initself has asked for the wisdom of the Perl Monks concerning the following question:

Venerable Monks,

I am using MP3::Tag for a very simple operation: parsing the id3v2 tag format into a %02d format for use with id3v1, like so:

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;

use MP3::Tag;

my $filename = $ARGV[0];
die "Usage: $0 filename" unless $filename;

my $mp3 = MP3::Tag->new($filename);
$mp3->get_tags();
my $id3v1 = $mp3->{ID3v1} if exists $mp3->{ID3v1};
my $id3v2 = $mp3->{ID3v2} if exists $mp3->{ID3v2};

# Convert id3v2 track format to id3v1
my $track = $id3v2->track;
$track = sprintf( "%02d", (split(/\//, $track))[0] );
$id3v1->track($track);

$id3v1->write_tag();

[download]

Dumping $id3v1 right before the write_tag function shows the track field populated with "01". Dumping the tag after the write shows "1". I have torn apart both MP3::Tag and MP3::Tag::ID3v1 and I just cannot find anything that is truncating this data, althought MP3::Tag does refer to it:

2-digit numbers, or I<number1/number2> with number1,2 up to 999 are allowed for the track number (the leading 0 is stripped);

I just can't find where that is actually implemented! Can anyone help me?

Replies are listed 'Best First'.
Re: MP3::Tag - Leading zeros on 'track' truncated
by martin (Friar) on Jan 17, 2008 at 11:50 UTC
    The track number available in an ID3v1 tag is a small unsigned integer number stored in a single byte. If you retrieve this information once MP3::Tag has stored it in such a tag you will only get a numerical value back, without fancy formatting.

    The documentation you quote refers to parse_rex() -- a method not related to your code.

    If you want to find out about the conversion from a Perl scalar holding some numerical value into a raw byte look out for the pack function.

    Example: 

    $x = pack 'C', 65;
print "$x\n";  # prints the letter A
$n = unpack 'C', $x;
print "$n\n";  # prints the number 65

    [download]
[reply]
[d/l]
Re: MP3::Tag - Leading zeros on 'track' truncated
by Ryszard (Priest) on Jan 17, 2008 at 10:38 UTC
    perhaps its not explicitly stripped, it could be that the truncation you see is a result of a math operation..?
[reply]
      Quite possibly as a result of getting the next track number. It's easy to use math ($t=$t+1) instead of the increment operator ($t++) causing the leading zeroes to be removed. Example:

      $ perl -e 'my $t="00";print ++$t'
01
$ perl -e 'my $t="00";print $t+1'
1

      [download]
      ...roboticus
[reply]
[d/l]
Re: MP3::Tag - Leading zeros on 'track' truncated
by Anonymous Monk on Jan 17, 2008 at 20:22 UTC
    I think you meant to say "venerable," not veritable. Not that I am monk or anything though ...
[reply]

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