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13. Game

Exercise 130

Navigating the code

Background: Pac-Man project

The project is split into three parts (solutions).

  • libpacman - This is where the entire game lives
  • pacman - This is the project that generates the executable, the only thing main does is to create the game instance and call the run() function
  • pacman_unit_tests - All the unit tests for the game
libpacman

The libpacman solution has 3 main parts.

  • Game - A class that holds the Canvas and GameState objects
  • Canvas - A class that uses the SFML library to draw objects to the screen. Also manages the game window
  • GameState - A class that holds all of the state for Pac-Man. Contains classes like the Ghosts (Blinky, Inky and Pinky), PacMan and the Pellets PacMan eats.
Updating the game state

Within the Game.cpp file there is a function called run(), this function draws the current game state to screen canvas.update(gameState); but it only calls the game state update function at a fixed interval, it also sends this interval to the game state update function gameState.step(delta_time);

Then in the step() function in GameState.cpp the magic happens. Each object in the game world is updated and the state of the game world is checked. Did Pac-Man just eat a pellet? Are the ghosts touching Pac-Man? Which direction is Pac-Man moving?

The game board

The game board is stored as a two-dimensional array of integers in the Board.cpp file. The array itself is only accessible within that file, which is why we export helper functions that use the array as a global variable. This exporting happens in Board.hpp

Ghosts

All ghosts inherit from the parent Ghost class defined in Ghost.hpp and Ghost.cpp, there are a couple of pure virtual functions those ghosts need to implement, otherwise you are not able to create an instance of them. A ghost stores its current location, its current target and other state regarding if it is frightened or not. At each update() call a ghost needs to decide what its target is and which direction it should turn next.

Pac-Man

PacMan has similar state to a Ghost, like direction and position. Pac-Man's behavior is also similar in update() as we need to determine where to place Pac-Man in the next game state. But the information on which direction to turn comes from the player (or a bot).

Unit tests

Because updates of game state and drawing the game on screen are completely separate, we can create game state within unit tests and give it arbitrary delta time and call whatever functions we want to simulate the game being played. Most unit tests only focus on one single class but some (like testFruits.cpp) use the GameState class.

Exercise

  1. Familiarize yourself with the codebase by trying to find the files that contain the code mentioned in the slides
  2. In which file is the logic for the ghost behavior implemented?
  3. What information is tracked as part of the score?
Solution

  1. The behavior of the ghosts is mostly implemented in Ghost.cpp, but each of the ghosts has its own file that implements its personality.

  2. The score keeps track of the number of lives, pellets and fruits, as well as the number of points earned by the player.