Encode a HTTP URL

Problem/Question/Abstract:

How to encode a HTTP URL?

Answer:

function HTTPEncode(const AStr: string): string;
const
  NoConversion = ['A'..'Z', 'a'..'z', '*', '@', '.', '_', '-'];
var
  Sp, Rp: PChar;
begin
  SetLength(Result, Length(AStr) * 3);
  Sp := PChar(AStr);
  Rp := PChar(Result);
  while Sp^ <> #0 do
  begin
    if Sp^ in NoConversion then
      Rp^ := Sp^
    else if Sp^ = ' ' then
      Rp^ := '+'
    else
    begin
      FormatBuf(Rp^, 3, '%%%.2x', 6, [Ord(Sp^)]);
      Inc(Rp, 2);
    end;
    Inc(Rp);
    Inc(Sp);
  end;
  SetLength(Result, Rp - PChar(Result));
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Edit1.Text := HTTPEncode(Edit1.Text);
end;