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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-302) class Text (replacement for class UTF8) was: HADOOP-136
Date Mon, 10 Jul 2006 18:00:31 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-302?page=comments#action_12420135 ] 

Hairong Kuang commented on HADOOP-302:

If we use standard UTF8, comparison on the binary form does not produce the same results as
the string comparison. See the following example provided by Addison:

> Consider the sequence U+D800 U+DC00 (a surrogate pair). In String comparison, this compares
as less than U+E000 (since 
> D800 < E000). In UTF-8 byte comparisons it is greater than E000 (because the lead
byte of the Unicode character U+10000 
> encoded by the surrogate pair is 0xF0, which is bigger than lead byte of U+E000, which
is 0xEE). 

Is it an issue? 

> class Text (replacement for class UTF8) was: HADOOP-136
> -------------------------------------------------------
>          Key: HADOOP-302
>          URL: http://issues.apache.org/jira/browse/HADOOP-302
>      Project: Hadoop
>         Type: Improvement

>   Components: io
>     Reporter: Michel Tourn
>     Assignee: Hairong Kuang

> Just to verify, which length-encoding scheme are we using for class Text (aka LargeUTF8)

> a) The "UTF-8/Lucene" scheme? (highest bit of each byte is an extension bit, which I
think is what Doug is describing in his last comment) or 
> b) the record-IO scheme in o.a.h.record.Utils.java:readInt 
> Either way, note that: 
> 1. UTF8.java and its successor Text.java need to read the length in two ways: 
>   1a. consume 1+ bytes from a DataInput and 
>   1b. parse the length within a byte array at a given offset 
> (1.b is used for the "WritableComparator optimized for UTF8 keys" ). 
> o.a.h.record.Utils only supports the DataInput mode. 
> It is not clear to me what is the best way to extend this Utils code when you need to
support both reading modes 
> 2 Methods like UTF8's WritableComparator are to be low overhead, in partic. there should
be no Object allocation. 
> For the byte array case, the varlen-reader utility needs to be extended to return both:

>  the decoded length and the length of the encoded length. 
>  (so that the caller can do offset += encodedlength) 
> 3. A String length does not need (small) negative integers. 
> 4. One advantage of a) is that it is standard (or at least well-known and natural) and
there are no magic constants (like -120, -121 -124) 

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