StringReplace alternatives to improve performance

Question

I am using StringReplace to replace > and < by the char itself in a generated XML like this:

StringReplace(xml.Text,\'>\',\'>\',[rfReplaceA         


        

Answer 1

Systools (Turbopower, now open source) has a ReplaceStringAllL function that does all of them in a string.

Answer 2

Try FastStrings.pas from Peter Morris.

Answer 3

Untested conversion of the C# code written by Jorge Ferreira.

function ReplaceLtGt(const s: string): string;
var
  inPtr, outPtr: integer;
begin
  SetLength(Result, Length(s));
  inPtr := 1;
  outPtr := 1;
  while inPtr <= Length(s) do begin
    if (s[inPtr] = '&') and ((inPtr + 3) <= Length(s)) and
       (s[inPtr+1] in ['l', 'g']) and (s[inPtr+2] = 't') and
       (s[inPtr+3] = ';') then
    begin
      if s[inPtr+1] = 'l' then
        Result[outPtr] :=  '<'
      else
        Result[outPtr] := '>';
      Inc(inPtr, 3);
    end
    else begin
      Result[outPtr] := Result[inPtr];
      Inc(inPtr);
    end;
    Inc(outPtr);
  end;
  SetLength(Result, outPtr - 1);
end;

Answer 4

The problem is that you are iterating the entire string size twice (one for replacing &gt; by > and another one to replace &lt; by <).

You should iterate with a for and simply check ahead whenever you find a & for a gt; or lt; and do the immediate replace and then skipping 3 characters ((g|l)t;). This way it can do that in proportional time to the size of the string xml.Text.

---

A simple C# example as I do not know Delphi but should do for you to get the general idea.

String s = "<xml>test</xml>";
char[] input = s.ToCharArray();
char[] res = new char[s.Length];
int j = 0;
for (int i = 0, count = input.Length; i < count; ++i)
{
    if (input[i] == '&')
    {
        if (i < count - 3)
        {
            if (input[i + 1] == 'l' || input[i + 1] == 'g')
            {
                if (input[i + 2] == 't' && input[i + 3] == ';')
                {
                    res[j++] = input[i + 1] == 'l' ? '<' : '>';
                    i += 3;
                    continue;
                }
            }
        }
    }

    res[j++] = input[i];
}
Console.WriteLine(new string(res, 0, j));

This outputs:

<xml>test</xml>

Answer 5

You should definitely look at the Fastcode project pages: http://fastcode.sourceforge.net/

They ran a challenge for a faster StringReplace (Ansi StringReplace challenge), and the 'winner' was 14 times faster than the Delphi RTL.

Several of the fastcode functions have been included within Delphi itself in recent versions (D2007 on, I think), so the performance improvement may vary dramatically depending on which Delphi version you are using.

As mentioned before, you should really be looking at a Unicode-based solution if you're serious about processing XML.

Answer 6

When you are dealing with a multiline text files, you can get some performance by processing it line by line. This approach reduced time in about 90% to replaces on >1MB xml file.

procedure ReplaceMultilineString(xml: TStrings);
var
  i: Integer;
  line: String;
begin
  for i:=0 to xml.Count-1 do
  begin
    line := xml[i];
    line := StringReplace(line, '>', '>', [rfReplaceAll]);
    line := StringReplace(line, '&lt;', '<', [rfReplaceAll]);
    xml[i] := line;
  end;
end;