Astronomically Obfuscated:
James Bonfield
MRC Laboratory of Molcular Biology
Hills Road,
Cambridge
CB2 2QH
England
Judges' Comments:
To build:
make rince
To run (using an X windows server):
./rince
Also try:
make dmy2jd
./rince 0.001 `./dmy2jd 7.8 1 1610`
This program may have you going around in circles :-) Try running it
first before reading the Selected Author's Comments below.
The judges verified the correctness of output for recent dates
thru visual observation.
Selected Author's Comments:
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). Eg:
./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] 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.
"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 in to 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 program [3] 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:
http://aa.usno.navy.mil/AA/faq/docs/millennium.html
Assumptions
-----------
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 [3] 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 ability to further obfuscate.
2. convert ANSI tri-graphs 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 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
op-codes (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 op-code 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 supported on many systems. It may be implemented
using select() or poll() if required, or as a final solution you
may 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 op-codes designed to be space
saving. They are all one byte, but many are also considered as
white-space 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. Eg
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's a couple of pieces of code whose
appearance is just there to mislead the unfirm 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.
[3] Skymap Pro, by Chris Marriott
http://www.skymap.com/