This particular solution does satisfy correctness, although I can't share the source code (as the competition is ongoing). Feel free to provide your input data and I'll run it to compare with your expected result.
It is surprising to me that C# and .NET is comparatively under-represented in North America where it was born. Nothing but good experience in developing and operating software with it, both desktop apps (Win/Mac) and backend services (Linux).
I like .Net and C#, but I've had a lot of experiences that were not good.
I personally hated working with Visual Studio 2017. 2019 was an improvement, but one so small it was still awful. I recently used it again (2021?) and was cynically surprised that they FINALLY got scrolling that isn't forcibly line-based. Why did they even bother now?
This and some other small things that used to make the experience painful are now solved, but they still haven't really gotten on top of the freezes and crashes.
Visual Studio being bad is one thing, it being the only officially supported IDE is the other. They should not only officially support third party efforts like Jetbrains Rider, in the past I've gone as far as saying they should discontinue Visual Studio in favor of it.
Now for the nice part: Almost all "new .NET" announcements/tutorials focus on VS Code, an IDE that sucks a whole lot less.
On another note: Transforming a 20 line T4 template took 4-5x as long as the actual compilation on the last .Net project I've seen.
I don't think it is that under-represented in North America. There are large swaths of "flyover country" that are deeply invested in .NET and C#. It was never "hip" or "cool" for Silicon Valley, but a lot of the Fortune 500 outside of FAANG/GAFAM/what-have-you is quietly built on it. Scott Hanselman termed it the "dark matter developer" experience that most .NET stuff just doesn't trend on Reddit or HN, it just kind of quietly exists and gets things done.
The differences were always there, but they change over time. I remember seeing a map ~15 years ago showing market shares of dev stacks in different countries. If I remember correctly, Europe on it was predominantly Java and Delphi, Australia and NZ were mostly .NET, and US and Canada were a mix Java and .NET with the former having a slight edge.
FAST FIX protocol is terrible performance-wise, its format requires multiple branching at every field parsing. Even "high-performance" libraries like mFAST are slow: I recently helped a client to optimize parsing for several messages and got 8x speed improvement over mFAST (which is a big deal in HFT space).
