use strict;
use warnings;

use MCE::Loop;
use MCE::Candy;

my $input_file  = shift || 'input.txt';
my $output_file = shift || 'output.txt';

open my $ofh, ">", $output_file
   or die "cannot open '$output_file' for writing: $!\n";

MCE::Loop::init {
   use_slurpio => 1, chunk_size => 400, max_workers => 4,
   gather => MCE::Candy::out_iter_fh($ofh),
   RS => "\nINTERPOLATED HYDROGRAPH",
};

## Each worker receives many records determined by chunk_size.
## Output order is preserved via MCE::Candy::out_iter_fh

mce_loop_f {
   my ( $mce, $chunk_ref, $chunk_id ) = @_;

   open my $ifh, "<", $chunk_ref;
   my $output = "";

   while ( my $line = <$ifh> ) {
      chomp $line; # remove newline character from end of line
      if ( $line =~ /INTERPOLATED HYDROGRAPH AT (\w+)$/ ) {
         $output .= $1;
         $line = <$ifh> for 1..6; # skip 5 lines
         my $val2 = (split / /, $line)[1]; # get the second column
         $output .= " $val2";
         $line = <$ifh> for 1..2; # skip one line
         chomp $line;
         my $val3 = (split / /, $line)[-1]; # get the last column
         $output .= " $val3\r\n";
      }
   }

   close $ifh;

   MCE->gather( $chunk_id, $output );

} $input_file;

close $ofh;