Capstone: Mini Pong

You've shipped a game loop, input, ECS queries, collision detection, and a physics integrator. Time to put them together.

Pong is the world's smallest real game:

• Two paddles, one ball, a field, a score.
• Per tick: read input, move paddles, step the ball, handle wall and paddle collisions, score if the ball escapes.

It exercises every engine subsystem in 50 lines of code.

The world

• Field 100 wide x 50 tall. Origin (0,0) top-left.
• Left paddle at x=2, right paddle at x=96. Both height 10. Both initial top y=20.
• Ball at (50, 25) with velocity (+1, +1).
• Score starts (0, 0).

The tick

1. Input: UP_L, DOWN_L, UP_R, DOWN_R — move the addressed paddle by 1 in y, clamped to [0, 40]. NONE — no paddle change.
2. Physics: ball.x += vx; ball.y += vy.
3. Wall collision: if y < 0 or y > 49, bounce vy.
4. Paddle collision: if ball reaches a paddle's x and its y is within the paddle's vertical span, flip vx.
5. Score: if the ball escapes left or right, increment the score and reset the ball.

The snapshot

A SNAPSHOT command dumps the world state in a deterministic textual format:

BALL <x> <y>  L <l_top> R <r_top>  SCORE <sL> <sR>

Note the double spaces before L and before SCORE — that's the format the test runner expects.

Why a textual snapshot?

Real engines render pixels. But for testing and replays, a textual dump of world state is invaluable: it's deterministic, diffable, and machine-checkable. Many AAA engines have an internal "world snapshot" facility for exactly this purpose.

The Practice problem

Implement the rules above. Pass the tests, and you've shipped a working game engine — game loop, input, physics, collision, scoring, and a render snapshot, all in one file.