Restore lost CPZ boss feature

(Original guide by Esrael)

At one point of development, there was to be an intended feature involving the Chemical Plant Zone boss, which would see Robotnik accidentally pour chemicals onto himself. This feature was taken out at some point, and has never been seen in any official build. However, a search through the ROM would reveal that the pieces needed for said feature still exist in the game. Now, this guide will show you how to make Dr. Eggman look like a dumbass and pour chemicals onto himself! (It isn't 100% certain if this was the EXACT intended way for this to occur, but if you're going to nitpick at that, go get bent.)

First, find the code:

 loc_2DAD4:
        move.w  d0,(a1)
        subq.b  #1,objoff_3E(a0)
        bne.s   return_2DAE8
        move.b  #$F,collision_flags(a0)
        bclr    #1,objoff_2D(a0)

return_2DAE8:
        rts

and replace it with this

 loc_2DAD4:
        move.w  d0,(a1)
        bset    #1,objoff_2D(a0)   ; Add this line to code
        subq.b  #1,objoff_3E(a0)
        bne.s   return_2DAE8
        move.b  #$F,collision_flags(a0)
        bclr    #1,objoff_2D(a0)

return_2DAE8:
        rts

After you made the changes. Give it a go. Wait for robotnik to fill the bottle, then hit him. You will see the once missing animation working again, though it only works when the bottle is fully filled, and you hit him at the right time. Now, thats all well and good, but... there are other things we need to fix before we call it a day.

One thing, though - the existing code seems to display the art slightly too far to the right for some reason. 3 pixels to be exact! So, we need to shift ol' Robotnik 3 pixels to the left.

There are 2 ways to do this. Here is the ASM fix:

Just edit mappings into the following file: Obj5D_MapUnc_2EADC: BINCLUDE "mappings/sprite/obj5D_a.bin"

The last 3 mappings in the file is animation: Change this:

               0001E00A004A0025FFF0
               0001E80D00530029FFF0
               0001E80D005B002DFFF0

to this:

               0001E00A004A0025FFED
               0001E80D00530029FFED
               0001E80D005B002DFFED

Alternatively just load the mapping file along with the CPZ Boss art file on SonMapEd and nudge the last three sprites three pixels left.

Now, there is one more issue with this feature. The animation won't turn around when Robotnik's ship does, leaving the goo-with-eyes occasionally floating away from Robotnik's head. Lets fix that now...

To fix this in the 2007 disassembly, find:

loc_2E790:

        subi.b  #1,anim_frame_duration(a0)
        bpl.s   loc_2E7B6

Or in the GitHub disassembly:

Obj5D_Gunk_6:
	subi_.w	#1,Obj5D_timer2(a0)
	bpl.s	+

and add the following code to the start of loc_2E790:

                
                cmpi.b  #$25, mapping_frame(A0)
                blt.s   _CPz_2
                move.w  (MainCharacter+x_pos).w, D0
                sub.w   x_pos(A0), D0
                bgt.s   _CPz_1
                bclr    #$00, render_flags(A0)
                bra.s   _CPz_2
_CPz_1:
                bset    #$00, render_flags(A0)                   
_CPz_2:

|Restore lost CPZ boss feature]]