use strict;
use warnings;

use feature 'say';

use Digest::CRC;

sub Left_Pad {
    my $value = shift;
    
    if (length($value) % 2) {$value = '0'.$value;}
    return $value;
}

sub Bitwise_Reverse {
    my $value = shift;
    
    $value = Left_Pad($value);
    my $offset = length($value);
    my $reversed;
    while ($offset > 0) {
        $offset -= 2;
        my $string = substr($value,$offset,2);
        $reversed .= $string;
    }
    return $reversed;
}

sub CRC_DNP_ASCII {
    my $value = shift;
    
    my $ctx = Digest::CRC->new(width=>16, init=>0x0, xorout=>0xffff, refin=>1,
                               refout=>1, poly=>0x3d65, cont=>0);
    $ctx->add($value);
    my $digest = $ctx->hexdigest;
    my $crc = Bitwise_Reverse($digest);
    return $crc;
}

sub CRC_DNP_HEX {
    my $value = shift;
    
    my $ctx = Digest::CRC->new(width=>16, init=>0x0, xorout=>0xffff, refin=>1,
                               refout=>1, poly=>0x3d65, cont=>0);
    my $offset = 0;
    while ($offset < length($value)) {
        my $string = substr($value,$offset,2);
        
        $ctx->add(chr(hex($string)));
        $offset += 2;
    }
    my $digest = $ctx->hexdigest;
    my $crc = Bitwise_Reverse($digest);
    return $crc;
}

my @data_list = ('056405F201000000','56405F201000000');
foreach my $data (@data_list) {
    say "Calculating CRC-DNP checksum for '$data':";
    my $ascii = CRC_DNP_ASCII($data);
    say "    ASCII input:  $ascii";
    my $hex = CRC_DNP_HEX($data);
    say "      Hex input:  $hex\n";
}