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Back after taking way too long to get Touhou Patch Center's MediaWiki update feature complete… I'm still waiting for more translators to test and review the new translation interface before delivering and deploying it all, which will most likely lead to another break from ReC98 within the next few months. For now though, I'm happy to have mostly addressed the nagging responsibility I still had after willing that site into existence, and to be back working on ReC98. 🙂
As announced, the next few pushes will focus on TH04's and TH05's bullet
spawning code, before I get to put all that accumulated TH01 money towards
finishing all of konngara's code in TH01. For a full
picture of what's happening with bullets, we'd really also like to
have the bullet update function as readable C code though.
Clearing all bullets on the playfield will trigger a Bonus!! popup, displayed as 📝 gaiji in that proportional font. Unfortunately, TLINK refused to link the code as soon as I referenced the function for animating the popups at the top of the playfield? Which can only mean that we have to decompile that function first…
So, let's turn that piece of technical debt into a full push, and first
decompile another random set of previously reverse-engineered TH04 and TH05
functions. Most of these are stored in a different place within the two
MAIN.EXE binaries, and the tried-and-true method of matching
segment names would therefore have introduced several unnecessary
translation units. So I resorted to a segment splitting technique I should
have started using way earlier: Simply creating new segments with names
derived from their functions, at the exact positions they're needed. All
the new segment start and end directives do bloat the ASM code somewhat,
and certainly contributed to this push barely removing any actual lines of
code. However, what we get in return is total freedom as far as
decompilation order is concerned,
📝 which should be the case for any ReC project, really.
And in the end, all these tiny code segments will cancel out anyway.
If only we could do the same with the data segment…
The popup function happened to be the final one I RE'd before my long break
in the spring of 2019. Back then, I didn't even bother looking into that
64-frame delay between changing popups, and what that meant for the game.
Each of these popups stays on screen for 128 frames, during which, of course, another popup-worthy event might happen. Handling this cleanly without removing previous popups too early would involve some sort of event queue, whose size might even be meaningfully limited to the number of distinct events that can happen. But still, that'd be a data structure, and we're not gonna have that! Instead, ZUN simply keeps two variables for the new and current popup ID. During an active popup, any change to that ID will only be committed once the current popup has been shown for at least 64 frames. And during that time, that new ID can be freely overwritten with a different one, which drops any previous, undisplayed event. But surely, there won't be more than two events happening within 63 frames, right?
The rest was fairly uneventful – no newly RE'd functions in this push,
after all – until I reached the widely used helper function for applying
the current vertical scrolling offset to a Y coordinate. Its combination of
a function parameter, the
pascal calling convention, and no
stack frame was previously thought to be undecompilable… except that it
isn't, and the decompilation didn't even require any new workarounds to be
developed? Good thing that I already forgot how impossible it was to
decompile the first function I looked at that fell into this category!
Oh well, this discovery wasn't too groundbreaking. Looking back at all the other functions with that combination only revealed a grand total of 1 additional one where a decompilation made sense: TH05's version of
snd_kaja_interrupt(), which is now compiled from the same C++
file for all 4 games that use it. And well, looks like some quirks really
remain unnoticed and undocumented until you look at a function for the 11th
time: Its return value is undefined if BGM is inactive – that is, if the
user disabled it, or if no FM board is installed. Not that it matters for
the original code, which never uses this function to retrieve anything from
KAJA's drivers. But people apparently do copy ReC98 code into their own
projects, so it is something to keep in mind.
All in all, nothing quite at jank level in this one, but we were surely grazing that tag. Next up, with that out of the way: The bullet update/step function! Very soon in fact, since I've mostly got it done already.