Re: State machines and Goto's (was Re: Should internet support software be written in Ada?)

[ncohen@watson.ibm.com (Norman H. Cohen)]

In article <3m0nv5$67l@theopolis.orl.mmc.com>, Theodore Dennison
<dennison@escmail.orl.mmc.com> writes: 

|> I have now worked under 3 different Ada coding standards. Every one
|> of them had a "no goto's" rule. Since I'm currently editing our next
|> coding standard, perhaps someone could suggest a replacement rule?

Throughout your coding standard, change all occurrences of "rule" to
"guideline" and all occurrences of "shall" to "shall generally".  Once
these global changes are made, no further local changes are needed for
your treatment of gotos.

Here's an example to ponder:  Consider the declarations

   type Node_Type;

   type Tree_Type is access Node_Type;

   type Node_Type is
      record
         Count       : Natural := 0;
         Left, Right : Tree_Type;
      end record;

   type Bit_Vector_Type is array (Natural range <>) of Boolean;

   type Bit_Vector_Pointer_Type is access Bit_Vector_Type;

   Root : Tree_Type;
   BV   : Bit_Vector_Pointer_Type;

BV points to a bit vector describing a path (False = left, True = right)
through the tree pointed to by Root.  We are counting the number of times
a given bit vector is encountered in the Count component of the
corresponding tree node.  Root starts out pointing to a tree consisting
only of a root node, and new nodes are added to the tree as needed.  The
loop below has three exits, and the actions appropriate when one of the
exits is taken is different from the actions appropriate when the other
two are taken, so a goto comes in handy.  Here's the code: 

      Node := Root;
      Bit := BV'First;
      loop
         if Bit > BV'Last then
            goto Increment;
         end if;
         if BV(Bit) then
            if Node.Left = null then
               Node.Left := new Node;
               Node := Node.Left;
               exit;
            else
               Node := Node.Left;
            end if;
         else
            if Node.Right = null then
               Node.Right := new Node;
               Node := Node.Right;
               exit;
            else
               Node := Node.Right;
            end if;
         end if;
         Bit := Bit + 1;
      end loop;

      -- We've reached a leaf.  Read the rest of BV.all while extending
      --    the tree along the corresponding path only.
      for I in Bit+1 .. BV'Last loop
         if BV(I) then
            Node.Left := new Node;
            Node := Node.Left;
         else
            Node.Right := new Node;
            Node := Node.Right;
         end if;
      end loop;

   <<Increment>>
      Node.Count := Node.Count + 1;

Knuth's article in the December 1974 Computing Surveys is still the best
discussion I've ever seen of the goto issue.  It ought to be required
reading early on in any Comp. Sci. curriculum.

--
Norman H. Cohen    ncohen@watson.ibm.com