PURGE isn't especially useful, while requiring some annoying
special handling in the file format. If you want no compression,
use NONE. If you want fast compression, use Zstandard.
Gets rid of the need to seek to the end of the file
when opening a file.
The downside of this is that we waste a little space,
since we can't know in advance exactly how much
space the compressed parts of the headers will need.
This is useful for the way Dolphin scrubs Wii discs.
The encrypted data is what gets zeroed out, but this
zeroed out data then gets decrypted before being stored,
and the resulting data does not compress well.
However, each block of decrypted scrubbed data is
identical given the same encryption key, and there's
nothing stopping us from making multiple group entries
point to the same offset in the file, so we only have
to store one copy of this data per partition.
For reference, wit zeroes out the decrypted data,
but Dolphin's WIA writer can't do this because it currently
doesn't know which parts of the disc are scrubbed.
This is also useful for things such as storing Datel discs
full of 0x55 blocks (repesenting unreadable blocks)
without compression enabled.
Fixes https://bugs.dolphin-emu.org/issues/10654.
To quote the documenation file included with the program tgctogcm:
"TGC's are miniaturized .gcm images with a 32kB header.
The embedded gcm contains some bogus data, namely:
-FST Location (0x424 in gcm)
-DOL Location (0x420 in gcm)
-FST File offsets (all files are offset/spoofed by a certain amount)"
Dolphin has been handling the values at 0x420 and 0x424 by simply
overwriting them with a working value (just like tgctogcm does),
but it has used a different approach for the file offsets in the FST.
Instead of changing the offsets that are stored in the FST, Dolphin
changed where the files actually are placed on the virtual disc.
My hope was that this would make the loading times more accurate to
how they are when running a TGC file as part of a larger disc.
However, there are TGC files where we would need to move files
backwards on the disc in order to do this (this is what issue
10654 is about), so the approach we have been using is flawed.
This change makes Dolphin overwrite offsets in the FST instead, like
tgctogcm does. Other than making Dolphin handle the affected TGC files
correctly, this change also makes it so that unnecessary padding data
isn't written if you use Dolphin to convert a TGC file to an ISO file.
This feature is not actually implemented in Dolphin as of now, but I'm
planning to add it in the near future as part of a larger feature.
JosJuice