Greets,
Discussion moved from the users list as per suggestion...
-- Marvin Humphrey
Begin forwarded message:
From: Marvin Humphrey <[EMAIL PROTECTED]>
Date: August 26, 2005 9:18:21 PM PDT
To: java-user@lucene.apache.org, [EMAIL PROTECTED]
Subject: Lucene does NOT use UTF-8.
Reply-To: java-user@lucene.apache.org
Greets,
[crossposted to java-user@lucene.apache.org and [EMAIL PROTECTED]
I've delved into the matter of Lucene and UTF-8 a little further, and
I am discouraged by what I believe I've uncovered.
Lucene should not be advertising that it uses "standard UTF-8" -- or
even UTF-8 at all, since "Modified UTF-8" is _illegal_ UTF-8. The
two distinguishing characteristics of "Modified UTF-8" are the
treatment of codepoints above the BMP (which are written as surrogate
pairs), and the encoding of null bytes as 1100 0000 1000 0000 rather
than 0000 0000. Both of these became illegal as of Unicode 3.1
(IIRC), because they are not shortest-form and non-shortest-form
UTF-8 presents a security risk.
The documentation should really state that Lucene stores strings in a
Java-only adulteration of UTF-8, unsuitable for interchange. Since
Perl uses true shortest-form UTF-8 as its native encoding, Plucene
would have to jump through two efficiency-killing hoops in order to
write files that would not choke Lucene: instead of writing out its
true, legal UTF-8 directly, it would be necessary to first translate
to UTF-16, then duplicate the Lucene encoding algorithm from
OutputStream. In theory.
Below you will find a simple Perl script which illustrates what
happens when Perl encounters malformed UTF-8. Run it (you need Perl
5.8 or higher) and you will see why even if I thought it was a good
idea to emulate the Java hack for encoding "Modified UTF-8", trying
to make it work in practice would be a nightmare.
If Plucene were to write legal UTF-8 strings to its index files, Java
Lucene would misbehave and possibly blow up any time a string
contained either a 4-byte character or a null byte. On the flip
side, Perl will spew warnings like crazy and possibly blow up
whenever it encounters a Lucene-encoded null or surrogate pair. The
potential blowups are due to the fact that Lucene and Plucene will
not agree on how many characters a string contains, resulting in
overruns or underruns.
I am hoping that the answer to this will be a fix to the encoding
mechanism in Lucene so that it really does use legal UTF-8. The most
efficient way to go about this has not yet presented itself.
Marvin Humphrey
Rectangular Research
http://www.rectangular.com/
#----------------------------------------
#!/usr/bin/perl
use strict;
use warnings;
# illegal_null.plx -- Perl complains about non-shortest-form null.
my $data = "foo\xC0\x80\n";
open (my $virtual_filehandle, "+<:utf8", \$data);
print <$virtual_filehandle>;
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