Balanced use of obfuscation - Gold award

Judges' comments:

To build:

make zeitak

To run:

./zeitak < file.c

where file.c is the file to be checked for nesting errors. For example, you may try incorrect.c and the program itself.

Try:

./zeitak < zeitak.c
./zeitak < incorrect.c

Selected Judges Remarks:

This is an extremely subtle and twisted piece of Gold award winning code!

The judges had spent a considerable amount of time analyzing this entry. At one point we spent 18 minutes just to understand 18 key characters of this code.

The file zeitak_deobfucate.c provides a version that has been slightly deobfuscated. You may find reading that file helpful in your attempt to understand this extremely subtle entry.

Author’s comments:

Nesting Errors Detector

What does it do

As you have probably understood by looking at the source*, this program has something to do with parenthesis (and equality of opening and closing parenthesis, if you look close enough). It goes over the file given to it and checks that every opening (, [, or { has a matching closing one and vice versa. It also checks that every “ or ‘ is closed.

If an error is detected, an error message will be printed. If the problem is a superfluous closing bracket, it will even print a few characters around it’s position.

Make sure you view the source with 4 spaces tab width.

Features

Ignores parenthesis inside strings or character constants, so no errors will be detected in the following line:
```
  printf(")");
```
Doesn’t get confused by the 1984/anonymous entry!

Mis-Features

Escapes (e.g. \") are ignored, so the following line will produce an error:
```
printf("\"");
```

Obfuscation

IOCCC winners already contain entries without digits, control-flow keywords and certain operators in their source. This entry has an even more limited source, that is:

Without any digits.
Without any character constants.
Without using functions from headers other than stdio.
Without any control-flow keywords (not even the ?: operator).
Without any arithmetic or logic operators!

So, what’s left? Parenthesis, and lots of them, as looking at the source will reveal immediately. The main obfuscation is building the whole algorithm using only function calls, typecasts, array lookups and pointer operators.

Additional obfuscations include:

Extensive reuse of identifiers: Macros share names with variables, inner curly braces scopes contain variables with names identical to those in the outer scopes and some of the variables have different uses in different parts of the program. Even the large array has more than one use.
A complicated data structure with “unused” fields.
Mostly one-letter identifiers.
Complex recursion: A function might call itself once, twice, or not at all.

Compilation/Running Notes

The program requires char to be one byte and pointers to be at least two bytes long.
The program was tested on the following platforms:

Ubuntu 9.04 32-bit with GCC Windows 7 32-bit with GCC ** Windows 7 32-bit with OpenWatcom