[jira] [Created] (PDFBOX-5575) optimize LZWFilter

["Axel Howind (Jira)" <ji...@apache.org>]

Axel Howind created PDFBOX-5575:
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             Summary: optimize LZWFilter
                 Key: PDFBOX-5575
                 URL: https://issues.apache.org/jira/browse/PDFBOX-5575
             Project: PDFBox
          Issue Type: Improvement
    Affects Versions: 3.0.0 PDFBox
            Reporter: Axel Howind
         Attachments: optimize_LZWFilter.patch

I ran the PDFBox tests with a profiler and saw that LZWFilter used quite a bunch of time, so I thought I might look at the code. I just looked at it totally out of context and tried to understand what is done there and what could be changed without altering results.

 * made the private mehtods static
 * changed the variable/method parameter 'earlyChange' from integer to boolean because I thought tha would be more readable
 * some minor tweaks
 * it looks like codeTable is initialized quite often and everytime, 256 length 1 byte arrays are created, so I pre-allocate those byte arrays so that they can be shared by all code tables. [~tilman] I assumed the contents of the codeTable entries will not be changed, and my analysis of the code seems to prove that (also the passing unit tests). Just please have a look at this so I don't break anything.
 * it took me some time to fully understand what findPatternCode() does and why it checks the codeTable in reverse order. I more or less recreated that method from scratch and I think it should now always be faster: for patterns of length 1 no iteration is done, and for longer patterns iteration stops once the correct entry is found. As this is the most notable change, please take a closer look. Unit tests pass.




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