package Encode::Unicode;

use strict;

use warnings;

our $VERSION = do { my @r = ( q$Revision: 2.17 $ =~ /\d+/g ); sprintf "%d." . "%02d" x $#r, @r };

use XSLoader;

XSLoader::load( __PACKAGE__, $VERSION );

#

# Object Generator 8 transcoders all at once!

#

use Encode ();

our %BOM_Unknown = map { $_ => 1 } qw(UTF-16 UTF-32);

for my $name (

    qw(UTF-16 UTF-16BE UTF-16LE

    UTF-32 UTF-32BE UTF-32LE

    UCS-2BE  UCS-2LE)

  )

{

    my ( $size, $endian, $ucs2, $mask );

    $name =~ /^(\w+)-(\d+)(\w*)$/o;

    if ( $ucs2 = ( $1 eq 'UCS' ) ) {

        $size = 2;

    }

    else {

        $size = $2 / 8;

    }

    $endian = ( $3 eq 'BE' ) ? 'n' : ( $3 eq 'LE' ) ? 'v' : '';

    $size == 4 and $endian = uc($endian);

    my $obj = bless {

        Name   => $name,

        size   => $size,

        endian => $endian,

        ucs2   => $ucs2,

    } => __PACKAGE__;

    Encode::define_encoding($obj, $name);

}

use parent qw(Encode::Encoding);

sub renew {

    my $self = shift;

    $BOM_Unknown{ $self->name } or return $self;

    my $clone = bless {%$self} => ref($self);

    $clone->{renewed}++;    # so the caller knows it is renewed.

    return $clone;

}

1;

__END__

=head1 NAME

Encode::Unicode -- Various Unicode Transformation Formats

=cut

=head1 SYNOPSIS

    use Encode qw/encode decode/;

    $ucs2 = encode("UCS-2BE", $utf8);

    $utf8 = decode("UCS-2BE", $ucs2);

=head1 ABSTRACT

This module implements all Character Encoding Schemes of Unicode that

are officially documented by Unicode Consortium (except, of course,

for UTF-8, which is a native format in perl).

=over 4

=item L<http://www.unicode.org/glossary/> says:

I<Character Encoding Scheme> A character encoding form plus byte

serialization. There are Seven character encoding schemes in Unicode:

UTF-8, UTF-16, UTF-16BE, UTF-16LE, UTF-32 (UCS-4), UTF-32BE (UCS-4BE) and

UTF-32LE (UCS-4LE), and UTF-7.

Since UTF-7 is a 7-bit (re)encoded version of UTF-16BE, It is not part of

Unicode's Character Encoding Scheme.  It is separately implemented in

Encode::Unicode::UTF7.  For details see L<Encode::Unicode::UTF7>.

=item Quick Reference

                Decodes from ord(N)           Encodes chr(N) to...

       octet/char BOM S.P d800-dfff  ord > 0xffff     \x{1abcd} ==

  ---------------+-----------------+------------------------------

  UCS-2BE       2   N   N  is bogus                  Not Available

  UCS-2LE       2   N   N     bogus                  Not Available

  UTF-16      2/4   Y   Y  is   S.P           S.P            BE/LE

  UTF-16BE    2/4   N   Y       S.P           S.P    0xd82a,0xdfcd

  UTF-16LE    2/4   N   Y       S.P           S.P    0x2ad8,0xcddf

  UTF-32        4   Y   -  is bogus         As is            BE/LE

  UTF-32BE      4   N   -     bogus         As is       0x0001abcd

  UTF-32LE      4   N   -     bogus         As is       0xcdab0100

  UTF-8       1-4   -   -     bogus   >= 4 octets   \xf0\x9a\af\8d

  ---------------+-----------------+------------------------------

=back

=head1 Size, Endianness, and BOM

You can categorize these CES by 3 criteria:  size of each character,

endianness, and Byte Order Mark.

=head2 by size

UCS-2 is a fixed-length encoding with each character taking 16 bits.

It B<does not> support I<surrogate pairs>.  When a surrogate pair

is encountered during decode(), its place is filled with \x{FFFD}

if I<CHECK> is 0, or the routine croaks if I<CHECK> is 1.  When a

character whose ord value is larger than 0xFFFF is encountered,

its place is filled with \x{FFFD} if I<CHECK> is 0, or the routine

croaks if I<CHECK> is 1.

UTF-16 is almost the same as UCS-2 but it supports I<surrogate pairs>.

When it encounters a high surrogate (0xD800-0xDBFF), it fetches the

following low surrogate (0xDC00-0xDFFF) and C<desurrogate>s them to

form a character.  Bogus surrogates result in death.  When \x{10000}

or above is encountered during encode(), it C<ensurrogate>s them and

pushes the surrogate pair to the output stream.

UTF-32 (UCS-4) is a fixed-length encoding with each character taking 32 bits.

Since it is 32-bit, there is no need for I<surrogate pairs>.

=head2 by endianness

The first (and now failed) goal of Unicode was to map all character

repertoires into a fixed-length integer so that programmers are happy.

Since each character is either a I<short> or I<long> in C, you have to

pay attention to the endianness of each platform when you pass data

to one another.

Anything marked as BE is Big Endian (or network byte order) and LE is

Little Endian (aka VAX byte order).  For anything not marked either

BE or LE, a character called Byte Order Mark (BOM) indicating the

endianness is prepended to the string.

CAVEAT: Though BOM in utf8 (\xEF\xBB\xBF) is valid, it is meaningless

and as of this writing Encode suite just leave it as is (\x{FeFF}).

=over 4

=item BOM as integer when fetched in network byte order

              16         32 bits/char

  -------------------------

  BE      0xFeFF 0x0000FeFF

  LE      0xFFFe 0xFFFe0000

  -------------------------

=back

This modules handles the BOM as follows.

=over 4

=item *

When BE or LE is explicitly stated as the name of encoding, BOM is

simply treated as a normal character (ZERO WIDTH NO-BREAK SPACE).

=item *

When BE or LE is omitted during decode(), it checks if BOM is at the

beginning of the string; if one is found, the endianness is set to

what the BOM says.  

=item *

Default Byte Order

When no BOM is found, Encode 2.76 and blow croaked.  Since Encode

2.77, it falls back to BE accordingly to RFC2781 and the Unicode

Standard version 8.0

=item *

When BE or LE is omitted during encode(), it returns a BE-encoded

string with BOM prepended.  So when you want to encode a whole text

file, make sure you encode() the whole text at once, not line by line

or each line, not file, will have a BOM prepended.

=item *

C<UCS-2> is an exception.  Unlike others, this is an alias of UCS-2BE.

UCS-2 is already registered by IANA and others that way.

=back

=head1 Surrogate Pairs

To say the least, surrogate pairs were the biggest mistake of the

Unicode Consortium.  But according to the late Douglas Adams in I

Hitchhiker's Guide to the Galaxy> Trilogy, C

Universe was created. This has made a lot of people very angry and

been widely regarded as a bad move>.  Their mistake was not of this

magnitude so let's forgive them.

(I don't dare make any comparison with Unicode Consortium and the

Vogons here ;)  Or, comparing Encode to Babel Fish is completely

appropriate -- if you can only stick this into your ear :)

Surrogate pairs were born when the Unicode Consortium finally

admitted that 16 bits were not big enough to hold all the world's

character repertoires.  But they already made UCS-2 16-bit.  What

do we do?

Back then, the range 0xD800-0xDFFF was not allocated.  Let's split

that range in half and use the first half to represent the C

half of a character> and the second half to represent the C

half of a character>.  That way, you can represent 1024 * 1024 =

1048576 more characters.  Now we can store character ranges up to

\x{10ffff} even with 16-bit encodings.  This pair of half-character is

now called a I<surrogate pair> and UTF-16 is the name of the encoding

that embraces them.

Here is a formula to ensurrogate a Unicode character \x{10000} and

above;

  $hi = ($uni - 0x10000) / 0x400 + 0xD800;

  $lo = ($uni - 0x10000) % 0x400 + 0xDC00;

And to desurrogate;

 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);

Note this move has made \x{D800}-\x{DFFF} into a forbidden zone but

perl does not prohibit the use of characters within this range.  To perl,

every one of \x{0000_0000} up to \x{ffff_ffff} (*) is I.

  (*) or \x{ffff_ffff_ffff_ffff} if your perl is compiled with 64-bit

  integer support!

=head1 Error Checking

Unlike most encodings which accept various ways to handle errors,

Unicode encodings simply croaks.

  % perl -MEncode -e'$_ = "\xfe\xff\xd8\xd9\xda\xdb\0\n"' \

         -e'Encode::from_to($_, "utf16","shift_jis", 0); print'

  UTF-16:Malformed LO surrogate d8d9 at /path/to/Encode.pm line 184.

  % perl -MEncode -e'$a = "BOM missing"' \

         -e' Encode::from_to($a, "utf16", "shift_jis", 0); print'

  UTF-16:Unrecognised BOM 424f at /path/to/Encode.pm line 184.

Unlike other encodings where mappings are not one-to-one against

Unicode, UTFs are supposed to map 100% against one another.  So Encode

is more strict on UTFs.

Consider that "division by zero" of Encode :)

=head1 SEE ALSO

L<Encode>, L<Encode::Unicode::UTF7>, L<http://www.unicode.org/glossary/>,

L<http://www.unicode.org/unicode/faq/utf_bom.html>,

RFC 2781 L<http://www.ietf.org/rfc/rfc2781.txt>,

The whole Unicode standard L<http://www.unicode.org/unicode/uni2book/u2.html>

Ch. 15, pp. 403 of C<Programming Perl (3rd Edition)>

by Larry Wall, Tom Christiansen, Jon Orwant;

O'Reilly & Associates; ISBN 0-596-00027-8

=cut