ALL: Check for presence and validity of external data files

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Definitely sounds like a good idea. We should do that for all the external data
files, i.e.

kyra.dat
lure.dat
queen.tbl
sky.cpt

We should first check for the presence of the file, and if missing, show an
approriate error *dialog*. Next, run some simply integrity check on the files
(minimal file size, a fixed header, whatever), and if that fails, display
*another* dialog, telling that the data file is likely corrupt.
And ideally, a third dialog would be shown if the data file was outdated.

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data file structure:

TAG VER LENGTH data block MD5

I.e.:
1. Check TAG that file is valid
2. Check version
3. Check LENGTH
4. Calculate MD5 and match

This will be easy to implement and will be universal enough
to avoid any hardcoded file details.

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wouldn't that require an update to all data files?

I think lordhoto once mentioned that kyra.dat is the same
format as the kyra .pak files

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Yes, but that will be done only once and will save us from
these periodic bogus bugreports.

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The current Kyrandia implementation hardcodes the size in
the ScummVM sourcecode, if someones implements the described
'data file structure' in a generic way I'll change the
Kyrandia code to support it. I'm changing the 'Assigned To'
field to None for now, since I'm not working on a generic
way, so any other developer can feel free to implement it
and as I, said before, will change the Kyrandia code then.

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I would use a structure like this though:
TAG \ (maybe this field could be engine depended too)
VER | Datafile header
SIZE |
MD5 /

DATAFILE > Engine specific stuff

I don't see much sense behind adding the MD5 at the file end.

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We could also preserve at least some measure of backwards
compatability by trying to load the file as the old format
if it fails the integrity check, and just display a 'Your
copy of kyra.dat may be outdated or corrupted' message
(given that the current format of kyra.dat would use invalid
fields for the integrity check, and always fail it).

It doesn't really make sense to force people to download a
new version of kyra.dat for an integrity check when they
know their current one works fine :).

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maybe simply add a size-check for now, add the more
sophisticated check the next time kyra.dat changes (or if it
is stable, with the next stable version?)

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Clemmy: we have that size check by now.

We should just try to load kyra.dat/otherdatafile in the old
style way if just the TAG check failed, we should add a
return value in the loading function which tells what failed
while trying to load it the new way IMHO.