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Path: utzoo!linus!philabs!mcvax!steven
From: steven@mcvax.UUCP (Steven Pemberton)
Newsgroups: net.lang.c
Subject: Re: palindromic C programs
Message-ID: <5953@mcvax.UUCP>
Date: Mon, 3-Sep-84 11:45:31 EDT
Article-I.D.: mcvax.5953
Posted: Mon Sep  3 11:45:31 1984
Date-Received: Thu, 13-Sep-84 04:59:32 EDT
References: <155@ihnet.UUCP> <276@metheus.UUCP>
Organization: CWI, Amsterdam
Lines: 21

> Are there any programs out there that when program
> 'foo' is executed it creates source for program 'bar' and vice-
> versa?  The result would be a bistable self generating program.
> And if bistable is possible, how about tristable, etc.?

In issue 47 of the Algol Bulletin, Chris Thomson of the Chion Corporation
presented, apart from a self-replicating Algol 68 program, ones that
replicated after 2 cycles (thus a=>b=>a), 3 cycles, 9 cycles, and then
finally a program that replicates itself after a random-length cycle of
expected length 1000!

For those of you interested, here is that last one:

(.INT s:=    0,e:=123;.STRING a="(.INT s:=    0,e:=123;
  .STRING a="";s:=(s*9+7)%*1000000;print(2*(a[:9]+whole(
  (s%1000=e|0|s),5)+a[15:33]+2*a[33:]))";s:=(s*9+7)%*100000;
  print(2*(a[:9]+whole((s%100=e|0|s),-5)+a[15:33])+2*a[33:]))

Subscripts can be used to get a substring of a string, eg a[15:33].  a[33:]
means from 33 to the end. % is integer division, %* is mod, (b|v1|v2) is the
same as C's e?v1:v2 and whole converts a value to a string.