3.5 KiB
psrw
2D radom walk in Postscript
This is a simple Postscript hack to visualise a 2D random walk. The
interesting part is that you get a different trajectory of the random
walk every time you open the file psrw.ps.
You can open the file with gv:
gv -nosafer -nosafedir psrw.ps
where the extra options are needed to disable some security checks. You can also visualise the document with Ghostscript, if you like:
gs psrw.ps
Liked it? Now close the file and reopen it ;-)
Hint 1: If you enable the "Watch File" option in gv, you will get a
nice slideshow, for some definition of nice.
Hint 2: Running gv with -nosafer -nodirsafe might be a very bad
idea.
WTF?
Postscript is a Turing-complete language. This means that you can perform any feasible computation in Postscript. Hence, simulating a random walk in Postscript is not a big fuss at all, also because the standard Postscript definition already includes a pseudo-random number generator, so you don't need to implement it yourself. The only problem is that the pseudo-random number generator needs to be initialised with a new seed, otherwise you would always visualise the same trajectory.
The simple solution implemented in psrw.ps is to store the seed in the
same file as a comment, and update it at each run. In practice,
psrw.ps rewrites a slightly modified copy of itself every time you
"view" it, but a user would hardly notice it :-)
Why?
Well, you don't need a particular reason to write anything like
psrw.ps. I just tried to do something similar around 2001 or 2002,
when I was using Postscript quite heavily, and at that time I did not
find a proper way through. So the simplicity of the solution
implemented in psrw.ps scratches a long-standing personal itch, and
tells a lot about my very poor understanding of Postscript...
No seriously, WHY?
I just wanted to make a point about (not) trusting documents written in formats that you don't understand, or that are not freely accessible or are poorly or not documented.
Many text formats out there are Turing-complete or close-to, and some viewers (e.g., for PDF or OpenXML files) include interpreters for other Turing-complete languages (like Javascript or VBScript). This mean that these viewers can do almost anything when you "open" those "text" files. The only chance you have is to understand what is going on behind the scenes, or to trust the company that provided the smart viewer. But can you really trust them?
If it was so easy for a Postscript illiterate like me to craft a document that modifies itself by changing something that you cannot even visualise, what else can be done by "text" files saved in proprietary formats?
Well, at this point you should start thinking that you cannot really
trust me either, even if I sweared that psrw.ps does absolutely
nothing nasty when you "open" it. But how can you be sure I am telling
the truth? ;-)
Links
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A brief summary of Poscript commands
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A game of life written in Poscript
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An interesting proof showing that ed, the standard text editor is indeed Turing-complete
License
This program is (c) 2018 Vincenzo (KatolaZ) Nicosia. You can use, modify, and redistribute it under the terms of the GNU General Public License, either Version 3 of the License or, at your option, any later version.