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=head2 is_control_field()

Tells whether this field is one of the control tags from 001-009.

=cut

sub is_control_field {
    my $self = shift;
    return $self->{_is_control_field};
}

=head2 subfield(code)

When called in a scalar context returns the text from the first subfield
matching the subfield code.

    my $subfield = $field->subfield( 'a' );

Or if you think there might be more than one you can get all of them by
calling in a list context:

    my @subfields = $field->subfield( 'a' );

If no matching subfields are found, C<undef> is returned in a scalar context
and an empty list in a list context.

If the tag is a control field, C<undef> is returned and
C<$MARC::Field::ERROR> is set.

=cut

sub subfield {
    my $self = shift;
    my $code_wanted = shift;

    croak( "Control fields (generally, just tags below 010) do not have subfields, use data()" )
        if $self->is_control_field;

    my @data = @{$self->{_subfields}};
    my @found;
    while ( defined( my $code = shift @data ) ) {
        if ( $code eq $code_wanted ) {
            push( @found, shift @data );
        } else {
            shift @data;
        }
    }
    if ( wantarray() ) { return @found; }
    return( $found[0] );
}

=head2 subfields()

Returns all the subfields in the field.  What's returned is a list of
list refs, where the inner list is a subfield code and the subfield data.

For example, this might be the subfields from a 245 field:

        (
          [ 'a', 'Perl in a nutshell :' ],
          [ 'b', 'A desktop quick reference.' ],
        )

=cut

sub subfields {
    my $self = shift;

    if ($self->is_control_field) {
        $self->_warn( "Control fields (generally, just tags below 010)  do not have subfields" );
        return;
    }

    my @list;
    my @data = @{$self->{_subfields}};
    while ( defined( my $code = shift @data ) ) {
        push( @list, [$code, shift @data] );
    }
    return @list;
}

=head2 data()

Returns the data part of the field, if the tag number is less than 10.

=cut

sub data {
    my $self = shift;

    croak( "data() is only for control fields (generally, just tags below 010) , use subfield()" )
        unless $self->is_control_field;

    $self->{_data} = $_[0] if @_;

    return $self->{_data};
}

=head2 add_subfields(code,text[,code,text ...])

Adds subfields to the end of the subfield list.

    $field->add_subfields( 'c' => '1985' );

Returns the number of subfields added, or C<undef> if there was an error.

=cut

sub add_subfields {
    my $self = shift;

    croak( "Subfields are only for data fields (generally, just tags >= 010)" )
        if $self->is_control_field;

    push( @{$self->{_subfields}}, @_ );
    return @_/2;
}

=head2 delete_subfield()

delete_subfield() allows you to remove subfields from a field: