Calculate hierarchical labels for Google Sheets using native functions

Question

Using Google Sheets, I want to automatically number rows like so:

The key is that I want this to use built-in functions only.

I have an

Answer 1

Lol this is hilariously the longest (and very likely the most unnecessarily complicated way to combine formulas) but because I thought it was interesting that it does in fact work, so long as you just add a 1 in the first row then in the second row you add:

=if(row()=1,1,if(and(istext(D2),counta(split(A1,"."))=3),left(A1,4)&n(right(A1,1)+1),if(and(isblank(B2),isblank(C2),isblank(D2)),"",if(and(isblank(B2),isblank(C2),isnumber(indirect(address(row()-1,column())))),indirect(address(row()-1,column()))&"."&if(istext(D2),round(max(indirect(address(1,column())&":"&address(row()-1,column())))+0.1,)),if(and(isblank(B2),istext(C2)),round(max(indirect(address(1,column())&":"&address(row()-1,column())))+0.1,2),if(istext(B2),round(max(indirect(address(1,column())&":"&address(row()-1,column())))+1,),))))))

in my defense ive had a very long day at work - complicating what should be a simple thing seems to be my thing today :)