Adding project structure doc.

exercises/project-structure.md

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+# Pac-Man
+
+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 it does is to create the game instance and call the `run()` function
+- **pacman_tests** - All of 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, 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/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
+
+Pac-Man 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`) use the `GameState` class.