use strict;
use warnings;
use Math::Pari;

my $STRIDE = 8192;  # pulled out of a hat

mult( @ARGV );

sub mult {
  my ( $infile, $div, $rem ) = @_;
  my $n       = PARI $div;
  my $carry   = PARI $rem;
  my $outfile = "$infile.$$";
  open my $in,  '<', $infile  or die "Can't read $infile: $!\n";
  open my $out, '>', $outfile or die "Can't write to $outfile: $!\n";
  {
    defined( sysread $in, my $chunk, $STRIDE )
      or die "sysread error: $!\n";
    last unless length $chunk;
    my $num = ( $n * PARI( scalar reverse $chunk ) + $carry )->pari_print;

    print $out scalar reverse substr( $num, -$STRIDE, $STRIDE, '' );
    $carry = PARI( $num || 0 );
    redo;
  }
  ( my $last = $carry->pari_print ) =~ s/^0+//;
  print $out scalar reverse $last;
  close $in;
  close $out;
}
__END__