I don't think the "living nightmare" control flow graph is that bad? B is once again a while(input == 0) loop, that exits to D. D and C together form a while(

I don't think the "living nightmare" control flow graph is that bad? B is once again a while(input == 0) loop, that exits to D. D and C together form a while(input == 1) loop, that exits to A. And A is again a switch between two routes, although in this case, rather than dispatching to one of two independent routes, it switches between a full route (go to B) and a partial route (jump straight to D); basically you could consider B and C to exist together in an if-block of sorts.