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        return $class->$orig(flag => $_[0], label => $_[1], id => $_[2], id2 => $_[3]);
    }
    return $class->$orig(@_);
};
my @order = qw/flag label id id2/;
use overload '@{}' => sub { my $self = shift; [map { $self->$_ } @order] };

package AI::MXNet::RecordIO;

=head2 unpack

    unpack a MXImageRecord to a string

    Parameters
    ----------
    s : str
        string buffer from MXRecordIO.read

    Returns
    -------
    header : AI::MXNet::IRHeader
        header of the image record
    s : str
        unpacked string
=cut

method unpack(Str $s)
{
    my $h;
    my $h_size = 24;
    ($h, $s) = (substr($s, 0, $h_size), substr($s, $h_size));
    my $header = AI::MXNet::IRHeader->new(unpack('IfQQ', $h));
    if($header->flag > 0)
    {
        my $label;
        ($label, $s) = (substr($s, 0, 4*$header->flag), substr($s, 4*$header->flag));
        my $pdl_type = PDL::Type->new(DTYPE_MX_TO_PDL->{float32});
        my $pdl = PDL->new_from_specification($pdl_type, $header->flag);
        ${$pdl->get_dataref} = $label;
        $pdl->upd_data;
        $header->label($pdl);
    }
    return ($header, $s)
}

=head2 pack

    pack a string into MXImageRecord

    Parameters
    ----------
    $header : AI::MXNet::IRHeader or ArrayRef suitable for AI::MXNet::IRHeader->new(@{ ArrayRef })
        header of the image record.
        $header->label can be a number or an array ref.
    s : str
        string to pack
=cut

method pack(AI::MXNet::IRHeader|ArrayRef $header, Str $s)
{
    $header = AI::MXNet::IRHeader->new(@$header) unless blessed $header;
    if(not ref $header->label)
    {
        $header->flag(0);
    }
    else
    {
        my $label = AI::MXNet::NDArray->array($header->label, dtype=>'float32')->aspdl;
        $header->label(0);
        $header->flag($label->nelem);
        my $buf = ${$label->get_dataref};
        $s = "$buf$s";
    }
    $s = pack('IfQQ', @{ $header }) . $s;
    return $s;
}

package AI::MXNet::IndexedRecordIO;
use Mouse;
use AI::MXNet::Base;
extends 'AI::MXNet::RecordIO';

=head1 NAME

    AI::MXNet::IndexedRecordIO - Read/write RecordIO format data supporting random access.
=cut

=head2 new

    Parameters
    ----------
    idx_path : str
        Path to index file
    uri : str
        Path to record file. Only support file types that are seekable.
    flag : str
        'w' for write or 'r' for read
=cut

has 'idx_path'  => (is => 'ro', isa => 'Str', required => 1);
has [qw/idx
    keys fidx/] => (is => 'rw', init_arg => undef);

method open()
{
    $self->SUPER::open();
    $self->idx({});
    $self->keys([]);
    open(my $f, $self->flag eq 'r' ? '<' : '>', $self->idx_path);
    $self->fidx($f);
    if(not $self->writable)
    {
        while(<$f>)
        {
            chomp;
            my ($key, $val) = split(/\t/);
            push @{ $self->keys }, $key;
            $self->idx->{$key} = $val;
        }
    }
}