#!/usr/bin/perl -w

## Demonstration of event-driven interaction with a subprocess

## Event driven programming is a pain.  This code is not that readable
## and is not a good place to start, especially since few people (including
## me) are familiar with bc's nuances.

use strict ;

use IPC::Run qw( run ) ;

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 ;

## bc instructions to initialize two variables and print one out
my $stdin_queue = "a = i = $i ; i\n" ;

## Note the FALSE on failure result (opposite of system()).
die $! unless run(
   ['bc'],
   sub {
      ## Consume all input and return it.  This is used instead of a plain
      ## scalar because run() would close bc's stdin the first time the
      ## scalar emptied.
      my $r = $stdin_queue ;
      $stdin_queue = '' ;
      return $r ;
   },
   sub {
      my $out = shift ;
      print "bc said: ", $out ;

      if ( $out =~ s/.*?(\d+)\n/$1/g ) {
         ## Grab the number from bc.  Assume all numbers are delivered in
	 ## single chunks and all numbers are significant.
         if ( $out > $i ) {
	    ## i! is always >i for i > 0
	    print "result = ", $out, "\n" ;
	    $stdin_queue = undef ;
	 }
         elsif ( $out == '1' ) {
	    ## End of calculation loop, get bc to output the result.
	    $stdin_queue = "a\n" ;
	 }
	 else {
	    ## get bc to calculate the next iteration and print it out.
	    $stdin_queue = "i = i - 1 ; a = a * i ; i\n" ;
	 }
      }
   },
) ;