# Tackling the Logiker 2022 Vintage Computing Christmas Challenge – part 7

See also: part 1, part 2, part 3, part 4, part 5, part 6 and part 7.

• 2022-12-30 – Update to Jason’s final version to make it two bytes smaller.

In this final (?) installment, I wanted to share some other approaches that were taken to by members of the CoCo community draw this:

…including one that immediately was smaller than the version I did.

## Rick Adams – PDP8/I

Early on, a version was shared by legendary CoCo programmer Rick Adams. His version was not for the CoCo – he chose to do it “in a very primitive BASIC, BASIC8 on a simulated PDP8/I running the TSS8 OS”…

``````0 'RICK ADAMS
12 FOR B = 1 TO 4
14 GOSUB 2000
20 NEXT B
22 C = 0
24 D = 0
30 FOR I = 1 TO 9
32 READ A, B
34 GOSUB 1000
36 NEXT I
50 FOR B = 4 TO 1 STEP -1
52 GOSUB 2000
58 NEXT B
200 DATA 0, 17, 1, 15, 2, 13, 3, 11, 4, 9, 3, 11, 2, 13, 1, 15, 0, 17
300 STOP
1000 PRINT TAB(A);
1010 FOR J = 1 TO B
1020 PRINT "*";
1030 NEXT J
1040 PRINT TAB(A + B + C);
1050 FOR J = 1 TO D
1060 PRINT "*";
1070 NEXT J
1080 PRINT
1090 RETURN
2000 A = 4
2002 D = B
2010 C = 9 - 2 * B
2020 GOSUB 1000
2030 RETURN
2046 END``````

I am unfamiliar with the BASIC on this machine, but at least it doesn’t require using “LET“. This version can run on the CoCo as well, and correctly reproduces the pattern.

## Jim Gerrie – MC-10/CoCo

Next, take a look a this one by MC-10 BASIC-meister, Jim Gerrie:

His approach uses DATA statements and then draws the star in an interesting way.

## Jason Pittman

In the comments on an earlier installment, Jason shared his attempt. His approach was realizing that the shape was just “four overlapping right triangles.”

``1 FORX=64TO416STEP32:L=X/32:T\$=STRING\$(L,42):PRINT@X-28,T\$;:PRINT@(X-19-L),T\$;:PRINT@544-X+4,T\$;:PRINT@557-X-L,T\$;:NEXT:GOTO1``

This version is just 100 bytes! Due to the CoCo’s 32 column screen being too short, it doesn’t draw the top and end lines of the pattern, so it wouldn’t meet the challenge requirements. To fix that, he needed to add an IF:

``````1 FORX=32TO416STEP32:L=X/32:T\$=STRING\$(L,42):PRINT@X-28,T\$;:PRINT@(X-19-L),T\$;:IF X>32THEN PRINT@544-X+4,T\$;:PRINT@557-X-L,T\$;
2 NEXT
3 GOTO3``````

Since the CoC 3 also has a 40×24 and 80×24 screen, the entire pattern could fit on those screens. Version three looked like this:

``1 WIDTH40:FORX=1TO13:L\$=STRING\$(X,42):LOCATE14-X,X:PRINTL\$;:LOCATE14-X,18-X:PRINTL\$;:LOCATE5+L,X:PRINTL\$;:LOCATE5,18-X:PRINTL\$;:NEXT:GOTO1``

That one is a mere 88 bytes! And, the GOTO1 at the end is just to make it keep redrawing, else it stops near the top and would print the “OK” in the middle of the pattern.

I’d say the “WIDTH40:” is not required, since you could just say “run this from the 40 column screen.” And, to keep the loop, starting on LINE 0 allows just saying “GOTO” with no line number:

``0 FORX=1TO13:L\$=STRING\$(X,42):LOCATE14-X,X:PRINTL\$;:LOCATE14-X,18-X:PRINTL\$;:LOCATE5+L,X:PRINTL\$;:LOCATE5,18-X:PRINTL\$;:NEXT:GOTO``

By my count, that turns in to 83 bytes! Amazing.

UPDATE: L. Curtis Boyle pointed out there was an unnecessary “+L” left in the code, which can be removed to make this 81 bytes. More amazing!

``0 FORX=1TO13:L\$=STRING\$(X,42):LOCATE14-X,X:PRINTL\$;:LOCATE14-X,18-X:PRINTL\$;:LOCATE5,X:PRINTL\$;:LOCATE5,18-X:PRINTL\$;:NEXT:GOTO``

Here is what it looks like, though I paused it to capture the full image:

Please read his comments to part 1 for more background and earlier versions he shared.

I’m really blown away by this.

Are we done? Is this as small as it gets?

Unless there are more ideas, I think that is the end.

Merry Christmas, everyone!

# Tackling the Logiker 2022 Vintage Computing Christmas Challenge – part 6

See also: part 1, part 2, part 3, part 4, part 5, part 6 and part 7.

Just when I thought I was out… they pull me back in.

Michael Corleon, Godfather III

Sometimes clever isn’t as good as brute force. In this installment, I’ll present a hybrid approach to the challenge of displaying the Logiker 2022 holiday image.

Instead of writing code to handle each section of the pattern, perhaps taking the simpler approach of just doing Run Length Encoding (sorta) might be smaller. I do not know where I first learned about RLE, but I implemented a simple version in Sub-Etha Software’s graphical “CoCoFEST Simulation” text adventure back in the early 1990s. The images in the graphical adventure were large, and I came up with a simple way to represent repeated data in them. (I do not know if someone gave me this idea, or if I worked it out on my own; in those pre-Internet days, knowledge was alot more organic.)

Basically, RLE looks for repeating sequences and replaces them with a count value and the sequence. The Wikipedia article gives this example:

Consider a screen containing plain black text on a solid white background. There will be many long runs of white pixels in the blank space, and many short runs of black pixels within the text. A hypothetical scan line, with B representing a black pixel and W representing white, might read as follows:

`WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW`

With a run-length encoding (RLE) data compression algorithm applied to the above hypothetical scan line, it can be rendered as follows:

`12W1B12W3B24W1B14W`

– https://en.wikipedia.org/wiki/Run-length_encoding

Above, if there are twelve “W” characters in a row, you would encode the value 12 followed by the “W”.

I wrote a simple program that will input a string and then scan it looking for repeating characters. When it finds a new character, it prints the count of the previous characters as well as what character that was.

``````0 ' RLE.BAS
10 LINE INPUT "STRING:";A\$
20 LC=ASC(LEFT\$(A\$,1)):L=1
30 FOR I=2 TO LEN(A\$)
40 C=ASC(MID\$(A\$,I,1))
50 IF C<>LC THEN PRINT L;LC:L=0:LC=C
60 L=L+1:NEXT
70 PRINT L;LC
``````

I pasted this code in to chat.openai.com and it correctly recognized it as an RLE routine! Here is how it described my program:

This looks like a BASIC program that encodes a string using run-length encoding (RLE). RLE is a simple form of data compression, where runs of repeated characters are replaced by a count and the character. For example, the string “AAABBCCC” would be encoded as “3A2B3C”.

The program starts by prompting the user to input a string, and then assigns the first character of the string to the variable `LC` (which stands for “last character”). The variable `L` is set to 1, and then a `FOR` loop is used to iterate through the remaining characters in the string.

For each character in the string, the program checks if it is different from the last character. If it is, it prints the length of the previous run and the character itself, and then resets the length and last character variables. If the character is the same as the last character, the length is incremented. This continues until the end of the string is reached, at which point the final run is printed.

For example, if the input string is “AAABBCCC”, the output would be:

3 65
2 66
3 67

This indicates that there were three instances of the character “A” (ASCII value 65), two instances of the character “B” (ASCII value 66), and three instances of the character “C” (ASCII value 67).

– chat.openai.com

Wow. Just wow.

Here is an example of the program running:

Since the Logiker Challenge image only contains spaces and asterisks, we don’t need a full RLE to encode it. I chose to use alternating numbers to indicate how many spaces, then how many asterisks.

Ultimately, I tried to save code space by breaking the image in to sections, and using two main routines to print that section within FOR/NEXT loops.

But … is that overkill? Instead of using multiple PRINT routines, what if I only needed one? By expanding the image data so each line covers the entire width of the CoCo’s 32-column screen, I could do away with the “end of line” markers in the data, and replace them with a larger series of spaces that goes from the end of the data on that line to the start of the data on the next line:

```[12345678901234567890123456789012]
"           *       *            "
"           **     **            "
"           ***   ***            "
"           **** ****            "
"       *****************        "
"        ***************         "
"         *************          "
"          ***********           "
"           *********            "
"          ***********           "
"         *************          "
"        ***************         "
"       *****************        "
"           **** ****            "
"           ***   ***            "
"           **     **            "
"           *       *            "
[12345678901234567890123456789012]```

Above, at the end of the first line’s asterisks, there are 12 spaces to the end of that line. For the next line, there are 11 spaces to get to the start of the next asterisks. That means after printing the last asterisks in line 1 we can just print 23 spaces and be at the start of the next line.

Assuming we start with a SPACE then an ASTERISK then a SPACE and do on, the data for the first two lines would look like this:

```11 - print11 spaces
1 - print 1 asterisk
7 - print 7 spaces
1 - print 1 asterisk
23 - print 23 spaces (to move to the start of data in the second line)
2 - print 2 asterisks
5 - print 5 spaces
2 - print 2 asterisks
...and so on...```

I was going to convert all the PRINT lines of the original version I started with to DATA statements and write a program to count this for me, but that sounded complicated. I just counted, and came up with the following numbers:

```11 1 7 1 23 2 5 2 3 3 23 4 4 4 23 5 16 15 16 17 18 16 5 2 3 3 23 4 4 4 23 5 18 2 5 2 1 1 7 1
```

I could store those in a DATA statement:

``DATA 11,1,7,1,23,2,5,2,3,3,23,4,4,4,23,5,16,15,16,17,18,16,5,2,3,3,23,4,4,4,23,5,18,2,5,2,1,1,7,1``

But, that takes up alot of room. There is a comma between each number, so for 50 numbers we’d be adding 49 commas, basically doubling the size of the data. Also, two digit numbers like 10 take up two bytes. I thought about using HEX numbers (0-15 turns in to 0-F) but the data has some values that are larger than 15 (the highest value that fits in a single character of a HEX value).

HEX is BASE-16 (0-F to represent 0-15) and what I really need is at least BASE-23 (0-23, the larger number I need). Since there are 26 letters in the alphabet, I could use all of them and get BASE-26 leaving me room to spare!

If A=1, B=2 and so on, the above series of numbers could be turned in to:

```K A G A W B E B W C C C W D A D S Q P O R M T K V I V K T M R O P Q S D A D W C C C W B E B W A G A
```

I could then turn those in to DATA:

``DATA K,A,G,A,W,B,E,B,W,C,C,C,W,D,A,D,S,Q,P,O,R,M,T,K,V,I,V,K,T,M,R,O,P,Q,S,D,A,D,W,C,C,C,W,B,E,B,W,A,G,A``

…and read them as a string (READ A\$) and then convert that string to a number by subtracting 63 (ASCII for A is 64, so if I read an A and get 64, subtracting 63 turns that in to 1):

``````READ A\$
V=ASC(A\$)-64``````

While this saves a byte for every number that was two digits, the extra code to convert from ASCII to a number may be larger than what we saved.

Since we have 49 commas, we could get rid of those and add code to parse a long string. As long as that code is smaller than 49 bytes, we come out ahead.

``DATA KAGAWBEBWCCCWDADSQPORMTKVIVKTMROPQSDADWCCCWBEBWAGA``

Now I could read that as a string and parse it in to numbers:

``````0 'STRTONUM.BAS
20 FOR I=1 TO LEN(A\$)
30 PRINT ASC(MID\$(A\$,I,1))-64;
40 NEXT
``````

And, if I want to use that series of numbers in a loop that prints alternating strings of spaces and asterisks, I don’t even need to bother with it being in a DATA statement. I could just embed it directly in the MID\$() command and hard code the lengthof the string, like this:

``````0 'STRTONUM2.BAS
20 FOR I=1 TO 50
40 NEXT
``````

And if I can do that, the only thing left is to figure out when to print a space and when to print an asterisks.

An easy way to do that is looking at the I variable in the FOR/NEXT loop. As it counts from 1 to 2 to 3 to 4, I can use AND to check bit 1. For odd numbers, that bit is set. For even numbers, it is not.

```0 = 0000000
1 = 0000001
2 = 0000010
3 = 0000011
4 = 0000100
5 = 0000101
...and so on...```

This means a simple check for “I AND 1” in an IF statement can help me decide which to print. Something like:

`IF (I AND 1) THEN PRINT space ELSE PRINT asterisk`

That gets me to something like this:

``````0 ' LOGIKER-ALPHA2.BAS
10 FORI=1TO50
30 IF I AND 1 THEN PRINT STRING\$(L,32); ELSE PRINT STRING\$(L,42);
40 NEXT
``````

Perhaps I can get rid of one of those PRINT STRING\$ commands… Since I know a space is ASCII 32 and an asterisk is ASCII 42, I could start with the 32 and add 10 if it’s the asterisk case. To do that, I need to see the result that comes back from AND:

```PRINT 1 AND 1
1

PRINT 2 AND 1
0```

So if the condition is TRUE (bit 1 is set, meaning the value is odd), I get a 1. If the condition is FALSE (bit 1 is clear, meaning the value is even), I get a 0.

Since I want to print spaces on the odd values, I need to use the 1 (odd) to mean 32, and the 0 (even) to mean 42. I’ll reverse my logic a bit and always start with 42 (asterisks) and multiply it by 10 times the result of (I AND 1). Something like this should work:

``````0 ' LOGIKER-ALPHA3.BAS
10 FOR I=1 TO 50
30 PRINT STRING\$(L,42-(I AND 1)*10);
40 NEXT
``````

And that gives me the pattern I want, with far less code. I can remove unneeded spaces and combine everything in to one line and see how big it is.

## Unneeded Spaces

A quick thing about unneeded spaces. There are spaces that BASIC itself doesn’t need, but the tokenizer that turns what you type in to the program DO need. For example:

`FOR I=100 TO 5000`

None of those spaces are needed, because BASIC knows where a keyword ends (FOR) and can tell the variable will be whatever is there before the “=”. The same is true for the numbers, since it can tell where a number ends and know to look for “TO”.

``FORI=100TO5000``

BUT, if you were using variables in that loop…

`FOR I=B TO E`

…and you took the spaces out:

`FORI=BTOE`

…how does BASIC know what your variable is? Is it “B”? Or “BT”? Or maybe “BTOE”? You will get an “?SN ERROR” if you try that because BASIC sees a non-number after the “=” and switches to parsing it as if it were a variable. To get around this, we have to put a space after it like this:

`FORI=B TOE`

That allows the tokenizer to work fine.

However If you were manually creating the BASIC program by packing bytes together in a file, you could omit that space and it will run just fine. Utilities such as Carl England’s CRUNCH do this trick to save a byte. BUT, if you were to CRUNCH the program then try to EDIT that line, you’d no longer have code that would run because updating the line requires it to be re-tokenized. #TheMoreYouKnow

## Why is that important?

I mention this because in my above program, I wanted to remove spaces from this line:

``30 PRINT STRING\$(L,42-(I AND 1)*10);``

I can remove all but one, since I need a space between “I” and “AND” for the same reason I just mentioned:

``30 PRINTSTRING\$(L,42-(I AND1)*10);``

But… instead of “I AND 1” I could change it to “1 AND I” and get the same result, but no longer need the space because BASIC can tell where a number stops:

``30 PRINTSTRING\$(L,42-(1ANDI)*10);``

And that, my friends, is how you save one more byte.

Would it be possible to also get rid of those parenthesis? Right now, I need to take my asterisk value (42) and subtract either 0 or 10. I need the results of “1 AND I” multiplied by 10, and if I removed the parens…

`42-1 AND I*10`

…BASIC would do the math first (42-1 and I*10) and if “I” was 3 at the time, I would get this:

```42-1 AND 3*10
41 AND 30```

…and that’s not at all what we want.

Can it be done? I moved things around but it really looks like the result of “1 AND I” has to be in parens. Can you figure a way to save those two bytes?

With that said, I present this version:

``````10 FOR I=1 TO 50
30 PRINT STRING\$(L,42-(1ANDI)*10);
40 NEXT
``````

…which can be packed in to this version:

``10 FORI=1TO50:L=ASC(MID\$("KAGAWBEBWCCCWDADSQPORMTKVIVKTMROPQSDADWCCCWBEBWAGA",I))-64:PRINTSTRING\$(L,42-(1ANDI)*10);:NEXT``

And that shows up as 114 bytes!

Oh, one thing I should also mention — during last year’s challenge, a comment was made about how ASC() works. If you give it a string, it returns the ASCII value of the first character. So ASC(“A”) returns 64, just like ASC(“ALLEN”) does. They said instead of using MID\$(A\$,I,1) to get one character, you can leave off that third parameterand MID\$ returns the rest of the string:

```A\$="HELLO"
PRINT MID\$(A\$,2,1)
C

PRINT MID\$(A\$,2)
ELLO```

If we were trying to print or use just one letter, we need that third parameter. But since I am passing it in to ASC, I could still give it the longer string and it would work fine:

```PRINT ASC("E")
69

PRINT ASC("ELLO")
69```

Thus, I can leave off that third parameter and save the two bytes that “,1” took up.

Neat!

Are we done? Can we save any more?

Until next time…

# Tackling the Logiker 2022 Vintage Computing Christmas Challenge – part 5

See also: part 1, part 2, part 3, part 4, part 5, part 6 and part 7.

The challenge continues. From humble beginnings of using PRINT, to fancier methods of encoding the image as a series of spaces and asterisks, we eventually ended up with an even fancier method that used only 1/4 of the image data to represent the entire symmetrical image.

That approach could work for any image that is symmetrical vertically and horizontally, and typically general purpose routines are not as small as custom routines that know what they will be doing.

## Knowing what we now know…

WIth that said, looking at this image, there is another shortcut that I missed:

The entire image is centered over one column… This means the amount of spaces on the left is unimportant — we just need to center the following lines:

```*       *
**     **
***   ***
**** ****
*****************
***************
*************
***********
*********
***********
*************
***************
*****************
**** ****
***   ***
**     **
*       *```

And, since we know it’s symmetrical both vertically and horizontally, we really only need to worry about this:

```*    | <- 1 asterisk, 4 spaces
**   | <- 2 asterisks, 3 spaces
***  | <- 3 asterisks, 2 space
**** | <- 4 asterisks, 1 space
*****************| 17
***************| 15
*************| 13
***********| 11
*********| 9```

I’m not sure what the pattern is as I type this, but I am expecting there is one. Here is a quick program that prints the rows of the shape using FOR/NEXT loops (uncentered):

``````0 ' LOGIKER13.BAS
10 FOR I=1 TO 4
20 PRINT STRING\$(I,"*");STRING\$(1+(4-I)*2," ");STRING\$(I,"*")
30 NEXT
40 FOR I=17 TO 9 STEP-2
50 PRINT STRING\$(I,"*")
60 NEXT
70 FOR I=11 TO 17 STEP 2
80 PRINT STRING\$(I,"*")
90 NEXT
100 FOR I=4 TO 1 STEP-1
110 PRINT STRING\$(I,"*");STRING\$(1+(4-I)*2," ");STRING\$(I,"*")
120 NEXT``````

If each of those lines were centered, we’d have our shape. Let’s try that by creating a string for the row, and then using the LEN() of that string to know how to center it using TAB().

``````0 ' LOGIKER14.BAS
10 FOR I=1 TO 4
20 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*")
25 PRINT TAB(16-LEN(A\$)/2);A\$
30 NEXT
40 FOR I=17 TO 9 STEP-2
50 A\$=STRING\$(I,"*")
55 PRINT TAB(16-LEN(A\$)/2);A\$
60 NEXT
70 FOR I=11 TO 17 STEP 2
80 A\$=STRING\$(I,"*")
85 PRINT TAB(16-LEN(A\$)/2);A\$
90 NEXT
100 FOR I=4 TO 1 STEP-1
110 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*")
115 PRINT TAB(16-LEN(A\$)/2);A\$
120 NEXT
130 GOTO 130``````

That produces our desired shape (though it does leave a blank line at the end, which our original version avoided by having a semi-colon on the PRINT and just breaking lines when we went to the next one).

The first thing I see it that the centering code on line 25, 55, 85 and 115 is the same. Subroutine!

``````0 ' LOGIKER15.BAS
10 FOR I=1 TO 4
20 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*")
25 GOSUB 150
30 NEXT
40 FOR I=17 TO 9 STEP-2
50 A\$=STRING\$(I,"*")
55 GOSUB 150
60 NEXT
70 FOR I=11 TO 17 STEP 2
80 A\$=STRING\$(I,"*")
85 GOSUB 150
90 NEXT
100 FOR I=4 TO 1 STEP-1
110 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*")
115 GOSUB 150
120 NEXT
130 GOTO 130
150 PRINT TAB(16-LEN(A\$)/2);A\$:RETURN``````

Next, we see that the string building code for the top and bottom are the same, so 20 and 110 are the same (it’s the value of I that changes how it prints), and then 50 and 80 are the same. Subroutines!

``````0 ' LOGIKER16.BAS
10 FOR I=1 TO 4
20 GOSUB 200
25 GOSUB 150
30 NEXT
40 FOR I=17 TO 9 STEP-2
50 GOSUB 250
55 GOSUB 150
60 NEXT
70 FOR I=11 TO 17 STEP 2
80 GOSUB 250
85 GOSUB 150
90 NEXT
100 FOR I=4 TO 1 STEP-1
110 GOSUB 200
115 GOSUB 150
120 NEXT
130 GOTO 130
150 PRINT TAB(16-LEN(A\$)/2);A\$:RETURN
200 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*"):RETURN
250 A\$=STRING\$(I,"*"):RETURN``````

Next, I notice the subroutines of 200 and 250 both have the centering PRINT called after them, so maybe we change it up a bit…

``````0 ' LOGIKER17.BAS
10 FOR I=1 TO 4
20 GOSUB 200
30 NEXT
40 FOR I=17 TO 9 STEP-2
50 GOSUB 250
60 NEXT
70 FOR I=11 TO 17 STEP 2
80 GOSUB 250
90 NEXT
100 FOR I=4 TO 1 STEP-1
110 GOSUB 200
120 NEXT
130 GOTO 130
200 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*"):GOTO 300
250 A\$=STRING\$(I,"*")
300 PRINT TAB(16-LEN(A\$)/2);A\$:RETURN``````

What else? The FOR/NEXT loops are basically all the same, except for the start and end value and the step value… Maybe we could come up with a way to have only one, and feed it those values using DATA statements?

```10 FOR I=1 TO 4
...
40 FOR I=17 TO 9 STEP-2
...
70 FOR I=11 TO 17 STEP 2
...
100 FOR I=4 TO 1 STEP-1

500 DATA 1,4,1
510 DATA 17,9,-2
520 DATA 11,17,2
530 DATA 4,1,-1```

If they all went to the same GOSUB routine this would be easy, but they don’t. The go 200, 250, 250, 200. We could add a fourth element in the DATA that tells it which routine to go to and “IF X=1 THEN GOSUB Y ELSE GOSUB Z” or something. That adds more code. Perhaps we don’t need the DATA since we know it alternates? Still, we’d have to track it ourselves with an IF or something. For now, let’s just try this:

``````0 ' LOGIKER18.BAS
10 FOR J=1 TO 4
30 FOR I=A TO B STEP C
40 IF D=0 THEN GOSUB 200 ELSE GOSUB 250
50 NEXT I
60 NEXT J
70 GOTO 70
200 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*"):GOTO 300
250 A\$=STRING\$(I,"*")
300 PRINT TAB(16-LEN(A\$)/2);A\$:RETURN
500 DATA 1,4,1,0
510 DATA 17,9,-2,1
520 DATA 11,17,2,1
530 DATA 4,1,-1,0``````

And that still produces your original shape. But is it any smaller?

In part 4, we had a version that (using my default XRoar emulator running DISK EXTENDED COLOR BASIC) showed 22499 bytes free after loading. This new version shows 22567 bytes free. So yes, it is smaller! And, we can pack those lines and make it even smaller than that. (And NEXT doesn’t near the variable — in fact, using “NEXT I” is slower than just saying “NEXT” so I’ll remove those here as well.)

``````0 ' LOGIKER19.BAS
10 FOR J=1 TO 4:READ A,B,C,D:FOR I=A TO B STEP C:IF D=0 THEN GOSUB 200 ELSE GOSUB 250
50 NEXT:NEXT
70 GOTO 70
200 A\$=STRING\$(I,"*")+STRING\$(1+(4-I)*2," ")+STRING\$(I,"*"):GOTO 300
250 A\$=STRING\$(I,"*")
300 PRINT TAB(16-LEN(A\$)/2);A\$:RETURN:DATA 1,4,1,0,17,9,-2,1,11,17,2,1,4,1,-1,0``````

That version shows me 22609 free, which is even smaller — and we could still make this a bit smaller by getting rid of unnecessary spaces in the code.

Side note: I am being lazy and just showing the BASIC “PRINT MEM” values rather than calculating the actual size of the program. On my configuration, 22823 is how much memory is there on startup. So, 22823-22609 shows that this program is 214 bytes. It uses more memory for the strings when running, but I don’t think that matters for this challenge.

What else can we do to save a few bytes? Well, STRING\$() takes two parameters. The first is the count of how many times to repeat the second parameter. The second parameter can be a quoted character like “*”, or a number like 42 (the ASCII value of the asterisk). 42 is one by smaller than “*” so we can do that as well as use 32 (the ASCII value for space) instead of ” “:

``````200 A\$=STRING\$(I,42)+STRING\$(1+(4-I)*2,32)+STRING\$(I,42):GOTO 300
250 A\$=STRING\$(I,42)``````

Another thing we know is that in the shape there are always the same number of spaces before the top and bottom sections, so we really don’t need to center it. We could just hard code a PRINT TAB for that instead of building a string and calling a center subroutine:

``````200 PRINT TAB(11);STRING\$(I,42);STRING\$(1+(4-I)*2,32);STRING\$(I,42)
210 RETURN``````

The middle section is similar. Since we know the length, we could calculate how many spaces to tab using that number:

``````250 PRINT TAB(16-I/2);STRING\$(I,42)
260 RETURN``````

And that removes a subroutine, leaving us with this (not line packed yet):

``````0 ' LOGIKER20.BAS
10 FOR J=1 TO 4
30 FOR I=A TO B STEP C
40 IF D=0 THEN GOSUB 200 ELSE GOSUB 250
50 NEXT
60 NEXT
70 GOTO 70
200 PRINT TAB(11);STRING\$(I,42);STRING\$(1+(4-I)*2,32);STRING\$(I,42)
210 RETURN
250 PRINT TAB(16-I/2);STRING\$(I,42)
260 RETURN
500 DATA 1,4,1,0
510 DATA 17,9,-2,1
520 DATA 11,17,2,1
530 DATA 4,1,-1,0``````

Two FOR/NEXT loops, a READ, an IF, and two PRINT subroutines.

Maybe we don’t need those subroutines, now that we have an “IF” in line 40 that decides what to do?

``````0 ' LOGIKER21.BAS
10 FOR J=1 TO 4
30 FOR I=A TO B STEP C
40 IF D=0 THEN PRINT TAB(11);STRING\$(I,42);STRING\$(1+(4-I)*2,32);STRING\$(I,42) ELSE PRINT TAB(16-I/2);STRING\$(I,42)
50 NEXT
60 NEXT
70 GOTO 70
500 DATA 1,4,1,0
510 DATA 17,9,-2,1
520 DATA 11,17,2,1
530 DATA 4,1,-1,0``````

That’s an ugly line 40, but it got rid of two GOSUBs and two RETURNS. Plus every line takes up 5 bytes, so by removing two of those (each subroutine must start on its own line) we saved even more.

Not only is this smaller, it’s faster, too, since it doesn’t have to spend time seeking through the program looking for a subroutine line number for each line it prints.

If we pack this version, removing unnecessary spaces and semicolons and such, and remove the “GOTO” loop (the challenge does not require the screen to be cleared, and says it can return to a prompt after running), we get:

``````0 ' LOGIKER22.BAS (don't include this line)

1 FORJ=1TO4:READA,B,C,D:FORI=A TOB STEPC:IFD=0THENPRINTTAB(11)STRING\$(I,42)STRING\$(1+(4-I)*2,32)STRING\$(I,42)ELSEPRINTTAB(16-I/2)STRING\$(I,42)
2 NEXT:NEXT:DATA1,4,1,0,17,9,-2,1,11,17,2,1,4,1,-1,0``````

151 bytes of BASIC code!

Is there more that can be done? Comment with your ideas! I think I’m out of them, here.

Until next time…

# Tackling the Logiker 2022 Vintage Computing Christmas Challenge – part 4

See also: part 1, part 2, part 3, part 4, part 5, part 6 and part 7.

The story so far…

In the beginning Logiker offered up a programming challenge to produce this simple image:

From its humble beginnings as a bunch of PRINT statements, to a DATA driven output routine, we have now ended up with code that can print the top half of the symmetrical image by only having data to describe one quarter of the image.

When we last left off, I had code that would read this data, print the left half, then print the right half, mirroring the same data. It worked well, but only printed the top half of the image.

I showed that code to OpenAI and asked how it would accomplish the task of reversing the data to print the bottom portion:

To make the program print the rows in reverse order after it has finished printing all of the rows, you will need to make some changes to the code. One way to do this would be to add another loop after the main loop that reads the data from the DATA statements and prints the rows in reverse order. You can use the “REVERSE” statement in BASIC to reverse the order of the rows in the DATA statements, and then use the same code as in the main loop to read the data and print the rows. This will cause the program to print the rows in the original order, and then again in reverse order.

– chat.openai.com

I’ve never heard of the REVERSE keyword, and the CoCo’s BASIC doesn’t have it. But if I did have it, this task would be pretty easy. I informed the A.I. that I had no such command, and it said:

If your BASIC does not have a REVERSE command, you can still print the rows in reverse order by using a different approach. One way to do this would be to store the data from the DATA statements in an array, and then loop through the array in reverse order to print the rows. In BASIC, you can use the “DIM” statement to create an array, and the “READ” statement to read data from the DATA statements into the array. You can then use a loop to iterate through the array in reverse order, using the same code as in the main loop to print the rows. This will cause the program to print the rows in the original order, and then again in reverse order.

– chat.openai.com

That’s the approach I was thinking of taking, so… okay!

Side Note: I did ask the A.I. to modify my program to do this, and it gave me something that would have been a good starting point for the rest of this article. I could have then discussed things with the A.I. a bit more and encouraged it to evolve the program until it met the requirements. (One issue with BASIC is that the A.I. inevitably tries to do things from other dialects of BASIC that Color BASIC does not have, such as the REVERSE command.) I hope to demonstrate one of these co-programming sessions in a future article series, provided access to ChatGPT remains available to me.

I changed my routine so it loaded all the DATA values in to a dimension. Since I knew how many entries there would be (27), I could eliminate the need for having a -1 to mark the end of data.

Then, in the PRINT routine, instead READing A, B and C and looking at them, I’d use the D() dimension variable as D(x), D(x+1) and D(x+2) to get to the same three values.

At the end of each line (three numbers, or two numbers followed by a zero), I would increment my index by three so it starts at the next set of numbers in the array.

I used a new variable, M, to indicate the mode (forward or reverse). When it reaches the end of the data (D goes past the 27 entries), it sets that variable to 1, and other code now runs decrementing the index by 3 each time until it reaches the top (0).

Here is my new code:

``````0 ' LOGIKER11.BAS
10 CLS:DIM D(26)
11 FOR I=0 TO 26:READ D(I):NEXT:D=0:M=0
15 PRINTTAB(6);
20 PRINT STRING\$(D(D),32);
25 IF D(D+2)=0 THEN PRINT STRING\$(D(D+1)*2-1,42);STRING\$(D(D),32);:GOTO 40
30 PRINT STRING\$(D(D+1),42);STRING\$(D(D+2)*2-1,32);STRING\$(D(D+1),42);
40 IF M=0 THEN D=D+3:IF D<27 THEN PRINT:GOTO 15 ELSE M=1
41 IF D>0 THEN PRINT:D=D-3:GOTO 15
50 DATA 5,1,4
60 DATA 5,2,3
70 DATA 5,3,2
80 DATA 5,4,1
90 DATA 1,9,0
100 DATA 2,8,0
110 DATA 3,7,0
120 DATA 4,6,0
130 DATA 5,5,0
220 GOTO 220``````

OpenAI describes it as follows:

This program is similar to the previous ones, but it uses a slightly different approach to read the data from the DATA statements and print the rows. The program starts by defining an array `D` to store the data from the DATA statements, and then reads the data into the array. The program then enters a loop that prints the rows of the pattern. In each iteration of the loop, the program prints a row of the pattern by printing a string of spaces, followed by a string of asterisks, and then another string of spaces. The number of spaces and asterisks in each string is determined by the data in the array.

After the program has printed all of the rows in the original order, it enters another loop that prints the rows in reverse order. In each iteration of the loop, the program prints the same row as in the previous loop, but in reverse order. This causes the program to print the rows of the pattern in the original order, and then again in reverse order. However, without more information about the data in the DATA statements, it is not possible to say exactly what the output of the program would look like.

– chat.openai.com

Nicely done, A.I.

When I run this program, I get the desired output!

It is, of course, slower than the original brute-force PRINT version. But, we are going for size. And speaking of size, lets try to pack this down as much as possible and see where we stand:

``````0 ' LOGIKER12.BAS
10 CLS:DIM D(26):FOR I=0 TO 26:READ D(I):NEXT:D=0:M=0
15 PRINTTAB(6);STRING\$(D(D),32);:IF D(D+2)=0 THEN PRINT STRING\$(D(D+1)*2-1,42);STRING\$(D(D),32);:GOTO 40
30 PRINT STRING\$(D(D+1),42);STRING\$(D(D+2)*2-1,32);STRING\$(D(D+1),42);
40 IF M=0 THEN D=D+3:IF D<27 THEN PRINT:GOTO 15 ELSE M=1
41 IF D>0 THEN PRINT:D=D-3:GOTO 15
50 GOTO 50:DATA 5,1,4,5,2,3,5,3,2,5,4,1,1,9,0,2,8,0,3,7,0,4,6,0,5,5,0``````

Loading the first version gives me 22431 bytes free. Loading the second gives me 22499 – about 68 bytes smaller! Compare that to the original brute-force PRINT version (which was 22309 free), we have saved 190 bytes so far.

And, we could remove spaces, get rid of the REM comment at the start, and save even more.

But is that enough for the challenge? One thing the challenge says it you are not required to clear the screen, and you can return to an OK/Ready prompt. That means I could remove the CLS and the GOTO loop at the end, saving even more.

But saving even more is not enough. I think there’s a few more things we can do, especially now that I understand what it takes to draw this shape.

Until next time, take a look at what I have done in LOGIKER11.BAS and see what suggestions you can come up with.

# Tackling the Logiker 2022 Vintage Computing Christmas Challenge – part 3

See also: part 1, part 2, part 3, part 4, part 5, part 6 and part 7.

So far, we’ve taken a brute force PRINT program and turned it in to a less-brute force program that did the same thing using DATA statements:

``````0 ' LOGIKER8.BAS
10 CLS
15 CH=32:PRINTTAB(6);
20 READ A:IF A=-1 THEN 220
25 IF A=0 THEN PRINT:GOTO 15
30 PRINT STRING\$(A,CH);
35 IF CH=32 THEN CH=42 ELSE CH=32
40 GOTO 20
50 DATA 5,1,7,1,0
60 DATA 5,2,5,2,0
70 DATA 5,3,3,3,0
80 DATA 5,4,1,4,0
90 DATA 1,17,0
100 DATA 2,15,0
110 DATA 3,13,0
120 DATA 4,11,0
130 DATA 5,9,0
140 DATA 4,11,0
150 DATA 3,13,0
160 DATA 2,15,0
170 DATA 1,17,0
180 DATA 5,4,1,4,0
190 DATA 5,3,3,3,0
200 DATA 5,2,5,2,0
210 DATA 5,1,7,1,0
215 DATA -1
220 GOTO 220``````

All of this in an effort to try to print out this image:

While there are still many BASIC optimizations we could do (removing spaces, combining lines even further, renumbering by 1, etc.), those would apply to any version of the code we create. Instead of doing that, let’s look at some other ways we can represent this data.

## Simpsons Atari 2600 did it first.

Let no meme go to waste, I always say.

When the Atari VCS came out in 1977 (you younguns may only know it as the 2600, but it didn’t get that name until 1982 — five years after its release later), it required clever tricks to make games run in only 1K or 2K of ROM and with just 128 bytes of RAM.

The game Adventure was quite the challenge, since it had multiple screens representing different mazes, castles and areas.

Each screen was represented by only 21 bytes of ROM! If you follow that link, you can read more about my efforts to understand how this worked. Here is an example of how the castle room was represented:

```;Castle Definition
CastleDef:
.byte \$F0,\$FE,\$15 ;XXXXXXXXXXX X X X      R R R RRRRRRRRRRR
.byte \$30,\$03,\$1F ;XX        XXXXXXX      RRRRRRR        RR
.byte \$30,\$03,\$FF ;XX        XXXXXXXXXXRRRRRRRRRR        RR
.byte \$30,\$00,\$FF ;XX          XXXXXXXXRRRRRRRR          RR
.byte \$30,\$00,\$3F ;XX          XXXXXX    RRRRRR          RR
.byte \$30,\$00,\$00 ;XX                                    RR
.byte \$F0,\$FF,\$0F ;XXXXXXXXXXXXXX            RRRRRRRRRRRRRR   ```

There are three bytes to represent each line. Three bytes would only be able to represent 24 pixels (8 bits per byte), and the ASCII art shows the screen width is actually 40. Those three bytes cannot represent the entire row of pixels.

In fact, 4-bits of that isn’t used. Each set of three bytes represents halfa row (20 bits out of the 24 the three bytes represent). Look at the first entry:

`  .byte \$F0,\$FE,\$15 ;XXXXXXXXXXX X X X      R R R RRRRRRRRRRR   `

If you turn those bytes into binary, you get this pattern:

``` byte 1  byte 2  byte 3|
--------========--------
111100001111111000010101```

The Atari drew the first 8-bits from least significant bit to most. the second 8-bits from most significant to least, then the third from least significant to most. That makes it look like this, matching the ASCII art (skipping the unused 4-bits):

```000011111111111010101000
XXXXXXXXXXX X X X   ```

To represent a full screen, the Atari had a trick that would mirror or duplicate the other half of the screen. In the case of the castle, the right side was a mirror image. In the case of certain mazes, the data was duplicated.

Looking at our image here, since it is symmetrical, we could certainly use the same trick and only store half of the image.

```+----------------+
|           *    |
|           **   |
|           ***  |
|           **** |
|       *********|
|        ********|
|         *******|
|          ******|
|           *****|
|          ******|
|         *******|
|        ********|
|       *********|
|           **** |
|           ***  |
|           **   |
|           *    |
+----------------+```

Also, since the top and bottom are also mirror images, we could mirror those, too, and get away with only storing 1/4 of the image:

```+----------------+
|           *    |
|           **   |
|           ***  |
|           **** |
|       *********|
|        ********|
|         *******|
|          ******|
|           *****|
+----------------+```

Since the image is 17×17 (an odd number, so there is a halfway row and column), we’d actually need to just draw to that halfway row/column, then reverse back through the data.

We should be able to take our existing data and crop it down from this, which represents the full image:

```50 DATA 5,1,7,1,0
60 DATA 5,2,5,2,0
70 DATA 5,3,3,3,0
80 DATA 5,4,1,4,0
90 DATA 1,17,0
100 DATA 2,15,0
110 DATA 3,13,0
120 DATA 4,11,0
130 DATA 5,9,0
140 DATA 4,11,0
150 DATA 3,13,0
160 DATA 2,15,0
170 DATA 1,17,0
180 DATA 5,4,1,4,0
190 DATA 5,3,3,3,0
200 DATA 5,2,5,2,0
210 DATA 5,1,7,1,0
```

…to this, which represents the top left quarter-ish of the image:

```50 DATA 5,1,4,0   '     X    '
60 DATA 5,2,3,0   '     XX   '
70 DATA 5,3,2,0   '     XXX  '
80 DATA 5,4,1,0   '     XXXX '
90 DATA 1,9,0     ' XXXXXXXXX'
100 DATA 2,8,0    '  XXXXXXXX'
110 DATA 3,7,0    '   XXXXXXX'
120 DATA 4,6,0    '    XXXXXX'
130 DATA 5,5,0    '     XXXXX'```

That represents all the data up to the center row/column, and that seems to be a considerable savings in code space (removing eight lines).

But how do we draw that forward, then in reverse? There is no way to back up when using the READ command, so we’d have to remember what we just did. For a general purpose “compress 1-bit image” routine it would be more complex, but since we know the image we are going to produce, we can make an assumption:

1. The image never has more than three transitions (space, asterisk, space) in a line.
2. No line entry has more than 4 numbers total.

Knowing that, we could simply save up to three numbers in variables, so we would print them out A B C and then C B A. We won’t even need the zeros now, since we can read A,B,C and act on them (stopping if C is 0).

Neat!

A quick bit of trial and error gave me this code that will print the top half of the image:

``````0 ' LOGIKER10.BAS
10 CLS
15 CH=32:PRINTTAB(6);
20 READ A:IF A=-1 THEN 220
21 PRINT STRING\$(A,32);
25 IF C=0 THEN PRINT STRING\$(B*2-1,42);STRING\$(A,32):GOTO 15
30 PRINT STRING\$(B,42);STRING\$(C*2-1,32);STRING\$(B,42)
40 GOTO 15
50 DATA 5,1,4
60 DATA 5,2,3
70 DATA 5,3,2
80 DATA 5,4,1
90 DATA 1,9,0
100 DATA 2,8,0
110 DATA 3,7,0
120 DATA 4,6,0
130 DATA 5,5,0
215 DATA -1
220 GOTO 220``````

It creates this:

I can now say “we are halfway there!”

But now I have another issue to solve. How do I back up? There is no way to READ data in reverse. It looks like I’m going to need to load all those numbers in to memory so I can reverse back through them.

To be continued…

# Tackling the Logiker 2022 Vintage Computing Christmas Challenge – part 2

See also: part 1, part 2, part 3, part 4, part 5, part 6 and part 7.

The design is one row taller than will fit on the CoCo’s 32×16 text screen, but it would easily fit on the 40 or 80 column screen of the CoCo 3. For this article, I am going to stick with the standard text screen and just let it scroll one row off the top of the screen. When I have something figured out, it might only require modifying the centering code to display on the 40/80 column screen.

## Let there be code!

At this stage, the design is being centered using the TAB command. Putting a “TAB(7)” at the start of each string takes up 3 bytes of programming space. It seems “TAB(” is tokenized, then there is the 3 character, followed by the “)” character. I had thought using PRINT@ might save some space, but the “@” takes a byte, then the screen position numbers follow it, and a comma is required. “PRINT@7,” takes the same amount of code space as “PRINTTAB(7)” so no savings there.

The biggest savings is going to come from eliminating the repeated use of the “* characters in the strings. Since the entire image is made up of spaces or asterisks, it could be represented by data that says how many spaces then how many asterisks then how many spaces, etc.

Here is what the image looks like centered to 32-columns:

```+--------------------------------+
|           *       *            |
|           **     **            |
|           ***   ***            |
|           **** ****            |
|       *****************        |
|        ***************         |
|         *************          |
|          ***********           |
|           *********            |
|          ***********           |
|         *************          |
|        ***************         |
|       *****************        |
|           **** ****            |
|           ***   ***            |
|           **     **            |
|           *       *            |
+--------------------------------+```

The first line has 11 spaces, 1 asterisks, 7 spaces, then 1 asterisk.

The second line has 11 spaces, 1 asterisks, 5 spaces, then 2 asterisks.

And so on… The first line could be represented in DATA to tell how many repeating groups of spaces and asterisks there are such as 11,1,7,1,0 (using 0, to indicate the end of the line).

For something this small, I could have just counted up the spaces and asterisks myself, but it’s more fun to spend ten minutes writing a BASIC program to do what would have taken me 60 seconds to do manually.

I converted the pattern in to DATA statements and created a program that would scan each line looking for repeated characters. When the character changes, it prints how many of the previous repeating characters it saw and resets the counter.

``````0 ' LOGIKER5.BAS
1 ' LC\$=LAST CHAR
2 ' DC=DUPLICATE CHAR COUNT
10 READ A\$:IF A\$="END" THEN END
11 LC\$=LEFT\$(A\$,1):DC=1
12 FOR I=2 TO LEN(A\$)
13 IF MID\$(A\$,I,1)=LC\$ THEN DC=DC+1 ELSE PRINT DC",";:DC=1:LC\$=MID\$(A\$,I,1)
14 NEXT
15 PRINT DC", 0"
16 LC\$=""
17 GOTO 10
50 DATA"    *       *"
60 DATA"    **     **"
70 DATA"    ***   ***"
80 DATA"    **** ****"
90 DATA"*****************"
100 DATA" ***************"
110 DATA"  *************"
120 DATA"   ***********"
130 DATA"    *********"
140 DATA"   ***********"
150 DATA"  *************"
160 DATA" ***************"
170 DATA"*****************"
180 DATA"    **** ****"
190 DATA"    ***   ***"
200 DATA"    **     **"
210 DATA"    *       *"
220 DATA "END"``````

Rather than explain how the program works, like I often try to do, I just submitted this code to the OpenAI ChatGPT system and let it do the work for me:

This BASIC program reads in a series of strings, one at a time, and outputs the number of times each character appears in the string.

Here is a brief explanation of the code:

• Lines 10-16: This is the main loop of the program, where each string is read in, processed, and output.
• Line 11: This line initializes the `LC\$` variable to the first character of the string and the `DC` variable to `1`.
• Lines 12-14: This is a `FOR` loop that iterates through each character of the string.
• Line 13: This line compares the current character with the last character (stored in `LC\$`). If they are the same, it increments the `DC` variable. If they are different, it outputs the value of `DC` and then resets `DC` to `1` and `LC\$` to the current character.
• Line 15: This line outputs the value of `DC` after the loop has finished processing the string.
• Lines 50-220: These are the data lines that contain the strings to be processed.
• Line 220: This is the sentinel value that indicates the end of the data.

Note that this code uses a number of built-in BASIC functions, such as `LEFT\$`, `MID\$`, and `LEN`, which may not be available in all versions of BASIC. You may need to modify the code to use equivalent functionality if it is not available in the version of BASIC you are using.

– chat.openai.com

Seriously, folks. You have to go play with ChatGPT. It’s amazingly fun.

## OpenAI ChatGPT Digression

After showing this program to ChatGPT, I asked “is there a way to do it better?” It responded with:

There are a few ways that this routine could be improved. Here are a few suggestions:

1. Use a `DO` loop instead of a `GOTO` statement to control the main loop. This would make the code easier to read and maintain.
2. Use a `SELECT CASE` statement to handle the different cases in line 13 instead of an `IF` statement. This would make the code more concise and easier to read.
3. Use a `DICTIONARY` or `MAP` data structure to store the counts of each character instead of using separate variables for each character. This would make the code more efficient and allow it to handle an arbitrary number of characters without having to hard-code variables for each one.
4. Add error-checking to the code to handle cases where the input data is invalid (e.g. empty strings, strings with invalid characters, etc.). This would make the code more robust and prevent it from crashing or producing incorrect results in these cases.

Here is an example of how the code could be modified to incorporate some of these changes:

– chat.openai.com

The code it provided was not something that would work on ancient Microsoft BASIC, but it was interesting to see:

``````DO
IF A\$="END" THEN EXIT DO

' Initialize the character counts to 0
CHAR_COUNTS = DICTIONARY()

' Loop through each character of the string
FOR i = 1 TO LEN(A\$)
ch = MID\$(A\$, i, 1)
IF ch = " " THEN CONTINUE ' Skip whitespace

' Increment the count for this character
IF CHAR_COUNTS.EXISTS(ch) THEN
CHAR_COUNTS[ch] = CHAR_COUNTS[ch] + 1
ELSE
CHAR_COUNTS[ch] = 1
END IF
NEXT

' Output the counts for each character
FOR ch = "A" TO "Z"
PRINT CHAR_COUNTS[ch]; ",";
NEXT
PRINT "0"
LOOP

50 DATA"    *       *"
60 DATA"    **     **"
70 DATA"    ***   ***"
80 DATA"    **** ****"
90 DATA"*****************"
100 DATA" ***************"
110 DATA"  *************"
120 DATA"   ***********"
130 DATA"    *********"
140 DATA"   ***********"
150 DATA"  *************"
160 DATA" ***************"
170 DATA"*****************"
180 DATA"    **** ****"
190 DATA"    ***   ***"
200 DATA"    **     **"
210 DATA"    *       *"
220 DATA "END"``````

I have no idea what version of BASIC that code would work on, and the program it wrote doesn’t really look like it’s doing the same thing, but still neat. In a future article (provided they continue to give us access to this AI), I plan to share some of the interesting things it can do with BASIC programs (including writing them from scratch).

But I digress…

## Driving Miss Data

When I run the program, it starts printing out rows of numbers. Since there are seventeen rows of the graphic, it prints out seventeen lines of numbers. I needed to pause the program mid-printing so I can write down the numbers before they scroll off. (A fancier way would have been to have the program write out an ASCII BASIC program with the line numbers and DATA statements already in it which I could load back later. I think I showed how I do that in an earlier article somewhere on this site.)

Now that we have numbers for our DATA statements, a new version of the program could be written to generate the image using them. I will make use of the STRING\$ that creates a string of a specific character of a specific length…

``````0 ' LOGIKER6.BAS
10 CLS
15 CH=32:PRINTTAB(7);
20 READ A:IF A=-1 THEN 220
25 IF A=0 THEN PRINT:GOTO 15
30 PRINT STRING\$(A,CH);
35 IF CH=32 THEN CH=42 ELSE CH=32
40 GOTO 20
50 DATA 4,1,7,1,0
60 DATA 4,2,5,2,0
70 DATA 4,3,3,3,0
80 DATA 4,4,1,4,0
90 DATA 17,0
100 DATA 1,15,0
110 DATA 2,13,0
120 DATA 3,11,0
130 DATA 4,9,0
140 DATA 3,11,0
150 DATA 2,13,0
160 DATA 1,15,0
170 DATA 17,0
180 DATA 4,4,1,4,0
190 DATA 4,3,3,3,0
200 DATA 4,2,5,2,0
210 DATA 4,1,7,1,0
215 DATA -1
220 GOTO 220
``````

Obviously those data statements could be combined in to fewer lines, but for this version I wanted them to match the same line number the original PRINT was on. You can easily compare the results:

```50 DATA"    *       *"
50 DATA 4,1,7,1,0

60 DATA"    **     **"
60 DATA 4,2,5,2,0

70 DATA"    ***   ***"
70 DATA 4,3,3,3,0

80 DATA"    **** ****"
80 DATA 4,4,1,4,0
```

Before I show you the results, can you see the flaw in my program?

I’ll give you a hint… Line 170.

## Close but no cigar

My program assumes each line starts with a space, so the first value will be printed as spaces, then the next value as asterisks, and so on. This causes a problem when it gets to the row that is entirely the asterisk it reads the first number and prints it as spaces, giving me this incorrect result:

I can think of several ways to solve this:

1. Use a different value other than 0 for “end-of-line” and make 0 mean “nothing to print, just switch to the astrisk”. That would change line 170 to be “DATA 0,17,X” (where “X” is the new end-of-line marker. This would probably require a new bit of IF logic to handle.
2. Make each group of data two bytes that specifies the character to print, and how many. Printing 17 asterisks would be “17,42”. Printing four spaces would be “4,32”. This would make the program logic simpler, but would double the size of the data. Depending on how much smaller the logic is, this might be a winner. (And I can think of optimizations to that as well, such as using 0 and 1 for the data to print and just printing “32+X*10” so it prints either 32 (if the value is 0) or 42 (if the value is 1). This is normally how I would have started, but I was trying to make the data as small as possible.
3. I could just encode the leading spaces at the start of each line rather than using TAB(7). By doing this, every line would start with a space. This would work for this specific challenge, but not be flexible for patterns that don’t start with a space.

For now, let’s make a quick change and try #3 by simply adding 7 to the first number in each DATA statement, and adding a 7 to line 170 which is the row that doesn’t have a space at the start. I think it would look like this:

``````0 ' LOGIKER7.BAS
10 CLS
15 CH=32
20 READ A:IF A=-1 THEN 220
25 IF A=0 THEN PRINT:GOTO 15
30 PRINT STRING\$(A,CH);
35 IF CH=32 THEN CH=42 ELSE CH=32
40 GOTO 20
50 DATA 11,1,7,1,0
60 DATA 11,2,5,2,0
70 DATA 11,3,3,3,0
80 DATA 11,4,1,4,0
90 DATA 7,17,0
100 DATA 8,15,0
110 DATA 9,13,0
120 DATA 10,11,0
130 DATA 11,9,0
140 DATA 10,11,0
150 DATA 9,13,0
160 DATA 8,15,0
170 DATA 7,17,0
180 DATA 11,4,1,4,0
190 DATA 11,3,3,3,0
200 DATA 11,2,5,2,0
210 DATA 11,1,7,1,0
215 DATA -1
220 GOTO 220``````

Running this program produces the desired results! But, it has a drawback:

The data size grew. Not only did we add “7,” (two bytes) to line 170, but eleven other lines went from a 1-digit value to a 2-digit value. This means our data grew by 13 bytes. If we saved 13 bytes in the decoding routine, this is a win. If we did not, it is not an acceptable fix.

When I load the previous version of the program in to the XRoar emulator and PRINT MEM, it shows 22425 free. When I do the same with this version, I get 22416 — less memory free, so a larger program. This is bad, but the previous version is still missing the code to handle that line 170.

Perhaps, instead of adding 7 to each line to center on the screen, each line could just add 1 (so it doesn’t create two-digit values) and we can use TAB(6). That would look like this:

``````0 ' LOGIKER8.BAS
10 CLS
15 CH=32:PRINTTAB(6);
20 READ A:IF A=-1 THEN 220
25 IF A=0 THEN PRINT:GOTO 15
30 PRINT STRING\$(A,CH);
35 IF CH=32 THEN CH=42 ELSE CH=32
40 GOTO 20
50 DATA 5,1,7,1,0
60 DATA 5,2,5,2,0
70 DATA 5,3,3,3,0
80 DATA 5,4,1,4,0
90 DATA 1,17,0
100 DATA 2,15,0
110 DATA 3,13,0
120 DATA 4,11,0
130 DATA 5,9,0
140 DATA 4,11,0
150 DATA 3,13,0
160 DATA 2,15,0
170 DATA 1,17,0
180 DATA 5,4,1,4,0
190 DATA 5,3,3,3,0
200 DATA 5,2,5,2,0
210 DATA 5,1,7,1,0
215 DATA -1
220 GOTO 220``````

Doing a PRINT MEM on that one shows 22421, so it is four bytes larger than the original, and still smaller than the “add 7” version. Perhaps that is good enough for now?

Combining all the lines to make a smaller program would look like this:

``````0 ' LOGIKER9.BAS
10 CLS
15 CH=32:PRINTTAB(6);
20 READ A:IF A=-1 THEN 220 ELSE IF A=0 THEN PRINT:GOTO 15
30 PRINT STRING\$(A,CH);:IF CH=32 THEN CH=42 ELSE CH=32
40 GOTO 20
50 DATA 5,1,7,1,0,5,2,5,2,0,5,3,3,3,0,5,4,1,4,0,1,17,0,2,15,0,3,13,0,4,11,0,5,9,0,4,11,0,3,13,0,2,15,0,1,17,0,5,4,1,4,0,5,3,3,3,0,5,2,5,2,0,5,1,7,1,0,-1
220 GOTO 220``````

Better! But we can make it more better.

In the next installment, we will do something that I learned from studying the Atari 2600’s Adventure program…

To be continued…

# Tackling the Logiker 2022 Vintage Computing Christmas Challenge – part 1

See also: part 1, part 2, part 3, part 4, part 5, part 6 and part 7.

Here we go again! Over in the Facebook Color Computer group, David M. shared a link to this year’s Vintage Computing Christmas Challenge from Logiker. Although I did not submit an entry, I did play with last year’s challenge on my TRS-80 Color Computer.

This year, the challenge is a bit more challenging. Per the challenge website, here is sample code for Commodore:

``` 10 print"{clear}"
20 print""
30 print""
40 print""
50 print"               *       *"
60 print"               **     **"
70 print"               ***   ***"
80 print"               **** ****"
90 print"           *****************"
100 print"            ***************"
110 print"             *************"
120 print"              ***********"
130 print"               *********"
140 print"              ***********"
150 print"             *************"
160 print"            ***************"
170 print"           *****************"
180 print"               **** ****"
190 print"               ***   ***"
200 print"               **     **"
210 print"               *       *"
220 goto 220```

Starting with that un-optimized version, I will change it to work on the CoCo 1/2/3’s 32-column screen by adjusting it to be properly centered on that display.

`````` 10 CLS
50 PRINT"           *       *"
60 PRINT"           **     **"
70 PRINT"           ***   ***"
80 PRINT"           **** ****"
90 PRINT"       *****************"
100 PRINT"        ***************"
110 PRINT"         *************"
120 PRINT"          ***********"
130 PRINT"           *********"
140 PRINT"          ***********"
150 PRINT"         *************"
160 PRINT"        ***************"
170 PRINT"       *****************"
180 PRINT"           **** ****"
190 PRINT"           ***   ***"
200 PRINT"           **     **"
210 PRINT"           *       *"
220 GOTO 220``````

Unfortunately, this design is 17 rows tall, and the CoCo’s standard display is only 16. It won’t fit:

We should still be able to enter the challenge by having the program print this pattern, even if it scrolls off the screen a bit. To get one extra line there, we can get rid of the line feed at the end of the final PRINT statement in line 210 by adding a semi-colon to the end:

``210 PRINT"           *       *";``

And so it begins…

## And so it begins

The goal is to make this as small as possible. There were many ways to approach last year’s Christmas tree challenge, and you can read about the results and a follow-up with suggestions folks gave to save a byte or two.

A simple thing is to remove the spaces at the front and replace them with the TAB() command:

`````` 10 CLS
50 PRINTTAB(7)"    *       *"
60 PRINTTAB(7)"    **     **"
70 PRINTTAB(7)"    ***   ***"
80 PRINTTAB(7)"    **** ****"
90 PRINTTAB(7)"*****************"
100 PRINTTAB(7)" ***************"
110 PRINTTAB(7)"  *************"
120 PRINTTAB(7)"   ***********"
130 PRINTTAB(7)"    *********"
140 PRINTTAB(7)"   ***********"
150 PRINTTAB(7)"  *************"
160 PRINTTAB(7)" ***************"
170 PRINTTAB(7)"*****************"
180 PRINTTAB(7)"    **** ****"
190 PRINTTAB(7)"    ***   ***"
200 PRINTTAB(7)"    **     **"
210 PRINTTAB(7)"    *       *";
220 GOTO 220
``````

Although this only looks like it saves a character per line (“TAB(8)” versus “seven spaces”), the code itself will be smaller since the TAB command tokenizes down to one (or maybe two?) bytes.

Also, the ending quote is not needed if it’s the last thing on a line, so they could be removed:

`````` 50 PRINTTAB(7)"    *       *
60 PRINTTAB(7)"    **     **
70 PRINTTAB(7)"    ***   ***``````

That would save one byte per line.

But, each line number consumes 5-bytes on it’s own, so a better way to save space would be to pack the lines together. Each line you eliminate saves five bytes. That would become pretty unreadable though, but let’s do it anyway:

``````10 CLS:PRINTTAB(7)"    *       *":PRINTTAB(7)"    **     **":PRINTTAB(7)"    ***   ***":PRINTTAB(7)"    **** ****":PRINTTAB(7)"*****************":PRINTTAB(7)" ***************":PRINTTAB(7)"  *************":PRINTTAB(7)"   ***********"
130 PRINTTAB(7)"    *********":PRINTTAB(7)"   ***********":PRINTTAB(7)"  *************":PRINTTAB(7)" ***************":PRINTTAB(7)"*****************":PRINTTAB(7)"    **** ****":PRINTTAB(7)"    ***   ***":PRINTTAB(7)"    **     **"
210 PRINTTAB(7)"    *       *";
220 GOTO 220
``````

That’s quite the unreadable mess!

This could still be made better, since the text lines were kept under the input buffer limitation size, but when you enter that line, BASIC compresses it (tokenizes keywords like PRINT, TAB and GOTO) making it take less space. You can then sometimes EDIT the line, Xtend to the end and type a few more characters.

That may or may not be allowed for the Logiker challenge. And since I want to provide code here you could copy and then load in to an emulator, I’ll keep it to the limit of what you could type in.

In the next installment, I’ll see if my brane can figure out a way to generate this code using program logic rather than brute-force PRINT statements.

Until then…

# More CoCo MC6847 VDG chip “draw black” challenge responses.

See also: challenge, responses, and more responses.

Today Sebastian Tepper submitted a solution to the “draw black” challenge. He wrote:

I think this is much faster and avoids unnecessary SETs. Instruction 100 will do the POKE only once per character block.

– Sebastian Tepper, 7/5/2022

The routine he presented (seen in lines 100 and 101) looked like this:

`10 CLS20 FOR A=0 TO 3130 X=A:Y=A:GOSUB 10040 NEXT50 GOTO 50100 IF POINT(X,Y)<0 THEN POKE 1024+Y*16+X/2,143101 RESET(X,Y):RETURN`

It did see the criteria of the challenge, correctly drawing a diagonal line from (0,0) down to (31,31) on the screen. And, it was fast.

POINT() will return -1 if the location is not a graphics character. On the standard CLS screen, the screen is filled with character 96 — a space. (That’s the value you use to POKE to the screen, but when printing, it would be CHR\$(32) instead.) His code would simply figure out which screen character contained the target pixel, and POKE it to 143 before setting the pixel.

So I immediately tried to break it. I wondered what would happen if it was setting two pixels next to each other in the same block. What would RESET do?

I added a few lines to the original test program so it drew the diagonal line in both directions PLUS draw a box (with no overlapping corners). My intent was to make it draw a horizontal line on an even pixel line, and odd pixel line, and the same for verticals. It looks like this (and the original article has been updated):

```10 CLS
20 FOR A=0 TO 15

30 X=A:Y=A:GOSUB 100
31 X=15-A:Y=16+A:GOSUB 100

32 X=40+A:Y=7:GOSUB 100
33 X=40+A:Y=24:GOSUB 100

34 X=39:Y=8+A:GOSUB 100
35 X=56:Y=8+A:GOSUB 100

40 NEXT
50 GOTO 50```

And this did break Sebastian’s routine… and he immediately fixed it:

```100 IF POINT(X,Y)<0 THEN POKE 1024+INT(Y/2)*32+INT(X/2),143
101 RESET(X,Y):RETURN```

I haven’t looked at what changed, but I see it calculates the character memory location by dividing Y by two (and making sure it’s an integer with no floating point decimals — so for 15 becomes 7 rather than 7.5), and then adds half of X. (Screen blocks are half as many as the SET/RESET pixels).

And it works. And it works well — all cases are satisfied.

And if that wasn’t enough, some optimizations came next:

And for maximum speed you could change line 100 from:

100 IF POINT(X,Y)<0 THEN POKE 1024+INT(Y/2)*32+INT(X/2),143

to:

100 IFPOINT(X,Y)<.THEN POKE&H400+INT(Y/2)*&H20+INT(X/2),&H8F

To time the difference, I added these extra lines:

15 TIMER=0

and:

45 PRINT TIMER

This lowers execution time from 188 to 163 timer units, i.e., down to 87% of the original time.

– Sebastian Tepper, 7/5/2022

Any time I see TIMER in the mix, I get giddy.

Spaces had been removed, 0 was changed to . (which BASIC will see a much faster-to-parse zero), and integer values were changed to base-16 hex values.

Also, in doing speed tests about the number format I verified that using hexadecimal numbers was more convenient only when the numbers in question have two or more digits.

– Sebastian Tepper, 7/5/2022

Awesome!

Perhaps final improvement could be to change the screen memory location from 1024/&H400 to a variable set to that value, the multiplication value of 32/&h20, as well as the 143/&H8F. Looking up a variable, if there are not too many of them in the list before the ones you’re looking up, can be even faster.

Using the timer value of 163 for our speed to beat, first I moved that extra space just to see if it mattered. No change.

Then I declared three new variables, and used DIM to put them in the order I wanted them (the A in the FOR/NEXT loop initially being the last):

```11 DIM S,C,W,A
12 S=1024:W=32:C=143
...
100 IFPOINT(X,Y)<.THENPOKES+INT(Y/2)*W+INT(X/2),C
101 RESET(X,Y):RETURN```

No change. I still got 163. So I moved A to the start. A is used more than any other variable, so maybe that will help:

`11 DIM A,S,C,W`

No change — still 163.

Are there any other optimizations we could try? Let us know in the comments.

Thank you for this contribution, Sebastian. I admire your attention to speed.

Until next time…

# CoCo MC6847 VDG chip “draw black” challenge responses.

See also: challenge, responses, and more responses.

Recently, I was annoyed to find that there did not seem to be any way to set a black pixel on the CoCo’s normal green background. I have since been schooled in the simplest way to make this work, which I will share after a long digressing ramble.

## Never the Same Color Twice

The CoCo’s MC6847 VDG chip provides nine colors. Commenter Jason wrote:

“It always bothered me that the CoCo had nine colors in semi-graphics modes. The number nine should raise a red flag for anyone who is familiar with computers and the tendencies for things to be powers of two.

“It’s interesting that seven of the eight colors are from the NTSC test pattern (https://en.wikipedia.org/wiki/SMPTE_color_bars) which leads me to believe they’re all a particular frequency distance from each other. This would make the circuitry simpler.”

– Jason

I suppose it’s actually eight foregrounds colors with a black background, which matches the black border of the screen. There was even a test pattern program included in one of Radio Shack’s quick reference guide that I still have:

## Color Adjustment Test Display

``````5 FOR X = 0 TO 63
10 FOR Y = 0 TO 31
15 C = INT(X/8+1)
20 SET(X,Y,C)
25 NEXT Y,X
30 GOTO 30``````

That produces the following output, showing the eight possible colors (plus the black background):

And here is the NTSC test pattern Jason referenced:

This made me want to alter the program to make it render something with the matching colors in the correct order (and with the Xroar emulator set to not emulate a crappy 1980s RF modulated TV signal):

The extra color that is not shown is orange. I wonder why those eight colors (plus black) were chosen? And what makes the colors used by the two PMODE high res graphics screens? I’ll have to revisit that in the future.

But I digress…

## You can’t, even if you SET your mind to it

My original article was written because I noticed you couldn’t SET a black pixel on a normal CoCo text screen. Even though the manual listed nine colors, with zero being black, attempting to do SET(X,Y,0) would result in that pixel being set to the green background color instead of black — the same as SET(X,Y,1). While other colors acted as you expected…

SET seemed to be treating color 0 (black) as 1 (green). Because reasons.

In order to SET a black pixel on the normal text screen, extra code would be needed.

## Ciaran Anscomb

Xroar emulator author Ciaran Anscomb was the first to respond with his GOSUB routine to achieve the desired effects:

I mean I think you’re making it harder by asking for it to work with
plain CLS and not CLS1, but in that case:

100 IFPEEK(1024+INT(Y/2)*32+INT(X/2))<128THENPOKE1024+INT(Y/2)*32+INT(X/2),143
110 RESET(X,Y):RETURN

– Ciaran Anscomb

His method would PEEK the character value at the 2×2 block that was being SET and, if that value was less than 128, it would change it to 143 and then use RESET… And it works:

``````10 CLS
20 FOR A=0 TO 31
30 X=A:Y=A:GOSUB 100
40 NEXT
50 GOTO 50
99 ' Ciaran Anscomb
100 IFPEEK(1024+INT(Y/2)*32+INT(X/2))<128THENPOKE1024+INT(Y/2)*32+INT(X/2),143
110 RESET(X,Y):RETURN``````

## Jim Gerrie

BASIC programmer extroidinaire Jim Gerrie provided his take on this routine:

100 SET(X,Y,1):SET(X-(X/2-INT(X/2)=.),Y,1):SET(X-(X/2-INT(X/2)=.),Y-(Y/2-INT(Y/2)=.),1):SET(X,Y-(Y/2-INT(Y/2)=.),1)
101 RESET(X,Y):RETURN

– Jim Gerrie

His version sets some pixels to color 1, and some to color 0 (using the “.” shortcut), based on some math with X and Y and divisions and integer conversions and … well, stuff I don’t grasp.

His also works! But as it draws, you can see it blipping surrounding pixels in the 2×2 block on then off. And while it passes the test case which drew a diagonal line, it doesn’t allow for setting arbitrary pixels near each other. They turn into full blocks.

However, he also added a second attempt:

I know that my first suggestion above is a bit of a cheat. Here’s a more robust suggestion:

10 CLS
20 FOR A=0 TO 31
30 X=A:Y=A:GOSUB 100
40 NEXT
50 GOTO 50
100 IFPOINT(X,Y)<.THENXX=(X/2-INT(X/2)=.)-(X/2-INT(X/2)>.):YY=(Y/2-INT(Y/2)=.)-(Y/2-INT(Y/2)>.):SET(X,Y,1):SET(X-XX,Y,1):SET(X-XX,Y-YY,1):SET(X,Y-YY,1)
101 RESET(X,Y):RETURN

– Jim Gerrie

This version passes the test as well, and looks like it better handles setting pixels at any position without impacting pixels around it.

## What’s the POINT?

Ciaran made use of PEEK to detect what was on the screen before adding something new, and Jim figured out what pixels to set back to the background color. Neither did it the way I was expecting — using POINT:

```POINT (X,Y) Tests whether specified graphics cell is on or off, x (horizontal) = 0-63; y (vertical) = 0-31. The value returned is -1 if the cell is in a text character mode; 0 if it is off, or the color code If it is on, See CLS for color codes.

IF POINT(10,10) THEN PRINT "ON" ELSE PRINT "OFF"```

I expected I’d see folks use this to see if a pixel was set, and handle accordingly. Somehow. But as I read this description (from the Quick Reference Guide), I see that note that says “The value returned is -1 if the cell is in a text character mode.”

Text character mode? It’s just the background, isn’t it?

## All green backgrounds are not the same

And that takes me back to Ciaran’s code:

`IF PEEK(1024+INT(Y/2)*32+INT(X/2))<128 . . .`

Less than 128 is a text character. The graphics blocks (2×2) start as 128. If the square is a text character then set it to 143. So what is that? That is a 2×2 graphics block that has all pixels set to the green color. And that green color is the same color as the background screen. Which isn’t 143 when you use CLS. Try this:

`CLS:PRINT PEEK(1024)`

If you clear the screen then PEEK to see what value is at the top left character (1024), it returns 96. 96 is the space character (yeah, ASCII is 32, but values in screen memory aren’t ASCII).

Ciaran’s code sees if it’s anything (including that green space), set it to 143, which is a green block that looks the same. Try this:

`CLS 1:PRINT PEEK(1024)`

That will print 143. Yet, visually, CLS and CLS 1 look the same. But, CLS is filling the screen with the space text character (96) and CLS 1 fills it with the green graphics character (143)! CLS 0-8 fill the screen with solid graphics characters, and CLS with no parameter is the space.

Now, I knew about character 143 looking like the normal space but not being one, because we used to use this as a cheap “copy protection” method. On DISK, you could save out a file like this:

`SAVE "HELLO.BAS"`

…and you’d get a file on BASIC called HELLO.BAS. But, if you did this:

`SAVE "HELLO"+CHR\$(143)+".BAS"`

…Disk BASIC would write out a file called HELLO(char 143).BAS. When you did a DIR they would look the same, but you couldn’t do a LOAD”HELLO.BAS” to get the one with the CHR\$(143) in it. Unless you exempted the disk directory bytes you would not know there was an “invisible” character at the end of the “HELLO” filename.

Sneaky. And I did this with some of my own programs.

But years later, when the CoCo 3 came out, it’s 40 and 80 column screen did NOT support the 2×2 graphics block characters, and this trick was no longer as sneaky since you would see “HELLO(some funky character).BAS” in the directory listing and know something weird had been done.

But I digress, again…

## Why do it the hard way, anyway?

It turns out, even though I knew about the “add CHR\$(143)” trick, I had forgotten (or never knew/realized) that CLS and CLS 1 filled the screen with different characters. And, if the screen has a graphics character at the position, RESET will then work to change that pixel back to black.

Ciaran got me exploring this because in his e-mail he added:

If you allow CLS1, the problem solves itself :)

– Ciaran Anscomb

I had to follow up and ask what he meant by this. And, well, nevermrind, then. All I needed to do was start the program with CLS 1 instead of CLS 0, and then I could use RESET() to set individual black pixels on the screen.

I could have a subroutine that expect X, Y and C (for color) and if the color was 0 (black), do a RESET, else do a SET:

``````0 REM RESET
10 CLS 1
20 FOR A=0 TO 31
30 X=A:Y=A:C=0:GOSUB 100
40 NEXT
50 GOTO 50
100 IF C=0 THEN RESET(X,Y) ELSE SET(X,Y,C)
110 RETURN``````

And that works fine on any CLS 0-8 screen. Remember, SET(X,Y,0) never gives you a black pixel. 0 seems to mean “background color” instead, while RESET(X,Y) seems to mean “set to black”.

To me, this is a bit counterintuitive, since today I would expect “reset” to mean “set to background color” but this isn’t a graphics mode — it’s just graphics characters on a screen, so the only way BASIC could have done this is if it remembered what the CLS # value was, and made RESET set to that color. Which would be extra ROM space for a simple enhancement that most could work around with RESET instead.

And that, my friends, is how a rabbit hole works.

Until next time…

# 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.)

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
20 PMODE0,1:PCLS0:SCREEN1,1
30 LINE(0,0)-(89,79),PSET,BF
40 PSET(X,Y,0)
50 X=X+XM:IF X<0 THEN XM=1:GOTO 50 ELSE IF X>=W THEN XM=-1:GOTO 50
60 Y=Y+YM:IF Y<0 THEN YM=1:GOTO 60 ELSE IF Y>=H THEN YM=-1:GOTO 60
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.

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
5 DIM TM,A
10 TIMER=0:TM=TIMER

15 W=89:H=79:X=0:Y=0:XM=1:YM=1
16 PMODE0,1:PCLS0:SCREEN1,1
17 LINE(0,0)-(89,79),PSET,BF

20 FORA=0TO7030

30 PSET(X,Y,0)
40 X=X+XM:IF X<0 THEN XM=1:GOTO 40 ELSE IF X>=W THEN XM=-1:GOTO 40
50 Y=Y+YM:IF Y<0 THEN YM=1:GOTO 50 ELSE IF Y>=H THEN YM=-1:GOTO 50

80 NEXT
90 PRINTTIMER:END```

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…