Mercury pwns Yuji Naka

A lot of people hate the Sonic 1 Special Stages. Watching YouTube playthroughs, you'll hear them profaned quite often.

I'm pretty good at beating them, myself, but I still hate them. Because I'm scientifically minded, I tried to figure out just why this is so.

Then it hit me - Sonic doesn't have a variable jump height. Unlike when he's in the normal zones, you can't just let go of the jump button to mitigate his upward velocity.

Having noticed this, it struck me that it was most likely the largest contributor to the hate for the Special Stages. Because you don't have as much control over Sonic as you've become accustomed to from playing the normal zones, it's frustrating and unfair. Trying to navigate through cramped paths winds up being an exercise in hitting every bumper and and reverse block through no fault of you own - Sonic just makes a full jump no matter what you do.

Well, it was clear - I had to remedy this. I looked in the disassembly, and lo! - there was an unused routine for reducing Sonic's jump height in the Special Stages. Of course, merely reactivating the routine would be far too simple a fix - there had to be more than that. After all, if it worked, why would it be deprecated in the first place?

It turns out the unused routine is the same as the one from Sonic's normal code. It's deprecated because, in the Special Stages, Sonic's in a rotating maze, where up isn't always up. The routine is simply impotent at any angle other than 0.

Well, Yuji Naka might be lazy, but I'm not - so I wrote a new routine that actually does work, at any angle.

This is what I did, and if you follow the steps, you can, too. This is for the SVN disassembly:

First, open "_incObj\09 Sonic in Special Stage.asm" and find the label "Obj09_InAir". Replace this line:

Code (ASM):

1.  
2. bsr.w   nullsub_2
3.  

with this one:

Code (ASM):

1.  
2. bsr.w   Obj09_JumpHeight
3.  

Now, find the "nullsub_2" label. This is the deprecated routine, and we need to replace it with the new one I wrote, "Obj09_JumpHeight". So replace these lines:

Code (ASM):

1.  
2. nullsub_2:              ; XREF: Obj09_InAir
3.         rts
4. ; End of function nullsub_2
5.  
6. ; ===========================================================================
7. ; ---------------------------------------------------------------------------
8. ; unused subroutine to limit Sonic's upward vertical speed
9. ; ---------------------------------------------------------------------------
10.         move.w  #-$400,d1
11.         cmp.w   obVelY(a0),d1
12.         ble.s   locret_1BBB4
13.         move.b  (v_jpadhold2).w,d0
14.         andi.b  #btnABC,d0
15.         bne.s   locret_1BBB4
16.         move.w  d1,obVelY(a0)
17.  
18. locret_1BBB4:
19.         rts
20.  

with these:

Code (ASM):

1.  
2. ; ---------------------------------------------------------------------------
3. ; Subroutine to limit Sonic's upward vertical speed
4. ; ---------------------------------------------------------------------------
5.  
6. ; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||
7.  
8.  
9. Obj09_JumpHeight:               ; XREF: Obj09_InAir
10.         move.b  (v_jpadhold2).w,d0  ; is the jump button up?
11.         andi.b  #btnABC,d0
12.         bne.s   locret_1BBB4        ; if not, branch to return
13.         btst    #7,obStatus(a0)     ; did Sonic jump or is he just falling or hit by a bumper?
14.         beq.s   locret_1BBB4        ; if not, branch to return
15.         move.b  (v_ssangle).w,d0    ; get SS angle
16.         andi.b  #$FC,d0
17.         neg.b   d0
18.         subi.b  #$40,d0
19.         jsr (CalcSine).l           
20.         move.w  obVelY(a0),d2       ; get Y speed
21.         muls.w  d2,d0               ; multiply Y speed by sin
22.         asr.l   #8,d0               ; find the new Y speed
23.         move.w  obVelX(a0),d2       ; get X speed
24.         muls.w  d2,d1               ; multiply X speed by cos
25.         asr.l   #8,d1               ; find the new X speed
26.         add.w   d0,d1               ; combine the two speeds
27.         cmpi.w  #$400,d1            ; compare the combined speed with the jump release speed
28.         ble.s   locret_1BBB4        ; if it's less, branch to return
29.         move.b  (v_ssangle).w,d0
30.         andi.b  #$FC,d0
31.         neg.b   d0
32.         subi.b  #$40,d0
33.         jsr (CalcSine).l
34.         muls.w  #$400,d1
35.         asr.l   #8,d1
36.         move.w  d1,obVelX(a0)
37.         muls.w  #$400,d0
38.         asr.l   #8,d0
39.         move.w  d0,obVelY(a0)       ; set the speed to the jump release speed
40.         bclr    #7,obStatus(a0)     ; clear "Sonic has jumped" flag
41.  
42. locret_1BBB4:
43.         rts
44.  

Next, go to the "Obj09_OnWall" label, and add this line directly after it:

Code (ASM):

1.  
2. bclr    #7,obStatus(a0) ; clear "Sonic has jumped" flag
3.  

Now, go to the "Obj09_Jump" label, and after this line:

Code (ASM):

1.  
2. bset    #1,obStatus(a0)
3.  

add this one:

Code (ASM):

1.  
2. bset    #7,obStatus(a0) ; set "Sonic has jumped" flag
3.  

Finally, go to the "Obj09_ChkBumper" label, and after this line:

Code (ASM):

1.  
2. bset    #1,obStatus(a0)
3.  

add this one:

Code (ASM):

1.  
2. bclr    #7,obStatus(a0) ; clear "Sonic has jumped" flag
3.  

Et voilà! Basically, what it does is it combines the X and Y speed using cos and sin to find how fast he's moving upward at the current angle. It compares this combined speed with the jump release speed ($400) and if it's greater, then it uses similar code to the normal jump to make Sonic move at $400 in the current angle. Simple.

When my friend (who's always been bad at the Sonic 1 Special Stages by her own admission, and hasn't played them in years) tested it out, she got the Chaos Emerald on her first try!

Yuji Naka has had to put up with a lot of shit in his time (not least of which the schoolyard bullies who taunted him in his youth for having a name that sounds for all the world like "huge knocker"), and now it's my turn to rub it in his face:

PWN, Yuji Naka!

+ - The false bravado in this post is for humorous purposes only. I actually admire Yuji Naka a lot, and I'm sure he could improve my code greatly after but a brief glance at it.  

So, let me know what you think of this, or if you find any bugs.

Here's the built rom with the fix applied.

 

What do you mean by the "8 bit style egg prison"? Forgive my ignorance.