90 degrees counterclockwise from south, which is east.

function dirPlus(dir, n) {
  var index = directionNames.indexOf(dir);
  return directionNames[(index + n + 8) % 8];
}

function WallFollower() {
  this.dir = "s";
}

WallFollower.prototype.act = function(view) {
  var start = this.dir;
  if (view.look(dirPlus(this.dir, -3)) != " ")
    start = this.dir = dirPlus(this.dir, -2);
  while (view.look(this.dir) != " ") {
    this.dir = dirPlus(this.dir, 1);
    if (this.dir == start) break;
  }
  return {type: "move", direction: this.dir};
};

The act method only has to “scan” the critter’s surroundings, starting from its left side and going clockwise until it finds an empty square. It then moves in the direction of that empty square.

What complicates things is that a critter may end up in the middle of empty space, either as its start position or as a result of walking around another critter. If we apply the approach I just described in empty space, the poor critter will just keep on turning left at every step, running in circles.

So there is an extra check (the if statement) to start scanning to the left only if it looks like the critter has just passed some kind of obstacle—that is, if the space behind and to the left of the critter is not empty. Otherwise, the critter starts scanning directly ahead, so that it’ll walk straight when in empty space.

And finally, there’s a test comparing this.dir to start after every pass through the loop to make sure that the loop won’t run forever when the critter is walled in or crowded in by other critters and can’t find an empty square.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.