Color BASIC challenge: ball bouncing WITHOUT any IFs?

It seems like all I’m doing lately is regurgitating things that Robin of 8-Bit Show and Tell has already done.

So let’s do that again.

Don’t blame me. Blame YouTube!

YouTube did what YouTube does and it showed me another of Robin’s well-done videos. This one caught my attention because it dealt with the Commodore VIC-20 and its Super Expander cartridge.

The main thing that pulled me away from Commodore was seeing the TRS-80 Color Computer’s Extended Color BASIC. The CoCo had simple commands to play music and draw lines, boxes and circles. It also had this wondrous ELSE command I’d only heard rumors about.

On the VIC-20, it seemed you needed to use POKE and PEEK for just about anything graphics or sound related. Thus I gave up a computer with programmable characters and a hardware sound chip for a machine that had neither. On my new CoCo, at least I could draw a circle and play music without needing pages of DATA statements and cryptic PEEKS and POKEs.

Commodore was aware of this shortcoming, and they sold the Super Expander as a way to make up for it. Not only did it provide an extra 3K of memory (giving a whopping 6.5K for BASIC), it also added new commands to do “high resolution” graphics including drawing lines and circles, as well as ways to PRINT music using simple notation.

I used the Super Expander to do TV titles for fishing videos my father shot and edited. It was a thrill to see my VIC-20 graphics on TV screens at the Houston Boat Show.

But no one else could run my programs unless they had purchased the Super Expander as well.

But I digress.

(And besides, the Commodore 64 was $600 when it came out, and I was able to get a 64K CoCo 1 for $300 at the time.)

Don’t blame YouTube. Blame Twitter.

Robin’s video was making use of the Super Expander to let the VIC-20 solve a challenge initiated by Twitter user Dataram_57. On June 8th, 2019, they wrote:

This was, more or less, a classic bouncing ball program very much like the ones I have been writing about lately. But, all mine certainly made use of IF. Here’s how mine started:

0 REM bounce.bas
10 CLS:X=0:Y=0:XM=1:YM=1
20 PRINT@Y*32+X,"O";
30 X=X+XM:IF X<1 OR X>30 THEN XM=-XM
40 Y=Y+YM:IF Y<1 OR Y>13 THEN YM=-YM
50 GOTO 20

That’s not a very good example. It doesn’t erase itself, nor does it use the bottom line to avoid screen scrolling when the ball hits the bottom right position. It does show how I would use X and Y coordinates then an XM (X movement) and YM (Y movement) variable to increment or decrement them based on if they hit an edge.

The parameters of Dataram_57’s challenge were as follows:

  • Width: 90
  • Height: 80
  • Starting Position: 0,0
  • Time: ???

I wrote a quick graphical program do do this using my X/Y/XM/YM method:

0 REM dataram_57 twitter challenge
1 POKE 65495,0
10 W=89:H=79:X=0:Y=0:T=0:XM=1:YM=1
30 LINE(0,0)-(89,79),PSET,BF
40 PSET(X,Y,0)
70 T=T+1:IF T=7031 THEN END
80 GOTO 40

The first thing to notice is that I draw a filled box from 0,0 to 89,79 and then set black pixels in it. This lets me visually verify my line is going all the way to the edge of the 90×80 target area. Also, I am using the CoCo 1/2 double speed poke since this is time consuming. If you do this on a CoCo 3, feel free to use POKE 65497,0 instead.

Twitter user Dataram_57’s challenge running on a CoCo.

Eventually the area should be entirely black when every dot has been erased.

How long has this been going on?

I did some tests and figured out that it takes 7032 iterations (0-7031) for the dot to cycle through the entire 90×80 area before it has erased all the other dots.

With that in mind, I propose we turn this into both a logic and optimization challenge. On the CoCo, let’s see if we can use the PMODE 0 screen (128×96 resolution with 1 color). We can put this in a modified version of benchmark framework for 7032 cycles and see how fast we can do it. (By modified, I am removing the outer “try this three times and average the results” loop.)

My example, using IFs, looks like this:

0 REM dataram_57 twitter challenge 2
1 POKE 65495,0

15 W=89:H=79:X=0:Y=0:XM=1:YM=1
17 LINE(0,0)-(89,79),PSET,BF

20 FORA=0TO7030

30 PSET(X,Y,0)


Mine, running in Xroar, displays 9808 at the end. And it’s not the correct way to meet the requirements of the challenge, so … the real versions may be faster, or slower.

Your challenge, should you decide to accept it…

Our challenge is to:

  1. Rewrite this to work WITHOUT using any “IFs”.
  2. Try to make it as fast as possible while keeping the benchmark code (lines 5, 10, 20, 80 and 90) intact. You can add variables to the DIM, but otherwise leave those lines alone.

What says you?

Credit where credit is due…

And lastly, for those who want to cheat, here is the solution that Robin came up with using the VIC-20 Super Expander cartridge…

Is his the only way? The best way? The fastest way?

Let the regurgitated challenge begin!

Until next time…

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