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P0217
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(Seihou) pbg...P0217
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Arandui
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First of all: This blog is now available as a web feed, in three different formats!

Thanks to handlerug for implementing and PR'ing the feature in a very clean way. That makes at least two people I know who wanted to see feed support, so there are probably a few more out there.

So, Shuusou Gyoku. pbg released the original source code for the first two Seihou games back in February 2019, but notably removed the crucial decompression code for the original packfiles due to… various unspecified reasons, considerations, and implications. :thonk: This vague language and subsequent rejection of a pull request to add these features back in were probably the main reasons why no one has publicly done anything with this codebase since.

The only other fork I know about is Priw8's private fork from 2020, but only because WishMakers informed me about it shortly after this push was funded. Both of them might also contribute some features to my fork in the future if their time allows it.
In this fork, Priw8 replaced packfile decompression with raw reads from directories with the pre-extracted contents of all the .DAT files. This works for playing the game, but there are actually two more things that require the original packfile code:

We can surely implement our own simple and uncompressed formats for these things, but it's not the best idea to build all future Shuusou Gyoku features on top of a replay-incompatible fork. So, what do we do? On the one hand, pbg expressed the clear wish to not include data reverse-engineered from the original binary. On the other hand, he released the code under the MIT license, which allows us to modify the code and distribute the results in any way we wish.
So, let's meet in the middle, and go for a clean-room implementation of the missing features as indicated by their usage, without looking at either the original binary or wangqr's reverse-engineered code.

With incremental rebuilds being broken in the latest Visual Studio project files as well, it made sense to start from scratch on pbg's last commit. Of course, I can't pass up a chance to use 📝 Tup, my favorite build system for every project I'm the main developer of. It might not fit Shuusou Gyoku as well as it fits ReC98, but let's see whether it would be reasonable at all…

… and it's actually not too bad! Modern Visual Studio makes this a bit harder than it should be with all the intermediate build artifacts you have to keep track of. In the end though, it's still only 70 lines of Lua to have a nice abstraction for both Debug and Release builds. With this layer underneath, the actual Shuusou Gyoku-specific part can be expressed as succinctly as in any other modern build system, while still making every compiler flag explicit. It might be slightly slower than a traditional .vcxproj build due to launching one cl.exe process per translation unit, but the result is way more reliable and trustworthy compared to anything that involves Visual Studio project files. This simplicity paves the way for expanding the build process to multiple steps, and doing all the static checking on translation strings that I never got to do for thcrap-based patches. Heck, I might even compile all future translations directly into the binary…

Every C++ build system will invariably be hated by someone, so I'd say that your goal should always be to simplify the actually important parts of your build enough to allow everyone else to easily adapt it to their favorite system. This Tupfile definitely does a better job there than your average .vcxproj file – but if you still want such a thing (or, gasp, 🤮 CMake project files 🤮) for better Visual Studio IDE integration, you should have no problem generating them for yourself.
There might still be a point in doing that because that's the one part that unfortunately sucks about this approach. Visual Studio is horribly broken for any nonstandard C++ project even in 2022:

In both cases, IntelliSense doesn't work properly at all even if it appears to be configured correctly, and Tup's dependency tracking appeared to be weirdly cut off for the very final .PDB file. Interestingly though, using the big Visual Studio IDE for just debugging a binary via devenv bin/GIAN07.exe suddenly eliminates all the IntelliSense issues. Looks like there's a lot of essential information stored in the .PDB files that Visual Studio just refuses to read in any other context. :thonk:

But now compare that to Visual Studio Code: Open it from the x64_x86 Cross Tools Command Prompt via code ., launch a build or debug task, or browse the code with perfect IntelliSense. Three small configuration files and everything just works – heck, you even get the Tup progress bar in the terminal. It might be Electron bloatware and horribly slow at times, but Visual Studio Code has long outperformed regular Visual Studio in terms of non-debug functionality.

On to the compression algorithm then… and it's just textbook LZSS, with 13 bits for the offset of a back-reference and 4 bits for its length? Hardly a trade secret there. The hard parts there all come from unexpected inefficiencies in the bitstream format:

  1. Encoding back-references as offsets into an 8 KiB ring buffer dictionary means that the most straightforward implementation actually needs an 8 KiB array for the LZSS sliding window. This could have easily been done with zero additional memory if the offset was encoded as the difference to the current byte instead.
  2. The packfile format stores the uncompressed size of every file in its header, which is a good thing because you want to know in advance how much heap memory to allocate for a specific file. Nevertheless, the original game only stops reading bits from the packfile once it encountered a back-reference with an offset of 0. This means that the compressor not only has to write this technically unneeded back-reference to the end of the compressed bitstream, but also ignore any potential other longest back-reference with an offset of 0 within the file. The latter can easily happen with a ring buffer dictionary.

The original game used a single BIT_DEVICE class with mode flags for every combination of reading and writing memory buffers and on-disk files. Since that would have necessitated a lot of error checking for all (pseudo-)methods of this class, I wrote one dedicated small class for each one of these permutations instead. To further emphasize the clean-room property of this code, these use modern C++ memory ownership features: std::unique_ptr for the fixed-size read-only buffers we get from packfiles, std::vector for the newly compressed buffers where we don't know the size in advance, and std::span for a borrowed reference to an immutable region of memory that we want to treat as a bitstream. Definitely better than using the native Win32 LocalAlloc() and LocalFree() allocator, especially if we want to port the game away from Windows one day.

One feature I didn't use though: C++ fstreams, because those are trash. :tannedcirno: These days, they would seem to be the natural choice with the new std::filesystem::path type from C++17: Correctly constructed, you can pass that type to an fstream constructor and gain both locale independence on Windows and portability to everything else, without writing any Windows-specific UTF-16 code. But even in a Release build, fstreams add ~100 KB of locale-related bloat to the .EXE which adds no value for just reading binary files. That's just too embarrassing if you look at how much space the rest of the game takes up. Writing your own platform layer that calls the Win32 CreateFileW(), ReadFile(), and WriteFile() API functions is apparently still the way to go even in 2022. And with std::filesystem::path still being a welcome addition to C++, it's not too much code to write either.

This gets us file format compatibility with the original release… and a crash as soon as the ending starts, but only in Release mode? As it turns out, this crash is caused by an out-of-bounds array access bug that was present even in the original game, and only turned into a crash now because the optimizer in modern Visual Studio versions reorders static data. As a result, the 6-element pFontInfo array got placed in front of an ECL-related counter variable that then got corrupted by the write to the 7th element, which subsequently crashed the game with a read access to previously deallocated danmaku script data. That just goes to show that these technical bugs are important and worth fixing even if they don't cause issues in the original game. Who knows how many of these will turn into crashes once we get to porting PC-98 Touhou?

So here we go, a new build of Shuusou Gyoku, compiled with Visual Studio 2022, and compatible with all original data formats:

:sh01: Shuusou Gyoku P0217

Inside the regular Shuusou Gyoku installation directory, this binary works as a full-fledged drop-in replacement for the original 秋霜玉.exe. It still has all of the original binary's problems though:

As well as some of its own:

So all in all, it's a strict downgrade at this point in time. :onricdennat: And more of a symbol that we can now start doing actual work on this game. Seihou has been a fun change of pace, and I hope that I get to do more work on the series. There is quite a lot to be done with Shuusou Gyoku alone, and the 21 GitHub issues I've opened are probably only scratching the surface.
However, all the required research for this one consumed more like 1⅔ pushes. Despite just one push being funded, it wouldn't have made sense to release the commits or this binary in any earlier state. To repay this debt, I'm going to put the next for Seihou towards the small code maintenance and performance tasks that I usually do for free, before doing any more feature and bugfix work. Next up: Improving video playback on the blog, and maybe delivering some microtransaction work on the side?