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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.
diff --git a/psrw.ps b/psrw.ps
new file mode 100644
index 0000000..41a430a
--- /dev/null
+++ b/psrw.ps
@@ -0,0 +1,105 @@
+%% 13331 
+%%
+%% Visualise a different 2D random walk every time the file is 
+%% opened
+%%
+%% (c) 2018 Vincenzo (KatolaZ) Nicosia <katolaz@freaknet.org>
+%%
+%% Use and distribute under the terms of the GNU General Public Licence
+%% Version 3 or, at your option, any later version.
+%%
+
+/myname (./psrw.ps) def
+/rwstartx 250 def
+/rwstarty 400 def
+/rwlength 500 def
+/rwstep 8 def
+
+/myreadlines {
+  dup 
+  255 string
+  readline
+  { 2 -1 roll myreadlines} {pop exit} ifelse
+} def
+
+/myloadfile {
+  (r) file 
+  {myreadlines} loop 
+  closefile 
+} def
+
+/mywritelines{
+    count
+    1 ne
+    {
+      dup 
+      count -1 roll
+      writestring
+      dup
+      (\n) writestring
+      mywritelines
+     } 
+     {exit} ifelse
+
+} def
+
+/mywritefile {
+ (w) file
+ {mywritelines} loop
+ closefile
+} def
+
+/replacefirst {
+  count -1 roll
+  pop
+  count 1 roll
+} def
+
+
+/get_seed {
+  myname (r) file
+  dup
+  255 string  
+  readline
+  pop
+  (%% ) anchorsearch
+  {pop cvi} {pop 201} ifelse 
+  exch closefile
+} def
+
+%% This comes from GNU Ghostscript
+/concatstrings % (a) (b) -> (ab)  
+   { exch dup length    
+     2 index length add string    
+     dup dup 4 2 roll copy length
+     4 -1 roll putinterval
+   } bind def 
+
+/rndangle { rand 360 mod 2 -1 roll pop } def
+
+/newrndpoint { 
+  rndangle 
+  dup 
+  cos rwstep mul 
+  2 -1 roll 
+  sin rwstep mul 
+} def
+
+erasepage
+
+newpath
+rwstartx rwstarty moveto
+2 setlinewidth
+
+get_seed
+
+srand
+
+1 1 rwlength { newrndpoint rlineto } for
+stroke
+
+myname myloadfile
+(%% ) rand 30 string cvs concatstrings 
+replacefirst
+
+myname mywritefile