ioccc/2000/rince

To build:

    make all

Bugs and (Mis)features:

The current status of this entry is:

    STATUS: INABIAF - please DO NOT fix

For more detailed information see 2000/rince in bugs.html.

To use:

    ./rince number number2

If DISPLAY is not set the program will very likely crash or do something different.

Try:

    ./try.sh

This will run the program several times with different parameters to show different types of things. See the author's remarks for more details.

Judges' remarks:

This program may have you going around in circles :-) Try running it first before reading the author's remarks below.

The judges verified the correctness of output for recent dates through visual observation.

Author's remarks:

How to use it

The simplest way to run it is with no arguments. Just type:

    ./rince

You will need to have access to X windows, although it doesn't require desperately high resolution and there are no known limitations on colour depth.

The display will change slowly over time, although it may take 10 minutes or more to be visible. For those that cannot wait that long the program has a built in time machine. Indeed the program knows the speed of light, which is used to ensure that we see the "live" pictures. Simply specify the fraction of a day to skip forward for each screen refresh. I suggest:

    ./rince 0.01

To display the output (still guessing?) for a given date, specify two arguments. The first argument is as before and the second argument is a Julian day number (given as a fractional component). E.g.:

    ./rince 0 2441193.6
    ./rince 0.01 2441193.6

A Julian day is defined to be the number of days since January 1st in the year -4712 (4713BC), starting at 12 noon GMT.^{1 2} To aid conversion from our time (GMT) to Julian day there is a supplementary program named: dmy2jd.c. Run it with day_float month year as arguments. See below for an example.

What it does

This program plots the positions of the four Galilean moons of Jupiter, as seen from Earth. It doesn't correct for your location on Earth, but I assure you that the parallax is minimal!

The moons are labelled with Roman numerals from I to IV, I being the closest to the planet and IV being the furthest. They are:

    I     Io
    II    Europa
    III   Ganymede
    IV    Callisto

Amazingly, all it takes is a pair of binoculars to see the Galilean moons, so the results can be experimentally checked. (Note that at present, the best views of Jupiter are over the autumn/winter.)

The positions of the moons should be correct for binoculars, but possibly not if you have a telescope or if you are in the southern hemisphere. In that case I suggest that to get the correct image you look at the screen from inside the monitor whilst standing on your head. In case of confusion, please consult the following chart (*):

               N
               ^
               |
         E <---+---> W
               |
               v
               S

(*) Not to scale.

History

Galileo discovered the moons in January 1610. https://web.archive.org/web/20001204212000/http://www-isds.jpl.nasa.gov/cwo/cwo_54ga/html/cd/galileo.htm states the following:

    By early 1610, Galileo had further improved his telescope by
    double its power, so it now enlarged objects a thousand
    times. He began looking at the stars and couldn't believe how
    many more he could now see! The number seemed infinite. He was
    beginning to realize the fixed stars weren't a flat distant
    wall on a globe, but a three-dimensional expanse bigger than
    anyone had ever thought!

    Then, on January 7, about an hour after sunset, Jupiter rose
    in the east, and Galileo happened to turn his powerful scope
    on its bright disk. It wasn't the first time he had looked at
    Jupiter, but before it was with one of his lesser-powered
    telescopes. This time, though, he noticed three small but
    bright stars near the planet, two to one side and one to the
    other. He wondered at how bright the small stars were and how
    they were arranged in what seemed like a straight line. The
    next night he decided to look at Jupiter again, and to his
    surprise, the stars had moved. But not in such a way as he
    would expect. If Jupiter was moving as it normally should, the
    stars would be east of Jupiter. But all three were to the
    west! The next night was cloudy, but on January 10, Galileo
    was able to observe the planet again. One of the stars was
    gone, and the other two were now on the east side. As the
    stars moved around Jupiter on each succeeding night, and were
    finally joined by a fourth star, it dawned on Galileo that
    what he had found were moons orbiting Jupiter!  Almost at once
    he realized that this was proof that not all objects orbited
    the Earth. Here were four bodies that were orbiting
    Jupiter. One argument against the sun-centered theory was, if
    the Earth orbited the sun, wouldn't the moon get left behind?
    Here he could see for himself that somehow, a planet could
    orbit the sun, and not leave a moon behind!

Thus we can plot Galileo's discoveries using the following three plots:

    ./rince 0 $(./dmy2jd  7.8 1 1610)
    ./rince 0 $(./dmy2jd  8.8 1 1610)
    ./rince 0 $(./dmy2jd 10.8 1 1610)

However we can go one better by seeing what Galileo would have seen if it wasn't cloudy on the 9th. Try:

    ./rince 0 $(./dmy2jd 9.85 1 1610)

to see what he was missing out on. I wonder what he'd have made of that!

Galileo discovered the fourth moon a few days later.

I've tried typing these dates into two Java based Jovian moon plotters, but both I tried had problems dealing with dates earlier than 1900. More professional software, such as the SkyMap gets these values correct and this compares well with the above description and with this IOCCC entry.

The year 4713BC was chosen by Joseph Justus Scaliger in the 16th Century, so clearly this program has a long history :-) A more complete description of the origins of Julian Days can be found at https://web.archive.org/web/20001206152200/http://aa.usno.navy.mil/AA/faq/docs/millennium.html.

Assumptions

This program assumes:

1. The ASCII character set is used.

2. That an enormous comet hasn't changed the course of Jupiter (or Earth!).

3. That the date has not been specified as the gap between the Julian and Gregorian calendar.

4. That the DISPLAY environment variable is set and the X display can opened. If this is not the case the program will crash.

5. There is a year 2000BC problem :-)

   More specifically, the plots for dates that far back are too inaccurate to be of realistic use. Comparisons against SkyMap Pro show that there is a good similarity (but not much more) up to about 500BC.

6. The code definitely is not portable - it will produce incorrect results if run from the surface of Mars.

Obfuscations

In addition to the normal obfuscations, I feel that the code contains some less common tricks. Taken through the standard rounds listed in the guidelines, the code attempts to pass these as follows:

1. Look at the original source

   With luck, pure casual inspection should not reveal much except that the program contains the X Windows logo (a big clue) and contains a date. This date may give a clue to a few people that this is an astronomical program.

   Unfortunately I was stretched for space which meant that the large string constant could not be split up using a "partA" "partB" method which clearly limits the code layout. However the space saved by using a single long string constant is more than outweighed by the ability to further obfuscate.

2. convert ANSI trigraphs to ASCII

   These are not used.

3. C pre-process the source ignoring #include lines

4. C pre-process the source ignoring #define and #include lines

   This does a good job of expanding out the code, but it does not clarify things greatly. The use of #defines has been restricted primarily to a space-saving mechanism.

   Additionally, the use of typedefs means that the types used are still obscured unless further replacement edits are made.

5. Run it through a C beautifier.

   GNU indent(1) seems to do a reasonable job, provided that the #defines are expanded first. This doesn't lay the algorithm particularly bare though as most of the obfuscations are not simple code layouts.

6. Examine the algorithm.

   The algorithm operates on two levels. The main visible level is by taking a large string and breaking this down into a series of opcodes (which I have termed J-code). These are then executed.

   To get to the real method of how it works, you need to decode the opcode string (which deliberately contains as many ;, { and } symbols as possible), which will give you a Reverse Polish notation chunk of mathematics.

   Sensible people amongst you will simply refer to the referenced book :-)

7. Lint it.

   I no longer have lint, although I do have access to Purify. I'm happy to say that purify does not detect any memory errors or memory leaks.

   See below for compilation warnings.

8. Compile it.

   I'm sorry to say that there are several warnings issued by the compiler.

   Firstly, the return type for X() is not specified, so it defaults to int. Horror of all horrors - this function does not return an integer type (it doesn't contain any specific returns). The return value from X() is not accessed. The same problem occurs in main() - it does not return. However main() never exits unless there is a serious error.

   The 6th argument of XDrawText (which should be XTextItem) is passed as a compatible structure, but without casting. This is to reduce code space. A more elegant solution is to remove the struct {...} Z completely and use XTextItem f={i-127+r/3*2,r%3+r/3+1,0,E} at the start of XDrawText block. Although this is accepted by several compilers, it is not strict ANSI due to the non-constant initialisers. A pity.

   Note that I make use of usleep(). This is a POSIX call (it is BSD), but it's supported on many systems. It may be implemented using select() or poll() if required, or as a final solution you can change the usleep(10000) to sleep(1). To compile on systems which do not have usleep(), but do have select(), the following may suffice as an additional compiler argument:

       -Dusleep={time_t v[2]={0,10000};select(0,0,0,0,v);}
   

   gcc complains about "unreachable code at beginning of switch statement", although removing some of this code will prevent the algorithm from working.

   gcc also suggests that I should be using extra parenthesis, but I'm taking this as good comment :-)

In addition to the above, the program has the following useful(?) obfuscations:

1. The algorithm requires many floating point numbers (about 50). These are encoded as ASCII.

2. The J-code algorithm has opcodes designed to be space saving. They are all one byte, but many are also considered as whitespace by the rules. This algorithm forms part of the same string that the floating point constants are embedded within. What does the ; command do in J-code?

3. The J-code language is both stack based and variable based. This "uncleanliness" is not ideal from a computer science angle, but when has cleanliness been considered as good in IOCCC? :-)

4. Association of unconnected data by implied function usage. E.g. use of strlen() and sizeof() simply to return useful numerical values, using irrelevant arguments.

5. As we already have a dependency on ASCII this has been abused further. Specifically some integer constants are specified as ASCII and some ASCII constants are integers. This hides the true meaning for the values.

6. Red herrings: there are a couple of pieces of code whose appearance is just there to mislead the infirm of mind. I've only tried to include such examples in places where at first glance the code appears to be executed.

References

---

1. Astronomical algorithms (1st ed): page 59 Jean Meeus. Published by Willmann-Bell Inc. ↩︎

2. Astronomical algorithms (1st ed): pages 285-299. Jean Meeus. Published by Willmann-Bell Inc. ↩︎