People make ROM Hacks using buggy tools, and then one day they find their game freezing on a pokemon evolution, or battle, or whatever. Since the ROM file is just a big mess and they have no idea how to fix it, they either start over or give up. I sometimes fix other people's ROM hacks.
My usual method was going close to the point of failure (freeze, reboot, whatever), making vba-sdl-h start generating a trace, activating that failure and then looking at the 200+ MB trace file using split(1) and less(1) (basic gnu tools).
Yesterday, this method proved more ineficcient than usual, so I came up with another one. I created 2 directories, a and b. In a I put a clean Pokemon ruby ROM and the broken hack, named r.gba and d.gba. In b I created the following script:
#!/bin/sh
cp ../a/* .
dd if=r.gba of=d.gba skip=$OFFSET seek=$OFFSET \
count=$SIZE bs=1 conv=notrunc
What this does is basically to copy SIZE bytes from r.gba to d.gba starting from byte OFFSET, using dd(1). I would run it as (for example):
$ OFFSET=$((0x416000)) SIZE=$((0x1000)) ./do.sh
I had a savestate to run just before the crash, so I just had to change OFFSET and SIZE, run vba-sdl-h, press F1, and see if it worked.
Starting with 0x600000 bytes from the ROM start fixed the bug, as expected, but that would be replacing too many things from the hack with stuff from the original game. So I narrowed it down by halving and tweaking those values until I ended up with a that 0x1000 byte copy shown above—which fixes evolution in the hack and (hopefully) doesn't break anything important in doing so.
The result of all this is here: What would have been, after some more work, Pokemon Dark Blue's beta 2.