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Unicode/Unicode.pm  view on Meta::CPAN

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<upper
half of a character> and the second half to represent the C<lower
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