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 # 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 ;-)
+
+## WTF?
+
+Postscript is a Turing-complete language. This means that you can do 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 after every run.  In a word,
+`psrw.ps` rewrites itself at each run, changing the seed and allowing to
+generate a _new_ random walk trajectory every time you open the file. 
+
+## Why?
+
+Well, there is no 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.  The simplicity of the solution implemented in `psrw.ps`
+scratches a long-standing personal itch, and tells a lot about my poor
+knowledge of Postscript...
+
+## No really, 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
+not documented.  Many _text_ formats out there are Turing-complete or
+close-to, and some visualiser (e.g., for PDF or OpenXML) include
+interpreters for other Turing-complete languages (like Javascript or
+VBScript). This mean that they can do almost anything when you "_open_"
+those "_text_" files. 
+
+If it's so easy to craft a document that modifies itself to change a
+comment that you can't visualise, what else can be done by "_text_"
+files saved in proprietary formats?
+
+## Links
+
+- A brief [summary of Poscript
+  commands][http://www.math.ubc.ca/~cass/courses/ps.html]
+
+- A [game of life][https://www.tjhsst.edu/~edanaher/pslife/] written in
+Poscript
+
+- An interesting 
+  [proof][https://nixwindows.wordpress.com/2018/03/13/ed1-is-turing-complete/ 
+] showing that [ed, the standard text
+  editor][http://wiki.c2.com/?EdIsTheStandardTextEditor] 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.