#!/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";
            }
        }
    },
);