⮜ Blog

📝 Posted:
🚚 Summary of:
P0153, P0154, P0155, P0156
Commits:
624e0cb...d05c9ba, d05c9ba...031b526, 031b526...9ad578e, 9ad578e...4bc6405
💰 Funded by:
Ember2528
🏷 Tags:
rec98 th01 gameplay boss konngara danmaku-pattern waste rng

📝 7 pushes to get Konngara done, according to my previous estimate? Well, how about being twice as fast, and getting the entire boss fight done in 3.5 pushes instead? So much copy-pasted code in there… without any flashy unused content, apart from four calculations with an unclear purpose. And the three strings "ANGEL", "OF", "DEATH", which were probably meant to be rendered using those giant upscaled font ROM glyphs that also display the STAGE # and HARRY UP strings? Those three strings are also part of Sariel's code, though.

On to the remaining 11 patterns then! Konngara's homing snakes, shown in the video above, are one of the more notorious parts of this battle. They occur in two patterns – one with two snakes and one with four – with all of the spawn, aim, update, and render code copy-pasted between the two. :zunpet: Three gameplay-related discoveries here:

  • The homing target is locked once the Y position of a snake's white head diamond is below 300 pixels.
  • That diamond is also the only one with collision detection…
  • …but comes with a gigantic 30×30 pixel hitbox, reduced to 30×20 while Reimu is sliding. For comparison: Reimu's regular sprite is 32×32 pixels, including transparent areas. This time, there is a clearly defined hitbox around Reimu's center pixel that the single top-left pixel can collide with. No imagination necessary, which people apparently 📝 still prefer over actually understanding an algorithm… Then again, this hitbox is still not intuitive at all, because…

    … the exact collision pixel, marked in red, is part of the diamond sprite's transparent background :tannedcirno:


This was followed by really weird aiming code for the "sprayed pellets from cup" pattern… which can only possibly have been done on purpose, but is sort of mitigated by the spraying motion anyway.
After a bunch of long if(…) {…} else if(…) {…} else if(…) {…} chains, which remain quite popular in certain corners of the game dev scene to this day, we've got the three sword slash patterns as the final notable ones. At first, it seemed as if ZUN just improvised those raw number constants involved in the pellet spawner's movement calculations to describe some sort of path that vaguely resembles the sword slash. But once I tried to express these numbers in terms of the slash animation's keyframes, it all worked out perfectly, and resulted in this:

Yup, the spawner always takes an exact path along this triangle. Sometimes, I wonder whether I should just rush this project and don't bother about naming these repeated number literals. Then I gain insights like these, and it's all worth it.


Finally, we've got Konngara's main function, which coordinates the entire fight. Third-longest function in both TH01 and all of PC-98 Touhou, only behind some player-related stuff and YuugenMagan's gigantic main function… and it's even more of a copy-pasta, making it feel not nearly as long as it is. Key insights there:

  • The fight consists of 7 phases, with the entire defeat sequence being part of the if(boss_phase == 7) {…} branch.
  • The three even-numbered phases, however, only light up the Siddhaṃ seed syllables and then progress to the next phase.
  • Odd-numbered phases are completed after passing an HP threshold or after seeing a predetermined number of patterns, whatever happens first. No possibility of skipping anything there.
  • Patterns are chosen randomly, but the available pool of patterns is limited to 3 specific "easier" patterns in phases 1 and 5, and 4 patterns in phase 3. Once Phase 7 is reached at 9 HP remaining, all 12 patterns can potentially appear. Fittingly, that's also the point where the red section of the HP bar starts.
    • Every time a pattern is chosen, the code only makes a maximum of two attempts at picking a pattern that's different from the one that Konngara just completed. Therefore, it seems entirely possible to see the same pattern twice. Calculating an actual seed to prove that is out of the scope of this project, though.
    • Due to what looks like a copy-paste mistake, the pool for the second RNG attempt in phases 5 and 7 is reduced to only the first two patterns of the respective phases? That's already quite some bias right there, and we haven't even analyzed the RNG in detail yet… :onricdennat: (For anyone interested, it's a LCG, using the Borland C/C++ parameters as shown here.)
  • The difficulty level only affects the speed and firing intervals (and thus, number) of pellets, as well as the number of lasers in the one pattern that uses them.
  • After the 📝 kuji-in defeat sequence, the fight ends in an attempted double-free of Konngara's image data. :godzun: Thankfully, the format-specific _free() functions defend against such a thing.
Seriously, 📝 line drawing was much harder to decompile.


And that's it for Konngara! First boss with not a single piece of ASM left, 30 more to go! 🎉 But wait, what about the cause behind the temporary green discoloration after leaving the Pause menu? I expected to find something on that as well, but nope, it's nothing in Konngara's code segment. We'll probably only get to figure that out near the very end of TH01's decompilation, once we get to the one function that directly calls all of the boss-specific main functions in a switch statement.

So, Sariel next? With half of a push left, I did cover Sariel's first few initialization functions, but all the sprite unblitting and HUD manipulation will need some extra attention first. The first one of these functions is related to the HUD, the stage timer, and the HARRY UP mode, whose pellet pattern I've also decompiled now.

All of this brings us past 75% PI in all games, and TH01 to under 30,000 remaining ASM instructions, leaving TH03 as the now most expensive game to be completely decompiled. Looking forward to how much more TH01's code will fall apart if you just tap it lightly… Next up: The aforementioned helper functions related to HARRY UP, drawing the HUD, and unblitting the other bosses whose sprites are a bit more animated.