Question

I have a method where performance is really important (I know premature optimization is the root of all evil. I know I should and I did profile my code. In this application every tenth of a second I save is a big win.) This method uses different heuristics to generate and return elements. The heuristics are used sequentially: the first heuristic is used until it can no longer return elements, then the second heuristic is used until it can no longer return elements and so on until all heuristics have been used. On each call of the method I use a switch to move to the right heuristic. This is ugly, but work well. Here is some pseudo code

class MyClass
{
private:
   unsigned int m_step;
public:
   MyClass() : m_step(0) {};

   Elem GetElem()
   {
      // This switch statement will be optimized as a jump table by the compiler.
      // Note that there is no break statments between the cases.
      switch (m_step)
      {
      case 0:
         if (UseHeuristic1())
         {
            m_step = 1; // Heuristic one is special it will never provide more than one element.
            return theElem;
         }

         m_step = 1;

      case 1:
         DoSomeOneTimeInitialisationForHeuristic2();
         m_step = 2;

      case 2:
         if (UseHeuristic2())
         {
            return theElem;
         }

         m_step = 3;

      case 3:
         if (UseHeuristic3())
         {
            return theElem;
         }
         m_step = 4; // But the method should not be called again
      }

      return someErrorCode;
   };
}

As I said, this works and it’s efficient since at each call, the execution jumps right where it should. If a heuristic can’t provide an element, m_step is incremented (so the next time we don’t try this heuristic again) and because there is no break statement, the next heuristic is tried. Also note that some steps (like step 1) never return an element, but are one time initialization for the next heuristic.

The reason initializations are not all done upfront is that they might never be needed. It is always possible (and common) for GetElem to not get called again after it returned an element, even if there are still elements it could return.

While this is an efficient implementation, I find it really ugly. The case statement is a hack; using it without break is also hackish; the method gets really long, even if each heuristic is encapsulated in its own method.

How should I refactor this code so it’s more readable and elegant while keeping it as efficient as possible?

Answer 1

To my mind if you do not need to modify this code much, eg to add new heuristics then document it well and don’t touch it.

However if new heuristics are added and removed and you think that this is an error prone process then you should consider refactoring it. The obvious choice for this would be to introduce the State design pattern. This will replace your switch statement with polymorphism which might slow things down but you would have to profile both to be sure.