PATTERN

Archive contents:

-rw-rw-r-- 1 julie julie  1280 Dec 24 20:28 disc_image.ssd  Disc image
-rw-rw-r-- 1 julie julie  2771 Dec 24 20:20 id.diz          This file!
-rw-rw-r-- 1 julie julie 12974 Dec 24 20:22 listing1.png    BASIC listing pt1
-rw-rw-r-- 1 julie julie 14557 Dec 24 20:23 listing2.png    BASIC listing pt2
-rw-rw-r-- 1 julie julie    48 Dec 24 20:29 PATTERN         Machine code
-rw-rw-r-- 1 julie julie 10386 Dec 24 20:23 result.png      Result image
-rw-rw-r-- 1 julie julie   577 Dec 24 20:28 SOURCE          Source ASCII
-rw-rw-r-- 1 julie julie   487 Dec 24 20:29 SOURCE.tok      Source tokenised

Author: Julie Kirsty Louise Montoya <bluerizlagirl@gmail.com>
Category: Christmas Challenge
System:   BBC Micro
Language: 6502 Assembler -- BBC BASIC native assembler
Len source code: 487 bytes (Tokenised)
                 577 bytes (ASCII text)
Len exe file:    48 bytes
Len code only:   48 bytes

Instructions:

Install, if necessary, and launch a BBC emulator such as BeebEm.
Load the file `disc_image.ssd` as a disc image in drive 0.
Make sure drive 0 is not write protected.
Load and run the source code file:

CHAIN "SOURCE"

The machine code will be assembled and saved onto the disc.
Once the code has been assembled, run it by typing at the BASIC prompt

*PATTERN

You may optionally press CTRL+L to clear the screen before you press
RETURN to enter the command.

Description:

The program draws the pattern

   *     *     *
  * *   * *   * *
 *   * *   * *   *
*     *     *     *
 *   * *   * *   *
  * *   * *   * *
   *     *     *
  * *   * *   * *
 *   * *   * *   *
*     *     *     *
 *   * *   * *   *
  * *   * *   * *
   *     *     *
  * *   * *   * *
 *   * *   * *   *
*     *     *     *
 *   * *   * *   *
  * *   * *   * *
   *     *     *

on the screen.

A lookup table is used consisting of the following sequence:
19,20,21,22,21,20,19,20,21,22,21,20,19,20,21,22,21,20,19

For each of 19 rows, and each of 19 columns, the values in the table
corresponding to the row and column are added together, plus an extra
one; and if the sum is exactly 42, the ASCII code for a star, this is
printed.  For any other sum, a space is printed.  After each complete
row, a new line is started.  Once the complete image is printed, the
program returns to the BASIC prompt with the cursor at the beginning
of the next line.

Comments:

The program should also work fine on an Acorn Electron, though this has
not been tested.

Anything I tried like reversing halfway through to keep the table small
seemed to wind up taking more bytes in code than it saved in data.
Sometimes a static table is just smaller than doing complex maths .....

Making the table add to 42 if we need to print a star was a last-minute
byte-saving effort.