Question

To make life in our world more interesting, we will add the concepts of food and reproduction. Each living thing in the world gets a new property, energy , which is reduced by performing actions and increased by eating things. When the critter has enough energy, it can reproduce, generating a new critter of the same kind. To keep things simple, the critters in our world reproduce asexually, all by themselves.

If critters only move around and eat one another, the world will soon succumb to the law of increasing entropy, run out of energy, and become a lifeless wasteland. To prevent this from happening (too quickly, at least), we add plants to the world. Plants do not move. They just use photosynthesis to grow (that is, increase their energy) and reproduce.

To make this work, we’ll need a world with a different letAct method. We could just replace the method of the World prototype, but I’ve become very attached to our simulation with the wall-following critters and would hate to break that old world.

One solution is to use inheritance. We create a new constructor, LifelikeWorld , whose prototype is based on the World prototype but which overrides the letAct method. The new letAct method delegates the work of actually performing an action to various functions stored in the actionTypes object.

function LifelikeWorld(map, legend) {
  World.call(this, map, legend);
}
LifelikeWorld.prototype = Object.create(World.prototype);

var actionTypes = Object.create(null);

LifelikeWorld.prototype.letAct = function(critter, vector) {
  var action = critter.act(new View(this, vector));
  var handled = action &&
    action.type in actionTypes &&
    actionTypes[action.type].call(this, critter,
                                  vector, action);
  if (!handled) {
    critter.energy -= 0.2;
    if (critter.energy <= 0)
      this.grid.set(vector, null);
  }
};

The new letAct method first checks whether an action was returned at all, then whether a handler function for this type of action exists, and finally whether that handler returned true, indicating that it successfully handled the action. Note the use of call to give the handler access to the world, through its this binding.

If the action didn’t work for whatever reason, the default action is for the creature to simply wait. It loses one-fifth point of energy, and if its energy level drops to zero or below, the creature dies and is removed from the grid.

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.