Question

i have a function who’s job is to convert an ADO Recordset into html:

class function RecordsetToHtml(const rs: _Recordset): WideString;

And the guts of the function involves a lot of wide string concatenation:

   while not rs.EOF do
   begin
      Result := Result+CRLF+
         '<TR>';

      for i := 0 to rs.Fields.Count-1 do
         Result := Result+'<TD>'+VarAsWideString(rs.Fields[i].Value)+'</TD>';

      Result := Result+'</TR>';
      rs.MoveNext;
    end;

With a few thousand results, the function takes, what any user would feel, is too long to run. The Delphi Sampling Profiler shows that 99.3% of the time is spent in widestring concatenation (@WStrCatN and @WstrCat).

Can anyone think of a way to improve widestring concatenation? i don’t think Delphi 5 has any kind of string builder. And Format doesn’t support Unicode.

---

And to make sure nobody tries to weasel out: pretend you are implementing the interface:

IRecordsetToHtml = interface(IUnknown)
    function RecordsetToHtml(const rs: _Recordset): WideString;
end;

---

Update One

I thought of using an IXMLDOMDocument, to build up the HTML as xml. But then i realized that the final HTML would be xhtml and not html – a subtle, but important, difference.

Update Two

Microsoft knowledge base article: How To Improve String Concatenation Performance

Answer 1

i found the best solution. The open source HtmlParser for Delphi, has a helper TStringBuilder class. It is internally used to build what he calls DomStrings, which is actually an alias of WideString:

TDomString = WideString;

With a little bit of fiddling of his class:

TStringBuilder = class
public
   constructor Create(ACapacity: Integer);
   function EndWithWhiteSpace: Boolean;
   function TailMatch(const Tail: WideString): Boolean;
   function ToString: WideString;
   procedure AppendText(const TextStr: WideString);
   procedure Append(const value: WideString);
   procedure AppendLine(const value: WideString);
   property Length: Integer read FLength;
end;

The guts of the routine becomes:

while not rs.EOF do
begin
   sb.Append('<TR>');

   for i := 0 to rs.Fields.Count-1 do
      sb.Append('<TD>'+VarAsWideString(rs.Fields[i].Value));

   sb.AppendLine('</TR>');

   rs.MoveNext;
end;

The code then feels to run infinitely afaster. Profiling shows much improvement; the WideString manipulation and length-counting became negligible. In its place was FastMM’s own internal operations.

Notes

1. Nice catch on the mistaken forcing of all strings into current code-page (VarAsString rather than VarAsWideString)
2. Some HTML closing tags are optional; omitted ones that logically make no sense.