[jira] [Commented] (PDFBOX-1000) Conforming parser

["Adam Nichols (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PDFBOX-1000?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13052652#comment-13052652 ] 

Adam Nichols commented on PDFBOX-1000:
--------------------------------------

I haven't had time to go back to this lately, but I'm still following the mailing list.  The comments in PDFBOX-1016 do a great job at explaining how the xref tables should be read/parsed.  So when I (or anyone else) comes back to the conforming parser, it'd be good to confirm that we're doing it properly and old references are overwritten by new ones (PDFBOX-1042).  I think the current code (attached above) is only reading the last xref table and ignoring all the previous ones, which is very wrong.  However, it should be easy to put a loop in there to handle this.  Linearized documents will be another thing to add support for in the future, but we'll cross that bridge when we come to it.

> Conforming parser
> -----------------
>
>                 Key: PDFBOX-1000
>                 URL: https://issues.apache.org/jira/browse/PDFBOX-1000
>             Project: PDFBox
>          Issue Type: New Feature
>          Components: Parsing
>            Reporter: Adam Nichols
>            Assignee: Adam Nichols
>         Attachments: COSUnread.java, ConformingPDDocument.java, ConformingPDFParser.java, ConformingPDFParserTest.java, XrefEntry.java, conforming-parser.patch, gdb-refcard.pdf
>
>
> A conforming parser will start at the end of the file and read backward until it has read the EOF marker, the xref location, and trailer[1].  Once this is read, it will read in the xref table so it can locate other objects and revisions.  This also allows skipping objects which have been rendered obsolete (per the xref table)[2].  It also allows the minimum amount of information to be read when the file is loaded, and then subsequent information will be loaded if and when it is requested.  This is all laid out in the official PDF specification, ISO 32000-1:2008.
> Existing code will be re-used where possible, but this will require new classes in order to accommodate the lazy reading which is a very different paradigm from the existing parser.  Using separate classes will also eliminate the possibility of regression bugs from making their way into the PDDocument or BaseParser classes.  Changes to existing classes will be kept to a minimum in order to prevent regression bugs.
> [1] Section 7.5.5 "Conforming readers should read a PDF file from its end"
> [2] Section 7.5.4 "the entire file need not be read to locate any particular object"

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