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Allow clone CHDs to use parent CHDs as parents.
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* util/chd.cpp: Allow caller to provide a helper for finding parent CHDs
  and expose (recursive) missing parent status.
* emu/romload.cpp: Search parent systems/devices/software for parent
  CHDs on encountering a delta CHD.
* emu/romload.cpp: Report error on delta CHDs when parent can't be
  found.
* emu/romload.cpp: Check parents for matching CHDs with different names
  for devices as well as systems and software.
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@cuavas
cuavas committed Oct 19, 2023
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6 comments on commit d1172bf

@cuavas
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@cuavas cuavas commented on d1172bf Oct 19, 2023

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This has been requested several times. There’s probably an open issue for it, but I haven’t looked. It allows substantial space savings for a full media set in cases where a clone system/device/software item has a CHD that can share a lot of hunks with a CHD in its parent.

Using sf2049tea and sf2049te as an example, you can create a delta CHD like:

chdman copy -i sf2049tea.chd -op sf2049te.chd -o sf2049tea-delta.chd
mv sf2049tea.chd sf2049tea-abs.chd
mv sf2049tea-delta.chd sf2049tea.chd

You’ll now have a delta CHD that’s about 50 MB that can be used in place of the regular CHD that’s over 1 GB, provided MAME can find sf2049te.chd from the parent.

Of course, there are cases where few blocks can be shared – if you try this with fmtowns_cd:4ddrivinm you’ll only save about 2 MB.

@FoxhackDN
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Could this potentially save a lot of space for parent / clone CHD sets that have the same audio track data?

@cuavas
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@cuavas cuavas commented on d1172bf Oct 19, 2023

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Could this potentially save a lot of space for parent / clone CHD sets that have the same audio track data?

It depends. You have to try it and find out. The data must be identical and at identical block offsets, or it won’t help.

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Leaving this comment here for posterity: No, it does not, at least not on my limited testing on NeoGeo CD. :)

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I take it back, I actually managed to create a delta of fatfury3 for the NeoGeo CD that was less than 200kb. It didn't work with the game I tried first (Double Dragon) because the parent has damaged tracks 1 and 2, so everything was out of whack - but for CDs that are properly dumped, it should work.

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@cuavas
Thanks very much !

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