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Date: Fri, 11 Dec 2020 11:29:56 -0800 (PST)
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> Perform_Round( Source => Source , Result => Result , ... ); 
> declare Tmp: Chart_Array renames Source; 
> begin 
> Source := Result; 
> Result := Tmp; 
> end; 

To complete Jeff's answer, what you are doing is not a swap: it's equivalent to 
Source := Result;
Result := Source;
Ooops.
The ":=" makes a complete copy of the arrays.
So you'd need above "Tmp : Chart_Array := Source" to have a correct swap with 3 copies.
Of course it feels a bit heavy to make all those copies. And you understandably want to avoid playing with pointers. So a solution is to define:
Map : array (0 .. 1) of Chart_Array;
...
current := 0;
loop
  next := 1 - current;
  Perform_Round( Source => Map (current), Result => Map (next) , ... ); 
  current := next;
end loop;