Question

Before we can start on the World constructor, we must get more specific about the critter objects that will be living inside it. I mentioned that the world will ask the critters what actions they want to take. This works as follows: each critter object has an act method that, when called, returns an action. An action is an object with a type property, which names the type of action the critter wants to take, for example "move" . The action may also contain extra information, such as the direction the critter wants to move in.

Critters are terribly myopic and can see only the squares directly around them on the grid. But even this limited vision can be useful when deciding which action to take. When the act method is called, it is given a view object that allows the critter to inspect its surroundings. We name the eight surrounding squares by their compass directions: "n" for north, "ne" for northeast, and so on. Here’s the object we will use to map from direction names to coordinate offsets:

var directions = {
  "n":  new Vector( 0, -1),
  "ne": new Vector( 1, -1),
  "e":  new Vector( 1,  0),
  "se": new Vector( 1,  1),
  "s":  new Vector( 0,  1),
  "sw": new Vector(-1,  1),
  "w":  new Vector(-1,  0),
  "nw": new Vector(-1, -1)
};

The view object has a method look , which takes a direction and returns a character, for example "#" when there is a wall in that direction, or " " (space) when there is nothing there. The object also provides the convenient methods find and findAll . Both take a map character as an argument. The first returns a direction in which the character can be found next to the critter or returns null if no such direction exists. The second returns an array containing all directions with that character. For example, a creature sitting left (west) of a wall will get ["ne", "e", "se"] when calling findAll on its view object with the "#" character as argument.

Here is a simple, stupid critter that just follows its nose until it hits an obstacle and then bounces off in a random open direction:

function randomElement(array) {
  return array[Math.floor(Math.random() * array.length)];
}

var directionNames = "n ne e se s sw w nw".split(" ");

function BouncingCritter() {
  this.direction = randomElement(directionNames);
};

BouncingCritter.prototype.act = function(view) {
  if (view.look(this.direction) != " ")
    this.direction = view.find(" ") || "s";
  return {type: "move", direction: this.direction};
};

The randomElement helper function simply picks a random element from an array, using Math.random plus some arithmetic to get a random index. We’ll use this again later because randomness can be useful in simulations.

To pick a random direction, the BouncingCritter constructor calls randomElement on an array of direction names. We could also have used Object.keys to get this array from the directions object we defined earlier , but that provides no guarantees about the order in which the properties are listed. In most situations, modern JavaScript engines will return properties in the order they were defined, but they are not required to.

The “ || "s" ” in the act method is there to prevent this.direction from getting the value null if the critter is somehow trapped with no empty space around it (for example when crowded into a corner by other critters).

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.