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From "Sebb (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (IO-288) Supply a ReversedLinesFileReader
Date Tue, 15 Nov 2011 12:27:51 GMT

    [ https://issues.apache.org/jira/browse/IO-288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13150426#comment-13150426
] 

Sebb commented on IO-288:
-------------------------

Sorry, you're correct about needing to convert CR and LF. 
I was forgetting that BufferedReader.readLine() works on the decoded values, so does not need
to encode them for comparison.
Whereas your code works on bytes, and decodes later.

AFAICT, the code depends on the line end byte arrays being sorted order of descending length.

This should be documented. Hopefully it's not possible for an encoding to use different lengths
for CR and LF!
                
> Supply a ReversedLinesFileReader 
> ---------------------------------
>
>                 Key: IO-288
>                 URL: https://issues.apache.org/jira/browse/IO-288
>             Project: Commons IO
>          Issue Type: New Feature
>          Components: Utilities
>            Reporter: Georg Henzler
>             Fix For: 2.2
>
>         Attachments: ReversedLinesFileReader0.2.zip
>
>
> I needed to analyse a log file today and I was looking for a ReversedLinesFileReader:
A class that behaves exactly like BufferedReader except that it goes from bottom to top when
readLine() is called. I didn't find it in IOUtils and the internet didn't help a lot either,
e.g. http://www.java2s.com/Tutorial/Java/0180__File/ReversingaFile.htm is a fairly inefficient
- the log files I'm analysing are huge and it is not a good idea to load the whole content
in the memory. 
> So I ended up writing an implementation myself using little memory and the class RandomAccessFile
- see attached file. It's used as follows:
> int blockSize = 4096; // only that much memory is needed, no matter how big the file
is
> ReversedLinesFileReader reversedLinesFileReader = new ReversedLinesFileReader (myFile,
blockSize, "UTF-8"); // encoding is supported
> String line = null;
> while((line=reversedLinesFileReader.readLine())!=null) {
>   ... // use the line
>   if(enoughLinesSeen) {
>      break;  
>   }
> }
> reversedLinesFileReader.close();
> I believe this could be useful for other people as well!

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