#!/usr/bin/perl -w

## Demonstration using a pipe to send input to a child process

use strict ;

use IPC::Run qw( start pump finish ) ;

die "usage: $0 <num>\n\nwhere <num> is a positive integer\n" unless @ARGV ;
my $i = shift ;
die "\$i must be > 0, not '$i'" unless $i =~ /^\d+$/ && $i > 0 ;

my $out ;

my $h = start ['bc'], '<pipe', \*IN, '>', \$out ;
my $tmp = select IN ; $|= 1 ; select $tmp ;

print IN "a = i = $i ; i\n" ;

while () {
   $out = '' ;
   pump $h until $out =~ s/.*?(\d+)\n/$1/g ;
   print "bc said: $out\n" ;

   if ( $out > $i ) {
      ## i! is always >i for i > 0
      print "result = ", $out, "\n" ;
      close( IN ) ;
      last ;
   }
   elsif ( $out == '1' ) {
      ## End of calculation loop, get bc to output the result
      print IN "a\n" ;
   }
   else {
      print IN "i = i - 1 ; a = a * i ; i\n" ;
   }
}

finish $h ;