Long winter evenings

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From	Richard Heathfield <rjh@cpax.org.uk>
Newsgroups	comp.programming
Subject	Long winter evenings
Date	Mon, 4 Nov 2019 17:37:09 +0000
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I have just written a program in C, which for reasons that will remain 
shrouded in the mists of wossname I wanted to be as short as possible. 
The program solves a highly specific and relatively antiquated problem.

Now that the nights are drawing in, some of you might be at a loose end 
and wish for something to help you pass the time.

So here's the puzzle, which is in two parts:

(1) What, *exactly*, does the program do? That is, what is the 
significance of the printed result?

(2) Is there a way to make the program shorter, without compromising its 
rigorous adherence to ISO C90? (See below.)

#include <stdio.h>
int r(int*c){int f=0;if(*c%5==1)*c/= 5,*c*=4,f=1;return f;}
int t(int c){int f=1,i;for(i=0;f&&i<5;i++)f=r(&c);return f*!(c%5);}
int main(void){int n=0,d=0;while(!d)d=t(++n);printf("%d\n",n);return 0;}

Four obvious shortenings are excluded:
A. Lose the space between "#include" and "<stdio.h>";
B. Lose the second and third newlines;
C. Lose "void";
D. Lose "return 0;"

If nobody gets it within a week or so, I'll post the solution to (1). 
(If I could think of a solution to (2), I'd have written it that way to 
start with!)

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within

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Long winter evenings Richard Heathfield <rjh@cpax.org.uk> - 2019-11-04 17:37 +0000
  Re: Long winter evenings David Brown <david.brown@hesbynett.no> - 2019-11-04 22:39 +0100
    Re: Long winter evenings Richard Heathfield <rjh@cpax.org.uk> - 2019-11-04 21:51 +0000
      Re: Long winter evenings David Brown <david.brown@hesbynett.no> - 2019-11-05 10:42 +0100
    Re: Long winter evenings Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 22:24 +0000
  Re: Long winter evenings Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 22:19 +0000
    Re: Long winter evenings Richard Heathfield <rjh@cpax.org.uk> - 2019-11-04 22:30 +0000
      Re: Long winter evenings Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-04 23:59 +0000
        Re: Long winter evenings Richard Heathfield <rjh@cpax.org.uk> - 2019-11-05 01:03 +0000
          Re: Long winter evenings Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-05 04:01 +0000
            Re: Long winter evenings Richard Heathfield <rjh@cpax.org.uk> - 2019-11-05 07:42 +0000
  Re: Long winter evenings Melzzzzz <Melzzzzz@zzzzz.com> - 2019-11-04 23:00 +0000
  Re: Long winter evenings Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-06 04:58 -0800

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