Move the ship with the left and right arrow keys. Press Space to fire. Additional controls (e.g. navigation keys) are documented on-screen.
To reduce code duplication, behavior for elements whose locations need to be tracked and whose elements need to be rendered on-screen is pulled out into a Sprites module.
The score is a simple measure of the number of enemies killed. Levels advance as a function of the score, and the probability of an enemy leaving formation increases with the level, as does their probability of firing a bullet. In this way, the game gets more difficult as the user kills more enemies. Because the difficulty is generated dynamically, there is no upper limit on the number of levels (however, for ease of grading, the difficulty is set to increase fairly aggressively).
Bullets are cleared as they leave the screen to avoid performance issues from rendering many off-screen elements that have no longer have any bearing on the game.
Where possible, functions are written to be applied to singular data structures (e.g. collision detection simply detects a collision between two sprites, rather than trying to compare one sprite with list of potential dangers), and are simply applied in functor/applicative style or in a zipping context if evaluation against a list is necessary. This makes them more reusable and less context-dependent.
The use of Maybes is preferred over passing of in-band error or sentinel values, and is also preferred over if/then branching. For instance, the function to produce an enemy-fired bullet returns a Maybe Bullet that can be Nothing if the enemy has not left formation or if the angle of the bullet would be undesirable.
Lines of code are capped at 80 characters for readability. A brief line of documentation is provided for each function.
Enemies and ships explode when hit by a bullet. When the animation is complete, the explosion sprite is cleared from the screen.
Enemies follow a Bezier curve to the bottom of the screen. If they reach the bottom unscathed, then they are placed back into formation at the top of the screen. To ensure that they can be seamlessly reinserted, open spots in the formation are tracked and updated as the formation moves about. Enemies are assigned one of the defined paths at random to ensure variety of movement. Enemies store a data type called RogueInfo to track information about their trajectories. For example, the enemy's location when they first "go rogue" is stored so that the coordinates provided by the curve function can be treated as offsets. Additional implementation notes provided inline.
Enemies don't fire straight down, but rather at an angle that is calculated based on their current trajectory. To ensure that bullets at different angles travel at the same speed, the (deltaX, deltaY) vector is normalized, and then multiplied a constant "bullet speed" scalar. Outlandish trajectories (e.g. a mostly-horizontal path calculated from an enemy that happens to be moving horizontally) are filtered out.
A makefile is provided to clear out compilation artifacts.
maketo compilemake cleanto remove the executable, *.o, and *.hi files.