Author:

• Name: Diomidis Spinellis
  Location: GR - Hellenic Republic (Greece)

To build:

    make all

To use:

    ./dds basic_program
    ./a.out 2>/dev/null

Try:

The author suggests trying:

    ./dds LANDER.BAS
    ./a.out

Notice that a file a.c has been generated. Can you tell how a.c was produced? How does a.c relate to LANDER.BAS?

What happens if you give the program a C program like itself? Try:

    ./dds dds.c

You’ll get errors yes but what does the generated file look like? What about other types of files?

Judges’ remarks:

This obfuscated program translates BASIC programs into obfuscated C programs by way of an obfuscated algorithm.

Author’s remarks:

This program is a companion to the 1990/dds 1990 DDS-BASIC interpreter program. This compiles BASIC programs into compilable C code and then compiles it. The input format is almost identical to the input format of the DDS-BASIC interpreter. The program needs an executable C compiler called cc in your path in order to work.

Program commands:

• variable names A to Z
• FOR var = exp TO exp
• GOSUB exp
• GOTO exp
• INPUT variable
• PRINT exp
• REM any text
• variables initialized to 0 on RUN
• NEXT variable
• RETURN
• IF exp THEN exp
• PRINT string
• var = exp
• END

Expressions are the same as the expressions of the C language. Many system calls and C library calls can be used.

Input format:

• Free format positioning of tokens on the line.
• No space is allowed before the line number.
• Exactly one space is needed after the FOR and NEXT tokens.
• ALL INPUT MUST BE UPPERCASE.

Error checking / error reports:

The compiler silently ignores many errors. Other errors may produce errors in later phases of the compilation.

Can you figure out how the compiler works?

Hint:

The compiler is NOT written in C, so this is really a meta-obfuscated program. The C code is an interpreter for the four register, seven instruction COGNIMP$ machine that is contained in the s string. (COGNIMP$ is named after the symbolic names of the seven instructions it defines (Copy, Output, Goto, iNcrement, If, Match and Print).

The $ sign is used for labels. The actual compiler is written in COGNIMP$. Browsing through the COGNIMP$ code we encourage you to examine the loops for scanning the expression in the IF statement and the way a decision tree is implemented in order to match the statements. (Of course the s string is encoded by adding one to every character of it just to confuse you).

Primary files

• dds.c - entry source code
• Makefile - entry Makefile
• dds.orig.c - original source code
• LANDER.BAS - BASIC source code for Lunar Lander game

Secondary files

• 1991_dds.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• index.html - this web page

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