Monthly Archive for August, 2013

Obey the State!

When you hear the term State Machine, you may think of totalitarian societies as depicted in George Orwell’s 1984 run by ruthless dictators. However, in programming, state machines are quite benign and helpful in solving certain kinds of programming problems. Chapter 10 of Learning PHP Design Patterns, explains the State design pattern in full detail. In this post I want to introduce the concept of a state machine, but we will not see a true State design pattern just yet. Here, we’ll just look at a state machine and how it can be used in OOP programming in general. As you will see, we can use a state machine in standard OOP programming without a full State design pattern. In Part III, we’ll examine a full State Design Pattern.

Generally speaking in programming, a state is the current value of a variable. A state machine (or finite state machine) is a system where each state waits for a transition to another state. (In Learning PHP Design Patterns, I used the example of a light having states of “on” or “off,” with the transition through the trigger of a light switch.) In the game of Rock, paper, scissors, lizard, Spock (R-P-S-L-S), each of the five gestures represents a state, and the transition occurs each time the players throw a gesture. (See PHP Game Making Part I.) To get started, play the revised version of the game and download the files for the State Machine version of the game:

A State Class

To get started, begin with an interface that encompasses all of the different states (moves) in R-P-S-L-S. The following listing, IPlayer, has methods for each move:

< ?php
interface IPlayer
{
    public function rockMove();
    public function spockMove();
    public function paperMove();
    public function lizardMove();
    public function scissorsMove();
}
?>

Note that all of the methods are public. This allows access to them through different implementations.

What we want to do with each method is to generate outcomes for all possible moves. Given that a player (human) is pitted against a computer that randomly makes moves, each state class with have outcomes for each of the methods based on the combination of what the player has done and what the computer will do. Take, for example, the Rock class. Each of the

< ?php
class Rock implements IPlayer
{
    public function rockMove()
    {
        return "Tie";
    }
 
    public function spockMove()
    {
        return "Computer wins!";
    }
 
    public function paperMove()
    {
        return "Computer wins!";
    }
 
    public function lizardMove()
    {
        return "Player wins!";
    }
 
    public function scissorsMove()
    {
        return "Player wins!";
    }  
}
?>

Essentially, you have self-aware state classes. For example, the Rock class is aware that if the opposition makes a Rock move, the result is a tie. Likewise, if the opposition chooses either a Lizard or Scissors move, Rock wins; but if the opposition makes either Paper or Spock moves, Rock loses. There are no conditional statements. That’s important as you will see when we move on the the State design pattern in Part III (or you saw in Chapter 10 of Learning PHP Design Patterns.

The State Machine

To understand a State Machine, it helps to use a statechart. A statechart identifies the different states in a system and the triggers that transition from one state to another. In the case of an R-P-S-L-S) finite state system, you have a triggering state (chant->throw) and the five individual states. Figure 1 shows the five states and the triggering state. Note that all changes from one state to another go through the trigger and none directly to another. (e.g., A Lizard state cannot go directly to a Paper state; it must go through the trigger.)

Figure 1: Statechart of the RPSLS State Machine

Importantly, the State Machine represents what actually happens in a game of R-P-S-L-S. Players enter the “chant” trigger and then throw a gesture. So we may actually refer to the RPSLS as a “state” used to transition between the five outcome states. The task now, is to implement these states. (Click below to continue.)
Continue reading ‘PHP Game Making Part II: The State Machine’