At the end of this chapter, I'll return to the now well-known SlowCode example. At the end of the previous chapter, we significantly adapted the code and ended with a version that calculates prime numbers with the Sieve of Eratosthenes (SlowCode_Sieve). That version processed 10 million numbers in 1,072 milliseconds. Let's see if we can improve that.

The obvious target for optimization is the Reverse function which creates the result by appending characters one at a time. We've seen in this chapter that modifying a string can cause frequent memory allocations:

function Reverse(s: string): string;
var
  ch: char;
begin
  Result := '';
  for ch in s do
    Result := ch + Result;
end;

Instead of optimizing this function, let's look at how it is used. The Filter method uses it to reverse a number:

reversed := StrToInt(Reverse(IntToStr(i)));

This statement brings in another memory allocation (in function IntToStr which creates a new string), and executes some code that has to...