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The International Obfuscated C Code Contest

2019/poikola - Most calendrical

← 2019/mills ↑ 2019 ↑ 2019/yang → C code Makefile Inventory ✓

Author:

To build:

    make

There is an alternate version of this program which changes the way the output is displayed. See Alternate code below.

NOTE: we disable the optimiser because the author stated that for GCC optimiser level 0 gave correct output. With clang [0123s] works which suggests that [123s] might or might not work with GCC. Likewise, we do similar for the standard, forcing -std=gnu11.

Bugs and (Mis)features:

The current status of this entry is:

STATUS: INABIAF - please DO NOT fix

For more detailed information see 2019/poikola in bugs.html.

To use:

    ./prog 512 ./prog

    sleep 86400; make clobber prog
    ./prog 512 ./prog

    sleep 86400; make clobber prog
    ./prog 512 ./prog

If you wish for some documentation:

    make docs

This will create the file poikola.pdf for your enjoyment.

Try:

If you don’t have a few days, try:

    ./try.sh

As the 2019/poikola in bugs.html says, this program will not validate input so it might get stuck or fail if invoked erroneously.

Alternate code:

This version prints a newline after each number for parsing in other ways than what can be done with the original entry.

Alternate build:

    make alt

Alternate use:

Use prog.alt as you would prog above.

Alternate try:

    ./try.alt.sh

Judges’ remarks:

Do you have the time to see what this program does? Think again, come back and try again.

Note that 2 can be said to be the least odd prime because it is the greatest even prime. Moreover 2 can be also said to be the greatest odd prime because it is the least even prime.

The source code layout and some of the variable names honors the ski jumping accomplishments of Matti Nykänen.

Author’s remarks:

How to build:

    gcc -O0 -o prog prog.c

or

    clang -O[0123s] -o prog prog.c

e.g.:

    clang -O1 -o prog prog.c

Poster:

You can generate an A3 sized poster by make docs. This command creates a pdf file poikola.pdf. This requires the pdfTeX tool.

What this entry does:

An old wise man mumbled to me:

    Implement a program, which can calculate SHA-3 checksum for a file --

I thought it was easy, but the wizard didn’t stop yet:

    and primes and Fibonacci numbers.

Still easy. But the Gandalf the Grey impersonator was not ready:

    But use only `main()`, and do not use `math.h` nor predefined stuff at all.

But wait, there are more rules: This work should reflect the extraordinary mind of Matti Nykänen, the all-time best ski jumper and amateur philosopher, who passed away on fourth of February 2019.

Oh boys:

So, I followed the rules given by Gandalf the White and the Judges of the IOCCC.

Output of iocccsize, using the 2019 version of iocccsize, is carefully selected. For the sake of clarity, I used single letter variables in the code. I also avoided unnecessary use of functions. Like a tripundra, this program has three levels. In order to reveal all of them, you have to compile this program on three consecutive days.

As this is a contest for obfuscated code, the program does not perform unnecessary checks, but either fails or gets stuck if invoked erroneously. The program is invoked in this way:

    ./prog <integer> <file>

where <integer> is 224, 256, 384, 512 or even 1024. Yes, I know 1024 is not a supported length of output, but as I said, the program does not validate its input in any way. These same parameters should be given in calculating a Fibonacci sequence or prime numbers. Please note that maximum file size is one gigabyte!

Obfuscation:

Matti Nykänen: Every time I jump, and I get that feeling of bon voyage, the feeling that I’ve been here before.

Matti Nykänen: When you are about to jump, you are all alone and have to make your own decisions. Up there, it’s all up yours.

This code has some jumping too. I used some gotos instead of longjmp(3). Some Finnish ski jumping sites are used as labels, however, there is lahti instead of salpausselkä.

SHA-3-512 Compatibility chart

Tested against test vectors.

Clang (version 4.0): For optimization levels [0123s], the program compiles and produces correct output for every tested vector using the C standards c11, c89, c90, c99, gnu11, gnu1x, gnu89, gnu90, gnu99, iso9899:1990, iso9899:199409, iso9899:1999 and iso9899:2011 (argument of -std=).

GCC (version 6): For optimization level 0, correct output using the C standards c11, c99, gnu11, gnu1x, gnu89, gnu90, gnu99, iso9899:1999 and iso9899:2011.

Missing a prime

For obvious reason, the oddest prime is missing from output.

Observations

gcc and clang are very fragile compilers. I spent numerous hours trying to achieve exactly the same output from different optimization levels and compilers. Small change in code can produce really unexpected results, i.e. the compiler can skip a few expressions or statements without obvious reason.

NOTICE to those who wish for a greater challenge:

If you want a greater challenge, don’t read any further: just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.

How this entry works:

It is commonly believed that Mike Keith’s algorithm published in Journal of Recreational Mathematics, Vol. 22, No. 4, 1990, p. 280, is the shortest way to calculate the day of the week for a given date.

Keith’s versus my algorithm

Kieth’s algorithm is (d+=m<3?y--:y-2,23*m/9+d+4+y/4-y/100+y/400)%7. I noticed that (23*m/9+(m<3?y--:y-2)+d+4+y/4-y/100+y/400)%7 is one character shorter. In my solution, m<3?y--:y-2 is used directly, instead of assigning a value to d.

Q is pronounced in Finnish exactly like kuu, a word for month or the Moon. So, Q = k is intended.

Last term of outputted Fibonacci sequence is the last one smaller than 264. It is a plain but funny coincidence that the value of ^ is 94 in ASCII.

For prime calculations: I recycled code from my high school C-programming course. Originally, I wrote it over 20 years ago. My teacher said that it is (too) complicated to use bits stored in arrays, so I did it.

Other stuff

Cody Boone Ferguson: I really like that you enjoyed my previous entry. I hope that you will like this entry too. Because I did not want to give too many clues to the Judges, I tried to write this entry in a different way than Most Stellar, but in this code there is at least one recycled thing from it. By the way, the answers to the questions I posed: (1) 255. (2) Try to compare binary representations of those floats and binary representation of the string “25th IOCCC!”. (3) I don’t know. I used standard trigonometric functions from math.h for rotating the Big Dipper and later replaced those functions with my own implementations. But there was a bug in my code and the effect was more beautiful than intended. I did not even debug this bug and now it works as a feature. (4) It is not possible. If you change a single bit, the Fletcher’s 16 checksum does not match anymore and goto jumps to the end of the code.

I see that one of the Volker Diels-Grabsch’s winning entries is awarded the category Most self-aware. In last year, my entry incorporates the Fletcher’s 16 algorithm to ensure integrity of code. One of my first ideas for this year was to do the same thing, but with SHA-3. Using Fletcher 16, calculating a checksum which matches with sleeping time factor took about one minute. I realized that SHA-3 even with 224-bit output could take too much CPU time.

I would like to say thanks to PSP, because there were not too many interruptions during the coding phase.

Inventory for 2019/poikola

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