notes.md

Notes

CBM BASIC gets very cryptic very quickly.

Variables

Game constants

• gr$(): Graphics data in the form of a 4x1 tile.
• lv$(): Level data strings. See below.
• ml: max levels
• w: level width in tiles
• h: level height in tiles
• mx: number of tiles per level, should not exceed 255
• mu: max undo tiles

Rules

• fnpp(x): Function that returns the rule for foreground tile at position x.
• ru%(0..31): Property rules for each object. Two bytes per.
  • where n contains a byte of tile data, r=ru%(nand31)orru%(n/32) sets r to the rule bits of both the foreground and background tile, so that any interaction works the same regardless of which object is in front.
• tr%(0..7): Transformation rules for each object.
• dr: Rule bit of an object being destroyed in a two-rule interaction

Level state

• l%: current level number
• `pf%(0..mx): Playfield tiles. Even though the compiled version only 1 byte per tile, it can allow multiple objects to overlap so that they can be tested for interaction.
  • n=pf%(x) and 31: set n to foreground tile at position x
    • 0 - 7 (nand24=0): Objects
    • 8 - 15 (nand24=8): Noun tiles
    • 16 - 31 (nand24>15): Property or verb tiles
  • int(pf%(x)/32): Background tile at position x
    • It can only be 0 through 7, because the rules cannot be modified such that text tiles can overlap.
• `ud%(0..mu,0..2): Undo stack, 3 bytes per tile changed
  • ud%(dl,0): location of changed tile
  • ud%(dl,1): old value of tile
  • ud%(dl,2): turn number when tile changed
• dl: next index to store a tile change in ud%()
• ud: TODO: something to do with the undo stack, kind of like dl?
• u%(0..mx): list of tiles with "YOU" property

Rule behaviors

Objects 0 through 7 are inert by default. Objects 1 through 7 may be granted the following properties through the following bits in the ru%() array:

• YOU (1): "YOU" tiles will move according to player input. If no on-screen tiles have the "YOU" property, then the player may not influence the level, and generally must undo or reset.
  • There are not yet any autonomous movement properties that would allow a level to be solved by temporarily relinquishing control.
• WIN (2): The player wins the level if the "YOU" tile overlaps a "WIN" tile.
• STOP (4): Objects cannot move onto a "STOP" tile. Any movement operation that encounters a "STOP" tile will be aborted.
• PUSH (8): If an object attempts to move into a tile with a "PUSH" object, the "PUSH" object will attempt to move in the same direction to get out of the way.
• SHUT (16): Behaves the same as a "STOP" tile, except that if an "OPEN" object moves onto it, both objects are destroyed.
  • we need to test that this works with a chain of PUSH blocks
• OPEN (32): see above
• SINK (64): If any object moves onto it, both objects are destroyed.
• LOSE (128): If any "YOU" object moves onto it, both objects are destroyed.
• HOT (256): Inert except to "MELT" objects
• MELT (512): Destroyed if it moves onto a "HOT" tile.
• there are several undefined properties

All text tiles are treated as "PUSH" objects with no other properties.

Rule definitions on the playfield

TODO

Level editor

Press "E" to enter the level editor. This will allow you to edit the playfield in place. You can return to the game at any time by pressing "F7".

A cursor will appear that looks like >@@<. When you move the cursor over a tile, it will display the contents of that tile as a two-letter code. The left character is the background and the right character is the foreground. For example. >@@< is an empty tile, >EA< is Baba standing on a water tile.

Editor controls are as follows:

• CRSR keys: move the cursor
• F1: Show a legend of supported tiles.
• @, A-Z, [, £, ], uparrow, leftarrow: put that object in the foreground.
• SHIFT-A through SHIFT-G: put that object in the background.
• DEL: remove foreground object. promote background to foreground.
• F5: Save current screen to memory.
• F3: Restore screen saved from memory.
• F7: Return to the game.

To add a level to the actual game, you'll have to use this workflow:

• Use E to enter the editor and start making changes.
• When you are ready to playtest, press F5 to save the screen, and then F7 to return to the game.
• To restart the level, press E to re-enter the editor, F3 to restore the screen, and then F7 to return to the game.
• When you're finished playtesting, press F7 to return to the game and press D to generate DATA statements.

The program will print two lines: a DATA statement containing the compressed level data, and replacement for line 15 redefining the maximum number of levels. It will exit to BASIC so that you can move the cursor up and add both of these lines to the program. Then you can SAVE the new program with the new level at the end.

Level string format

Reading

The DATA statements starting around line 9200 contain level data in a compressed format. When the player enters a new level, the data string is decompressed as follows:

• 119: entire playfield is filled with 0 (empty space)
• 120-135: variables are initialized
  • lv$ contains the current level
  • x contains the index being read from lv$ starting at 1
  • i contains the index being written info pf%() starting at 0
• 140-150: each PETSCII value from lv$ is read into t and parsed.
  • character 191 (CBM-B) seems to indicate that the level data continues into another DATA statement, so read the next array element into lv$ and continue.
    (I doubt it'll work because l2 gets reset at line 120 which is where the GOTO statement leads)
  • character 252 (PI) is treated as character 34 (double quotes), to accommodate the limitations of BASIC.
  • character 64 to 95 (unshifted letters): store the low 5 bits (0-31) to the playfield
  • character 192 to 223 (shifted letters): store the low 5 bits (0-31) to the playfield, and store the "IS" type to the next tile.
  • character 33 to 47 (punctuation): the next t-32 tiles are zero. (off by one?)
  • character 49 to 63 (punctuation): repeat the previous tile t-48 times.

Writing

TODO document the DATA statement generator