use warnings;
use strict;
use File::Copy;
use feature 'say';
use myconfig qw/read_config/;

my $D2sDataFile = read_config();

if(! -f $D2sDataFile){
  die "$D2sDataFile does not exist! Where can we get the checksum?";
}

my $all;
{
  local $/;
  open(my ${f}, '<', $D2sDataFile);
  $all = <$f>;
  close(${f});
}

#say 'in file checksum: ', +(unpack('H8', &get_actual_checksum()))[0];
say 'in file checksum: ', uc(&get_actual_checksum());
say 'my checksum:      ', sprintf('%08X', &get_your_checksum());

sub get_actual_checksum(){
  #CORE::say +(unpack('H32', $all))[0];
  #return pack('H*', substr((unpack('H32', $all))[0], 24, 8));
  return unpack('H*', substr($all, 12, 4));
}

sub get_your_checksum(){
  my $c = 0;
  my $addr = 0;
  my @cs = unpack('C*', $all);
  for my $ct(@cs){
    $ct += $c >> 31 & 1;
    $c = (($addr >> 2 == 3) ? 0 : $ct) + ($c << 1);
    $addr++;
  }
  return ($c & 0xff000000) >> 24 | ($c & 0xff0000) >> 8 | ($c & 0xff00) << 8 | ($c & 0xff) << 24;
}