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From "Michel Tourn (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-302) class Text (replacement for class UTF8) was: HADOOP-136
Date Wed, 14 Jun 2006 00:32:29 GMT
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
 Assigned to: Doug Cutting 


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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