PC World 2003 March/PCWorld_2003-03_cd.bin/Software/Topware/activeperl/ActivePerl/Perl/lib/Encode/Encoding.pm

Perl POD Document | 2002-06-19 | 7.6 KB | 294 lines

package Encode::Encoding;
# Base class for classes which implement encodings
use strict;
our $VERSION = do { my @r = (q$Revision: 1.30 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r };

require Encode;

sub Define
{
    my $obj = shift;
    my $canonical = shift;
    $obj = bless { Name => $canonical },$obj unless ref $obj;
    # warn "$canonical => $obj\n";
    Encode::define_encoding($obj, $canonical, @_);
}

sub name         { return shift->{'Name'} }
sub new_sequence { return $_[0] }

sub needs_lines { 0 };

sub perlio_ok {
    eval{ require PerlIO::encoding };
    return $@ ? 0 : 1;
}

# Temporary legacy methods
sub toUnicode    { shift->decode(@_) }
sub fromUnicode  { shift->encode(@_) }

#
# Needs to be overloaded or just croak
#

sub encode {
    require Carp;
    my $obj = shift;
    my $class = ref($obj) ? ref($obj) : $obj;
    Carp::croak $class, "->encode() not defined!";
}

sub decode{
    require Carp;
    my $obj = shift;
    my $class = ref($obj) ? ref($obj) : $obj;
    Carp::croak $class, "->encode() not defined!";
}

sub DESTROY {}

1;
__END__

=head1 NAME

Encode::Encoding - Encode Implementation Base Class

=head1 SYNOPSIS

  package Encode::MyEncoding;
  use base qw(Encode::Encoding);

  __PACKAGE__->Define(qw(myCanonical myAlias));

=head1 DESCRIPTION

As mentioned in L<Encode>, encodings are (in the current
implementation at least) defined as objects. The mapping of encoding
name to object is via the C<%Encode::Encoding> hash.  Though you can
directly manipulate this hash, it is strongly encouraged to use this
base class module and add encode() and decode() methods.

=head2 Methods you should implement

You are strongly encouraged to implement methods below, at least
either encode() or decode().

=over 4

=item -E<gt>encode($string [,$check])

MUST return the octet sequence representing I<$string>.

=over 2

=item *

If I<$check> is true, it SHOULD modify I<$string> in place to remove
the converted part (i.e.  the whole string unless there is an error).
If perlio_ok() is true, SHOULD becomes MUST.

=item *

If an error occurs, it SHOULD return the octet sequence for the
fragment of string that has been converted and modify $string in-place
to remove the converted part leaving it starting with the problem
fragment.  If perlio_ok() is true, SHOULD becomes MUST.

=item *

If I<$check> is is false then C<encode> MUST  make a "best effort" to
convert the string - for example, by using a replacement character.

=back

=item -E<gt>decode($octets [,$check])

MUST return the string that I<$octets> represents.

=over 2

=item *

If I<$check> is true, it SHOULD modify I<$octets> in place to remove
the converted part (i.e.  the whole sequence unless there is an
error).  If perlio_ok() is true, SHOULD becomes MUST.

=item *

If an error occurs, it SHOULD return the fragment of string that has
been converted and modify $octets in-place to remove the converted
part leaving it starting with the problem fragment.  If perlio_ok() is
true, SHOULD becomes MUST.

=item *

If I<$check> is false then C<decode> should make a "best effort" to
convert the string - for example by using Unicode's "\x{FFFD}" as a
replacement character.

=back

=head2 Other methods defined in Encode::Encodings

You do not have to override methods shown below unless you have to.

=over 4

=item -E<gt>name

Predefined As:

  sub name  { return shift->{'Name'} }

MUST return the string representing the canonical name of the encoding.

=item -E<gt>new_sequence

Predefined As:

  sub new_sequence { return $_[0] }

This is a placeholder for encodings with state. It should return an
object which implements this interface.  All current implementations
return the original object.

=item -E<gt>perlio_ok()

Predefined As:

  sub perlio_ok {
      eval{ require PerlIO::encoding };
      return $@ ? 0 : 1;
  }

If your encoding does not support PerlIO for some reasons, just;

 sub perlio_ok { 0 }

=item -E<gt>needs_lines()

Predefined As:

  sub needs_lines { 0 };

If your encoding can work with PerlIO but needs line buffering, you
MUST define this method so it returns true.  7bit ISO-2022 encodings
are one example that needs this.  When this method is missing, false
is assumed.

=back

=head2 Example: Encode::ROT13

  package Encode::ROT13;
  use strict;
  use base qw(Encode::Encoding);

  __PACKAGE__->Define('rot13');

  sub encode($$;$){
      my ($obj, $str, $chk) = @_;
      $str =~ tr/A-Za-z/N-ZA-Mn-za-m/;
      $_[1] = '' if $chk; # this is what in-place edit means
      return $str;
  }

  # Jr pna or ynml yvxr guvf;
  *decode = \&encode;

  1;

=head1 Why the heck Encode API is different?

It should be noted that the I<$check> behaviour is different from the
outer public API. The logic is that the "unchecked" case is useful
when the encoding is part of a stream which may be reporting errors
(e.g. STDERR).  In such cases, it is desirable to get everything
through somehow without causing additional errors which obscure the
original one. Also, the encoding is best placed to know what the
correct replacement character is, so if that is the desired behaviour
then letting low level code do it is the most efficient.

By contrast, if I<$check> is true, the scheme above allows the
encoding to do as much as it can and tell the layer above how much
that was. What is lacking at present is a mechanism to report what
went wrong. The most likely interface will be an additional method
call to the object, or perhaps (to avoid forcing per-stream objects
on otherwise stateless encodings) an additional parameter.

It is also highly desirable that encoding classes inherit from
C<Encode::Encoding> as a base class. This allows that class to define
additional behaviour for all encoding objects.

  package Encode::MyEncoding;
  use base qw(Encode::Encoding);

  __PACKAGE__->Define(qw(myCanonical myAlias));

to create an object with C<< bless {Name => ...}, $class >>, and call
define_encoding.  They inherit their C<name> method from
C<Encode::Encoding>.

=head2 Compiled Encodings

For the sake of speed and efficiency, most of the encodings are now
supported via a I<compiled form>: XS modules generated from UCM
files.   Encode provides the enc2xs tool to achieve that.  Please see
L<enc2xs> for more details.

=head1 SEE ALSO

L<perlmod>, L<enc2xs>

=begin future

=over 4

=item Scheme 1

The fixup routine gets passed the remaining fragment of string being
processed.  It modifies it in place to remove bytes/characters it can
understand and returns a string used to represent them.  For example:

 sub fixup {
   my $ch = substr($_[0],0,1,'');
   return sprintf("\x{%02X}",ord($ch);
 }

This scheme is close to how the underlying C code for Encode works,
but gives the fixup routine very little context.

=item Scheme 2

The fixup routine gets passed the original string, an index into
it of the problem area, and the output string so far.  It appends
what it wants to the output string and returns a new index into the
original string.  For example:

 sub fixup {
   # my ($s,$i,$d) = @_;
   my $ch = substr($_[0],$_[1],1);
   $_[2] .= sprintf("\x{%02X}",ord($ch);
   return $_[1]+1;
 }

This scheme gives maximal control to the fixup routine but is more
complicated to code, and may require that the internals of Encode be tweaked to
keep the original string intact.

=item Other Schemes

Hybrids of the above.

Multiple return values rather than in-place modifications.

Index into the string could be C<pos($str)> allowing C<s/\G...//>.

=back

=end future

=cut