Accessing file position information during a SAX parse

[Pete Hodgson]

Hi everyone,
I was hoping for some advice regarding a problem my team is facing 
related to SAX parsing in Xerces-C++. I'm new to Xerces, and SAX in 
general, so please forgive any stupidity!

The application we're developing is processing /very/ large XML files 
that contain time-series data looking something like this:

<Root>
  <Header>
    <SomeMetaData>
    <SomeMoreMetaData>
    ...
    ...
  </Header>
  <Frame id="1">
    <LotsOfData>
    <LotsMoreData>
    <YetMoreData>
    ...
  </Frame>
  <Frame id="2">
    ...
  </Frame>
  <Frame id="3">
    ...
  </Frame>
  ...
  ...
  ...
</Root>
We've been using progressive parsing SAX to read the <Frame> data from 
these XML files, which works great because we can deal with it as a 
stream without having to read the entire file up front.

We've also been using the MSXML DOM implementation to read <Header> data 
with the same Schema as the <Header> element in the time-series files, 
but from other, small files.

The problem now is that we wish to access the <Header> data in these 
extremely large files. We don't want to use DOM to parse the entire file 
(for efficiency issues), but we'd like to re-use the existing DOM-based 
implementation that we have for reading the <Header> schema (rather than 
implementing a new SAX parser for the <Header>).

So, I guess my question is, is there a way to discover the exact file 
location of an Element as it's encountered during a SAX parse? If we 
could get the location we could manually read the entire <Header> 
section into a string and DOM-parse the string. We'd also like to be 
able to access file location information for other reasons, such as to 
pre-parse the files and build a 'look up table' for the XML file, so 
that a particular section of the time series can be read in on demand 
with the help of a custom LocalFileSource.

The closest thing I've found is Locator, but that doesn't help because 
it gives you a line and column, rather than an absolute location within 
the file. I looked into peeking at the BinInputStream that the 
SAX2XMLReader is using, but that doesn't work because the stream is read 
in chunks, so calling BinInputStream::curPos() when the Header element 
is encountered doesn't supply the exact location either. I know that 
that would have been a kludgy solution anyways, but it would have served 
our purposes.

Any suggestions on how best to solve this one?
Many Thanks,
Pete Hodgson
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