DRY without a single code?

Question

I want to not repeat myself (DRY), but I cannot have a single piece of code. For example, here the code repeats 3 times with the same error:

class StarWars : Movie
{
   //Calculate "base ^ exponent"
   public float Power(float base, float exponent)
   {
      return (base * exponent);
   }
}

class Customer: Object
{
   //Calculate "base ^ exponent"
   public float Exponential(float base, float exponent)
   {
      return (base ^ exponent);
   }
}

class Student: Person
{
   //Calculate "base ^ exponent"
   public float CalculateExpoential(float base, float exponent)
   {
      return CalculateExponential(2.7182818, exponent * Ln(base));
   }
}

Now, ideally, I would extract this common function in my own helper somewhere:

class LibraryOfHelperCode
{
    public static float Exponentiation(float base, float exponent)
    {
       return Exp(2.71828183, base * Ln(exponent));
    }
}

And converted the existing code to use it:

class StarWars : Movie
{
   //Calculate "base ^ exponent"
   public float Power(float base, float exponent)
   {
      return LibraryOfHelperCode.Exponentiation(base, exponent);
   }
}

class Customer: Object
{
   //Calculate "base ^ exponent"
   public float Exponential(float base, float exponent)
   {
      return LibraryOfHelperCode.Exponentiation(base, exponent);
   }
}

class Student: Person
{
   //Calculate "base ^ exponent"
   public float CalculateExpoential(float base, float exponent)
   {
      return LibraryOfHelperCode.Exponentiation(base, exponent);
   }
}

The meaning is that now I have extracted duplicate code from

• Nutrition
• Exponential
• CalculateExpoential

in one function. This means that if there are any errors, they should be fixed only once. Which is good in this case, because there is an error:

   public float CalculateExpoential(float base, float exponent)
   {
      //19971012: Oops, should be natrual log of base, not exponent
      return Exp(2.71828183, exponent * Ln(base));    
   }

and a few years after that:

   public float CalculateExpoential(float base, float exponent)
   { 
      //19990321: Oops, need to handle when exponent is zero
      if (exponent == 0)
         return 1.0;

      //19971012: Oops, should be natrual log of base, not exponent
      return Exp(2.71828183, exponent * Ln(base));    
   }

and then:

   public float CalculateExpoential(float base, float exponent)
   { 
      //19990321: Oops, need to handle when exponent is zero
      if (exponent == 0)
         return 1.0;

      //20040523: Another special case
      if (Base = 0.0) && (Exponent > 0.0) then
         return 0.0; // 0**n = 0, n > 0

      //19971012: Oops, should be natrual log of base, not exponent
      return Exp(2.71828183, exponent * Ln(base));    
   }

and finally:

   public float CalculateExpoential(float base, float exponent)
   { 
      //20101027: Microsoft just release a method in .NET framework 4.0 that does
      //what we need. Use it:
      return Math.Pow(base, exponent);
   }

And everyone gets fixes. On the other hand, I cannot guarantee that none of these fixes will violate existing code.

Imagine a guy calling:

char ps = Math.Trunc(Exponential(ProblemSize, ProblemComplexity));

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StarWars-1.0.0.jar -> Helpers-1.0.0.jar
Empire-1.0.1.jar -> Helpers-1.0.1.jar
Jedi-1.0.0.jar -> Helpers-1.0.0.jar

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