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       Encode::Unicode -- Various Unicode Transformation Formats

           use Encode qw/encode decode/;
           $ucs2 = encode("UCS-2BE", $utf8);
           $utf8 = decode("UCS-2BE", $ucs2);

       This module implements all Character Encoding Schemes of
       Unicode that are officially documented by Unicode Consor-
       tium (except, of course, for UTF-8, which is a native for-
       mat in perl).

       <http://www.unicode.org/glossary/>; says:
           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 Encod-
           ing Scheme.  It is separately implemented in
           Encode::Unicode::UTF7.  For details see Encode::Uni-

       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   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

Size, Endianness, and BOM
       You can categorize these CES by 3 criteria:  size of each
       character, endianness, and Byte Order Mark.

       by size

       UCS-2 is a fixed-length encoding with each character tak-
       ing 16 bits.  It does not support surrogate pairs.  When a
       surrogate pair is encountered during decode(), its place
       is filled with \x{FFFD} if CHECK is 0, or the routine
       croaks if CHECK is 1.  When a character whose ord value is
       larger than 0xFFFF is encountered, its place is filled

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       with \x{FFFD} if CHECK is 0, or the routine croaks if
       CHECK is 1.

       UTF-16 is almost the same as UCS-2 but it supports surro-
       gate pairs.  When it encounters a high surrogate
       (0xD800-0xDBFF), it fetches the following low surrogate
       (0xDC00-0xDFFF) and "desurrogate"s them to form a charac-
       ter.  Bogus surrogates result in death.  When \x{10000} or
       above is encountered during encode(), it "ensurrogate"s
       them and pushes the surrogate pair to the output stream.

       UTF-32 (UCS-4) is a fixed-length encoding with each char-
       acter taking 32 bits.  Since it is 32-bit, there is no
       need for surrogate pairs.

       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
       short or long in C, you have to pay attention to the endi-
       anness 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}).

       BOM as integer when fetched in network byte order
                         16         32 bits/char
             BE      0xFeFF 0x0000FeFF
             LE      0xFFeF 0xFFFe0000

       This modules handles the BOM as follows.

       o   When BE or LE is explicitly stated as the name of
           encoding, BOM is simply treated as a normal character

       o   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.  If
           no BOM is found, the routine dies.

       o   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

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           encode() the whole text at once, not line by line or
           each line, not file, will have a BOM prepended.

       o   "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.

Surrogate Pairs
       To say the least, surrogate pairs were the biggest mistake
       of the Unicode Consortium.  But according to the late Dou-
       glas Adams in The Hitchhiker's Guide to the Galaxy Tril-
       ogy, "In the beginning the 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 "upper half of a character" and the second
       half to represent the "lower half of a character".  That
       way, you can represent 1024 * 1024 = 1048576 more charac-
       ters.  Now we can store character ranges up to \x{10ffff}
       even with 16-bit encodings.  This pair of half-character
       is now called a 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 a character.

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

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       Encode, Encode::Unicode::UTF7, <http://www.uni-
       code.org/glossary/>, <http://www.unicode.org/uni-

       RFC 2781 <http://rfc.net/rfc2781.html>;,

       The whole Unicode standard <http://www.unicode.org/uni-

       Ch. 15, pp. 403 of "Programming Perl (3rd Edition)" by
       Larry Wall, Tom Christiansen, Jon Orwant; O'Reilly & Asso-
       ciates; ISBN 0-596-00027-8

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