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From "Adrien Grand (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-4584) Compare the LZ4 implementation in Lucene against the original impl
Date Sun, 02 Dec 2012 14:01:58 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-4584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13508270#comment-13508270
] 

Adrien Grand commented on LUCENE-4584:
--------------------------------------

bq. You wouldn't need static files if you compared output lengths

Even the output length depends on the endianess: LZ4 uses a hash table without collision resolution
(it maps hash -> last offset that produced this hash) to find matchs of 4 consecutive bytes
in the input bytes, and this hash function is not endian-neutral (it interprets the 4 bytes
as an 32-bits int, multiplies it by a prime number and keeps the 12 first bits (13 if there
are less than 2^16 input bytes)), so the collisions won't be the same depending on the endianess
and LZ4 won't find the same matchs.
                
> Compare the LZ4 implementation in Lucene against the original impl
> ------------------------------------------------------------------
>
>                 Key: LUCENE-4584
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4584
>             Project: Lucene - Core
>          Issue Type: Task
>            Reporter: Adrien Grand
>            Assignee: Adrien Grand
>             Fix For: 4.1
>
>
> We should add tests to make sure that the LZ4 impl in Lucene compresses data the exact
same way as the original impl.

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