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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-302) class Text (replacement for class UTF8) was: HADOOP-136
Date Sat, 08 Jul 2006 07:49:31 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-302?page=comments#action_12419845 ] 

Doug Cutting commented on HADOOP-302:

I think we should use this opportunity to switch to standard UTF-8 for persistent data.  Optimized
code should try to avoid conversion of these to Java strings.  For example, comparision can
be done on the binary form (since this yeilds the same results as lexicographic unicode comparisons).

> 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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