Crunching BASIC programs

To fit more code into the BBC Micro's limited memory, BASIC programs can be crunched. Crunching reduces the size of a program without changing its meaning, but in the process makes it almost unreadable -- it is a form of obfuscation.

Crunching can be done by a utility ROM that compacts a program in memory, or more recently by sending a text listing to be compacted on another computer and reading it back in. Automated crunchers tend to concentrate on the following areas that yield big and easy savings:

• Removing redundant characters
• Joining lines together (a multi-byte line header is replaced by a colon)
• Shortening variable names.

Utility ROMs are able to remove spaces that were needed during keying, but are redundant now that the code has been tokenised. So they may produce a program that can't be typed back in from a listing.

As BASIC is an interpreted language, crunching delivers an overall time saving as well. The suggestions below are targeted at saving space more than time. See Chapter 32 of the B+ User Guide for tips on increasing the speed of a program.

Suggestions

The following does not apply to the characters within a string constant or a *command. Correctness is not guaranteed!

Trivial

• Delete empty lines.
• Delete leading and trailing spaces. A space between the line number and the code is stored in memory, and optional. E.g.

  1030 ENDPROC

  could be replaced by

  1030ENDPROC

  LISTO 1 makes LIST reinsert the space on the screen.
• Replace multiple spaces with a single space.
• Delete leading and trailing colons.
• Replace multiple colons with a single colon.
• Delete comments.

Easy

• Eliminate the keyword LET.
• Delete the keyword THEN except before a system variable assignment, unary operator, function return statement, *command or implied-GOTO line number.
• NEXT statements don't have to name the control variables. One NEXT statement can terminate several FOR loops, using commas.

  NEXT X%:NEXT Y%

  can be replaced with

  NEXT,

  If the program breaks when the control variable is removed, the FOR...NEXT loops are mis-nested!
• Functions with a single argument, except RND, don't need brackets around the argument. E.g.

  PRINT CHR$letter%, INKEY100, STR$~code%

• The result of a numeric function can be discarded with IF rather than assigning to a dummy variable:

  IFGET

  This only saves space at the end of a line.

Moderate

• Replace Teletext CHR$ functions with inline characters in strings, using SHIFT/CTRL and the function keys. The listing cannot be printed and re-typed after this.
• Express very large or very small real constants in scientific format:

  G=6.673E-11

• A zero before a decimal point can be eliminated.
• Express integer constants ≥ +1,000,000 as hexadecimal and the rest as decimal.
• VDU sequences may be shorter with some byte constants combined into word constants using semicolons. In particular 0; replaces 0,0,

Tedious

• Delete spaces:-
  • After characters "#$%'()*+,-./:;<=>[\]^{|}~
    • Preserve spaces between string constants so that they do not merge.
  • Before characters !"#&'()*+,-/:;<=>?@[\]^{|}~
    • Again preserve spaces between string constants.
  • Between numbers and other code, but not between two numbers.
  • After keywords, but not after real variable names.
    • Preserve the space in END ELSE, ERR OR, GET $, INKEY $, MOD E, OPT <keyword>, <assembler mnemonic> <keyword> and TO P if the listing is to be typed in.
• Replace long variable, procedure and function names with shorter ones.
  • Preserve the variable type; don't replace integers with reals or vice versa as rounding errors may result.
  • Use the resident integers A% to Z% for speed, but otherwise it is best to use names with a lowercase character to avoid collisions with keywords.
  • Automated crunchers should use all one-character names first, then two character names with the first character 'varying fastest', and so on.
  • @% is reserved if the program PRINTs variables.
  • A%, C%, X% and Y% are reserved if CALL or USR appear in the program (6502 BASIC).
  • O% and P% are reserved if the program contains assembly language.
  • A, X, Y, a, x and y are reserved inside assembly language segments as they are register indicators, not variable names (6502 BASIC).
• Replace multiple lines with fewer multi-statement lines.
  • The longest line that can be typed in is 240 characters including the line number.
  • Remember DATA and DEF must be at the beginning of a line.
  • DATA 1 (newline) DATA 2 becomes DATA 1,2.
  • Don't add to the end of a line containing ELSE, IF, ON, REM or a *command.
  • Keywords [, ELSE, REPEAT and THEN, and DEF... statements don't need a colon between them and the next statement.
• DATA strings don't need double quotes unless they contain double quotes, commas or leading spaces.
• When crunching a text listing, keywords can be abbreviated to their minimum forms. See Chapter 48 of the B+ User Guide.

Difficult

• Use operator precedence rules to find redundant brackets in expressions and remove them.
• Use intermediate variables to reduce the number of repeated sub-expressions.
• Refactor repeated segments of code into a function, procedure or subroutine.
• Find other ways of storing data in the program besides DATA; see Data without DATA.

References

Based on crunch.pl, packaged with EDOSPAT 4.40.

-- beardo 19:12, 11 October 2007 (BST)