# 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.