Fast String Append in Delphi

I don't normally put code in this blog, but bear with me on this one, it's a good one!

I needed to append a lot of strings together really quickly; I was sitting in a loop, getting strings from another function, and appending them to a result string. These strings weren't small, and the loop was going round 20,000 times; the operation took 10 minutes. Blah. There must be a better way.

I built the class below, and implemented using it. Now it takes about 2 seconds. Fantastic!

It's a simple thing, but pretty damned useful if you are building up big XML as I was.

Here it is:

TFastStringAppend = class

private

  arrchar: Array of Char;

  flength: integer;

  fcapacity: integer;

  function GetResultString: string;

public

  constructor create;

  procedure Append(astr: String);

  property ResultString: string read GetResultString;

end;



{ TFastStringAppend }



procedure TFastStringAppend.Append(astr: String);

var

  newlength: integer;

begin

  newlength := flength + length(astr);

  if newlength > fcapacity then

  begin

    fcapacity := round(newlength * 1.5);

    setlength(arrchar, fcapacity + 1);

  end;



  strpcopy(@arrchar[flength], astr);

  flength := newlength;

  arrchar[flength] := char(0);

end;



constructor TFastStringAppend.create;

begin

  setlength(arrchar, 1);

  arrchar[0] := char(0);

  flength := 0;

  fcapacity := 0;

end;



function TFastStringAppend.GetResultString: string;

begin

  result := pchar(@arrchar[0]);

end;

You use it as follows:

function GetAReallyBigString: string;

var

  fsa: TFastStringAppend;

begin

  try

    fsa := TFastStringAppend.create; 

    // fsa is initialised holding the empty string

    for t := 0 to 19999 do

    begin

      // function SomeFunkyString(i: integer): string;

      fsa.Append(SomeFunkyString(t));

    end;

    // and here's the mighty result string!

    result := fsa.ResultString;

  finally

    FreeAndNil(fsa.free); // FreeAndNil is in Sysutils

  end;

end;