(Wide)String - storing in TFileStream, Delphi 7. W

问题:

I'm using Delphi7 (non-unicode VCL), I need to store lots of WideStrings inside a TFileStream. I can't use TStringStream as the (wide)strings are mixed with binary data, the format is projected to speed up loading and writing the data ... However I believe that current way I'm loading/writing the strings might be a bottleneck of my code ...

currently I'm writing length of a string, then writing it char by char ... while loading, first I'm loading the length, then loading char by char ...

So, what is the fastest way to save and load WideString to TFileStream?

Thanks in advance

回答1:

Rather than read and write one character at a time, read and write them all at once:

procedure WriteWideString(const ws: WideString; stream: TStream);
var
  nChars: LongInt;
begin
  nChars := Length(ws);
  stream.WriteBuffer(nChars, SizeOf(nChars);
  if nChars > 0 then
    stream.WriteBuffer(ws[1], nChars * SizeOf(ws[1]));
end;

function ReadWideString(stream: TStream): WideString;
var
  nChars: LongInt;
begin
  stream.ReadBuffer(nChars, SizeOf(nChars));
  SetLength(Result, nChars);
  if nChars > 0 then
    stream.ReadBuffer(Result[1], nChars * SizeOf(Result[1]));
end;

Now, technically, since WideString is a Windows BSTR, it can contain an odd number of bytes. The Length function reads the number of bytes and divides by two, so it's possible (although not likely) that the code above will cut off the last byte. You could use this code instead:

procedure WriteWideString(const ws: WideString; stream: TStream);
var
  nBytes: LongInt;
begin
  nBytes := SysStringByteLen(Pointer(ws));
  stream.WriteBuffer(nBytes, SizeOf(nBytes));
  if nBytes > 0 then
    stream.WriteBuffer(Pointer(ws)^, nBytes);
end;

function ReadWideString(stream: TStream): WideString;
var
  nBytes: LongInt;
  buffer: PAnsiChar;
begin
  stream.ReadBuffer(nBytes, SizeOf(nBytes));
  if nBytes > 0 then begin
    GetMem(buffer, nBytes);
    try
      stream.ReadBuffer(buffer^, nBytes);
      Result := SysAllocStringByteLen(buffer, nBytes)
    finally
      FreeMem(buffer);
    end;
  end else
    Result := '';
end;

Inspired by Mghie's answer, have replaced my Read and Write calls with ReadBuffer and WriteBuffer. The latter will raise exceptions if they are unable to read or write the requested number of bytes.

回答2:

There is nothing special about wide strings, to read and write them as fast as possible you need to read and write as much as possible in one go:

procedure TForm1.Button1Click(Sender: TObject);
var
  Str: TStream;
  W, W2: WideString;
  L: integer;
begin
  W := 'foo bar baz';

  Str := TFileStream.Create('test.bin', fmCreate);
  try
    // write WideString
    L := Length(W);
    Str.WriteBuffer(L, SizeOf(integer));
    if L > 0 then
      Str.WriteBuffer(W[1], L * SizeOf(WideChar));

    Str.Seek(0, soFromBeginning);
    // read back WideString
    Str.ReadBuffer(L, SizeOf(integer));
    if L > 0 then begin
      SetLength(W2, L);
      Str.ReadBuffer(W2[1], L * SizeOf(WideChar));
    end else
      W2 := '';
    Assert(W = W2);
  finally
    Str.Free;
  end;
end;

回答3:

WideStrings contain a 'string' of WideChar's, which use 2 bytes each. If you want to store the UTF-16 (which WideStrings use internally) strings in a file, and be able to use this file in other programs like notepad, you need to write a byte order mark first: #$FEFF.

If you know this, writing can look like this:

Stream1.Write(WideString1[1],Length(WideString)*2); //2=SizeOf(WideChar)

reading can look like this:

Stream1.Read(WideChar1,2);//assert returned 2 and WideChar1=#$FEFF
SetLength(WideString1,(Stream1.Size div 2)-1);
Stream1.Read(WideString1[1],(Stream1.Size div 2)-1);

回答4:

You can also use TFastFileStream for reading the data or strings, I pasted the unit at http://pastebin.com/m6ecdc8c2 and a sample below:

program Project36;

{$APPTYPE CONSOLE}

uses
  SysUtils, Classes,
  FastStream in 'FastStream.pas';

const
  WideNull: WideChar = #0;

procedure WriteWideStringToStream(Stream: TFileStream; var Data: WideString);
var
  len: Word;
begin
  len := Length(Data);
  // Write WideString length
  Stream.Write(len, SizeOf(len));
  if (len > 0) then
  begin
    // Write WideString
    Stream.Write(Data[1], len * SizeOf(WideChar));
  end;
  // Write null termination
  Stream.Write(WideNull, SizeOf(WideNull));
end;

procedure CreateTestFile;
var
  Stream: TFileStream;
  MyString: WideString;
begin
  Stream := TFileStream.Create('test.bin', fmCreate);
  try
    MyString := 'Hello World!';
    WriteWideStringToStream(Stream, MyString);

    MyString := 'Speed is Delphi!';
    WriteWideStringToStream(Stream, MyString);
  finally
    Stream.Free;
  end;
end;

function ReadWideStringFromStream(Stream: TFastFileStream): WideString;
var
  len: Word;
begin
  // Read length of WideString
  Stream.Read(len, SizeOf(len));
  // Read WideString
  Result := PWideChar(Cardinal(Stream.Memory) + Stream.Position);
  // Update position and skip null termination
  Stream.Position := Stream.Position + (len * SizeOf(WideChar)) + SizeOf(WideNull);
end;

procedure ReadTestFile;
var
  Stream: TFastFileStream;

  my_wide_string: WideString;
begin
  Stream := TFastFileStream.Create('test.bin');
  try
    Stream.Position := 0;
    // Read WideString
    my_wide_string := ReadWideStringFromStream(Stream);
    WriteLn(my_wide_string);
    // Read another WideString
    my_wide_string := ReadWideStringFromStream(Stream);
    WriteLn(my_wide_string);
  finally
    Stream.Free;
  end;
end;

begin
  CreateTestFile;
  ReadTestFile;
  ReadLn;
end.