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From "Simo Chiegang, Boris Arthur" <Boris.S...@heidelberg.com>
Subject RE: [compress] uncompress a file containing lz4 stripes
Date Mon, 18 Sep 2017 12:09:51 GMT
Thanks, it works!

-----Original Message-----
From: Stefan Bodewig [mailto:bodewig@apache.org]
Sent: Saturday, September 16, 2017 6:39 PM
To: user@commons.apache.org
Subject: Re: [compress] uncompress a file containing lz4 stripes

On 2017-09-15, Simo Chiegang, Boris Arthur wrote:

> So, we develop a special kind of image file with the following structure:

> -          Header file [offset 0 to 4096 bytes]

> -          Stripe 1 [4096 - 20000 bytes] (LZ4 block compressed)

> -          Empty bloc [20000 - 22000 bytes]

> -          Stripe 2 [22000 - 35000] (LZ4 block compressed)

> -          ...

> Something like that. So now I want to read this file with the class
> BlockLZ4CompressorInputStream. It is possible?

In general yes, but probbaly not with a single instance of it (and you may need to adapt something).

BlockLZ4CompressorInputStream should be able to read a single block, but it wont be able to
detect the end of the LZ4 block unless the underlying stream is exhausted. There is no way
to identify the last sequence other than via EOF.

If your header contains information about the compressed size of your
block(s) then you can wrap the current input stream into a BoundedInputStream that returns
EOF once the block has been read completely. You then create a BlockLZ4CompressorInputStream
wrapped around the BoundedInputStream - and do this for each stripe.

This is pretty much is what FramedLZ4CompressorInputStream does as well.

Things get a bit more complex if match offsets of stripe 2 are allowed to point into the contents
of stripe 1. FramedLZ4CompressorInputStream handles this in the case where "block dependency"
is expected.

Stefan

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