#From $From_D2s to $To_D2s:
#- Added 99 to all skills
use warnings;
use strict;

#<config>
my $From_D2s = 'my_c.d2s';
my $To_D2s = 'my_d.d2s';
#</config>





my ($Letter_change) = $To_D2s =~ m#my_([a-z]).d2s#;

#Open for reading the original D2S file.
open(my ${f1}, '<', $From_D2s) or die $!;

sysseek(${f1}, 767, 0);
sysread(${f1}, my $s, 400);
my $skill_loc = index($s, 'if');
#print $skill_loc, "\n"; #40
$skill_loc += 767;
sysseek(${f1}, 0, 0);

#First calculate checksum
my $c = 0;  #checksum
my $loc = 0;
while(sysread(${f1}, my $s, 1)){
  my ($byte) = unpack('C', $s);
  if($loc == 12 || $loc == 13 || $loc == 14 || $loc == 15){
    $byte = 0;
  }
  elsif($loc == 23){  #my_d
    $byte = ord($Letter_change); 
  }
  elsif($loc >= $skill_loc + 2 && $loc < $skill_loc + 32){
    $byte = 99;
  }
  if($c < 0){
    $byte++;
  }
  $c = $byte + $c * 2;
  undef $byte;
  ($c) = unpack('l', pack('l', $c));
  $loc++;
}

#Now create my_b.d2s with the new checksum and new name.
$loc = 0;
sysseek(${f1}, 0, 0);
open(my ${f2}, '>', $To_D2s) or die $!;
while(sysread(${f1}, my $s, 1)){
  if($loc == 12){ #insert the checksum.
    syswrite(${f2}, pack('L', $c), 4);
    $loc++;
    next;
  }
  elsif($loc == 13 || $loc == 14 || $loc == 15){
    $loc++;
    next;
  }
  elsif($loc == 23){ #insert the name.
    syswrite(${f2}, $Letter_change, 1);
    $loc++;
    next;
  }
  elsif($loc >= $skill_loc + 2 && $loc < $skill_loc + 32){
    syswrite(${f2}, pack('C', 99), 1);
    $loc++;
    next;
  }
  syswrite(${f2}, $s, 1);
  $loc++;
}
undef $loc;
undef $c;
close ${f1};
close ${f2};

print <<EOF;
From $From_D2s to $To_D2s:
- Added 99 to all skills
EOF