use strict;
use warnings;
use Math::Pari;
use List::Util 'min';
use Fcntl qw( SEEK_SET SEEK_END );

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";

  my $in_offset  = sysseek $in, 0, SEEK_END or die "sysseek error: $!\
+n";
  my $out_offset = $in_offset + int( ( log $n )/log 10 );

  my $to_print;
  {
    my $to_read = min( $STRIDE, $in_offset );
    $in_offset -= $to_read;

    sysseek $in, $in_offset, SEEK_SET or die "sysseek error: $!\n";
    my $chunk = '';
    while ( $to_read ) {
      my $read = sysread( $in, my $buffer, $to_read );
      die "sysread error: $!\n" unless defined $read;
      $chunk .= $buffer;
      $to_read -= $read;
    }
    my $num = ( $n * PARI( $chunk ) + $carry )->pari_print;

    my $batch = substr( $num, -$STRIDE, $STRIDE, '' );
    $to_print = length $batch;
    die "internal error\n" if $to_print > $out_offset;
    $out_offset -= $to_print;
    seek $out, $out_offset, SEEK_SET or die "sysseek error: $!\n";

    print $out $batch;

    $carry = PARI( $num || 0 );

    last if $in_offset == 0;
    redo;
  }

  ( my $last = $carry->pari_print ) =~ s/^0+//;
  die if $out_offset != length $last;
  sysseek $out, 0, SEEK_SET or die "sysseek error: $!\n";
  print $out $last;

  close $in;
  close $out;
}
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