What is the secret operator?

Question

There is a secret operator $F$ that takes two whole numbers and outputs a new number. For example

$$\begin{split}F(6,9) &= 15\\ F(66,11) &= 110 \\ F(86,18) &= 179\end{split}$$

Can you find what $F$ does and what is the output of $F(601,81)$ ?

Hint:

F may not be defined for all inputs

Answer 1

The answer could be:

Rotating the whole numbers, rather parameters, and not the individual integers, by 180 degrees and adding them together.

$F( 6, 9) \quad\rightarrow 9 + 6 \quad= 15$
$F(66,11) \rightarrow 99 + 11 = 110$
$F(86,18) \rightarrow 98 + 81 = 179$

Therefore the output of $F(601, 81)$ would be

$109 + 18 = 127$

Answer 2

For what it's worth, here's the solution as a short C# program. No string operations necessary :)

void Main()
{
  F(6,9).Dump();      //  15
  F(66,11).Dump();    // 110
  F(86,18).Dump();    // 179
  F(601,81).Dump();   // 127
 }

 int F(int x, int y)
 {
  return rotate(x)+rotate(y);
 }

 int rotate(int num)
 {
  int result =0;
  for (; num > 0; num /= 10)
  {
      int digit = num % 10;
      switch (digit)
      {
          case 0: case 1: case 2: case 5: case 8: break;
          case 3: case 4: case 7: throw new ArgumentException($"can't flip {digit}");
          case 6: digit = 9; break;
          case 9: digit = 6; break;
          default: throw new ArgumentException("I can't even");
      }
      result = result * 10 + digit;
  }
  return result;
 }