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Abhinav Sarkar 2013-11-21 22:04:24 +05:30
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# Gravity Pong
Gravity Pong is the famous [Pong][pong] game with a twist, written in [ClojureScript][cljs] using the
[core.async][coreasync] library.
Along with the usual Pong ball and the paddles on both sides of the screen, there is also a mass at
the center of the screen which exerts a gravitational force on the ball causing it to change its path
from a usual straight line to a curve. And the user can control the gravitational force using the
keyboard to shape the ball's path as they want! It can be played [here][demo] (only works on
Google Chrome for now).
## Programming Details
It is written in the [Flow-based Programming][fbprog] paradigm where calculations of ball's position,
velocity etc are done by separate modular components running in their own threads of computation
(in this case, core.async go processes). These components are connected using signals (implemented
using core.async channels) to form a network - like an electronic circuit - to build the complete game.
The very [lose coupling][coupling] (data coupling) caused by this modular design allows one to add
new components in the network without changing other parts of it. In fact, this was first developed
as a usual Pong game and gravitation was added only as an afterthought, with very minimal changes
to the existing code.
Here is how the network of components interconnected with signals looks like:
<pre>
+-d-----------+--------------------------+
v | |
keyboard +-e-> sampler +---k---> paddle-postnr +-d-+ +-> gravitation +--+ |
^ | p a |
t | | | |
| | | | |
+-------+ +-p----------+-|---+ | d
| | +-a-------|-|------+---------------+ |
| v v | | | v
browser +-b--> ticker +-+--t--> ball-postnr +-p-+-|------|------------p-> renderer
^ | ^ | | | ^ ^
| | +-----|---------+ | | | |
s | | l +--d--+ | s t
| | | +-+-----|-------+ | | |
| | p l +-a-|-------|----+ | |
| | v v v v +-l--+ | |
| +---t-> collision-detr | |
| | ^ +-s--+-----------------------+ |
| | s | |
+-----|-------------+-------------+ |
| |
+------------------------------------------------------+
</pre>
Signals
- *b*: ticks from browser
- *t*: game ticks
- *e*: keyboard events
- *k*: keydowns
- *p*: ball position
- *l*: ball velocity
- *a*: ball acceleration
- *d*: paddle positions
- *s*: game state
Components
- *keyboard*: keyboard for user input
- *browser*: browser
- *ticker*: generates game ticks from browser ticks
- *sampler*: samples input signal based on a sampling signal
- *paddle-postnr*: paddle positioner, calculates the position of paddles
- *ball-postnr*: ball positioner, calculates the position of the ball
- *collision-detr*: collision detector, detects collision of the ball with the paddles and the
walls, also calculates the velocity of the ball
- *gravitation*: calculates the acceleration of the ball due to gravitation
- *renderer*: renders the game on screen
This project was written as an experiment to play with the core.async library and can serve as
a medium sized example project for the same. The main source can be found [here][src].
[pong]: https://en.wikipedia.org/wiki/Pong
[cljs]: https://github.com/clojure/clojurescript
[coreasync]: https://github.com/clojure/core.async
[demo]: http://abhinavsarkar.net/frpong
[fbprog]: https://en.wikipedia.org/wiki/Flow-based_programming
[coupling]: https://en.wikipedia.org/wiki/Coupling_(computer_programming)
[src]: https://github.com/abhin4v/frpong/blob/master/src/cljs/frpong/core.cljs