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-- Marvin Humphrey

Begin forwarded message:

From: Marvin Humphrey <[EMAIL PROTECTED]>
Date: August 26, 2005 9:18:21 PM PDT
Subject: Lucene does NOT use UTF-8.


[crossposted to 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


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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