📝 Posted:

2021-09-28 16:05 UTC

🚚 Summary of:

P0158, P0159

⌨ Commits:

bf7bb7e...c0c0ebc, c0c0ebc...e491cd7

💰 Funded by:

Yanga

🏷 Tags:

th01

rec98

gameplay

card-flipping

rng

score

bomb

animation

jank

waste

Of course, Sariel's potentially bloated and copy-pasted code is blocked by even more definitely bloated and copy-pasted code. It's TH01, what did you expect?

But even then, TH01's item code is on a new level of software architecture ridiculousness. First, ZUN uses distinct arrays for both types of items, with their own caps of 4 for bomb items, and 10 for point items. Since that obviously makes any type-related switch statement redundant, he also used distinct functions for both types, with copy-pasted boilerplate code. The main per-item update and render function is shared though… and takes every single accessed member of the item structure as its own reference parameter. Like, why, you have a structure, right there?! That's one way to really practice the C++ language concept of passing arbitrary structure fields by mutable reference…
To complete the unwarranted grand generic design of this function, it calls back into per-type collision detection, drop, and collect functions with another three reference parameters. Yeah, why use C++ virtual methods when you can also implement the effectively same polymorphism functionality by hand? Oh, and the coordinate clamping code in one of these callbacks could only possibly have come from nested min() and max() preprocessor macros. And that's how you extend such dead-simple functionality to 1¼ pushes…

Amidst all this jank, we've at least got a sensible item↔player hitbox this time, with 24 pixels around Reimu's center point to the left and right, and extending from 24 pixels above Reimu down to the bottom of the playfield. It absolutely didn't look like that from the initial naive decompilation though. Changing entity coordinates from left/top to center was one of the better lessons from TH01 that ZUN implemented in later games, it really makes collision detection code much more intuitive to grasp.

The card flip code is where we find out some slightly more interesting aspects about item drops in this game, and how they're controlled by a hidden cycle variable:

• At the beginning of every 5-stage scene, this variable is set to a random value in the [0..59] range
• Point items are dropped at every multiple of 10
• Every card flip adds 1 to its value after this mod 10 check
• At a value of 140, the point item is replaced with a bomb item, but only if no damaging bomb is active. In any case, its value is then reset to 1.

Then again, score players largely ignore point items anyway, as card combos simply have a much bigger effect on the score. With this, I should have RE'd all information necessary to construct a tool-assisted score run, though?
Edit: Turns out that 1) point items are becoming increasingly important in score runs, and 2) Pearl already did a TAS some months ago. Thanks to spaztron64 for the info!

The Orb↔card hitbox also makes perfect sense, with 24 pixels around the center point of a card in every direction.

The rest of the code confirms the card flip score formula documented on Touhou Wiki, as well as the way cards are flipped by bombs: During every of the 90 "damaging" frames of the 140-frame bomb animation, there is a 75% chance to flip the card at the [bomb_frame % total_card_count_in_stage] array index. Since stages can only have up to 50 cards 📝 thanks to a bug, even a 75% chance is high enough to typically flip most cards during a bomb. Each of these flips still only removes a single card HP, just like after a regular collision with the Orb.
Also, why are the card score popups rendered before the cards themselves? That's two needless frames of flicker during that 25-frame animation. Not all too noticeable, but still.

And that's over 50% of REIIDEN.EXE decompiled as well! Next up: More HUD update and rendering code… with a direct dependency on rank pellet speed modifications?