Delphi function comparing content of two TStream? - delphi

Question

I need to compare if two TStream descendant have the same content.
The only interesting result for me is the boolean Yes / No.
I'm going to code a simple loop checking byte after byte the streams content's.
But I'm curious to know if there is an already existing function. I haven't found any inside DelphiXE or JCL/JVCL libs.
Of course, the two streams have the same size !

Answer 1

Exactly, as Nickolay O. said you should read your stream in blocks and use CompareMem. Here is an example (including size test) ...
function IsIdenticalStreams(Source, Destination: TStream): boolean;
const Block_Size = 4096;
var Buffer_1: array[0..Block_Size-1] of byte;
Buffer_2: array[0..Block_Size-1] of byte;
Buffer_Length: integer;
begin
Result := False;
if Source.Size <> Destination.Size then
Exit;
while Source.Position < Source.Size do
begin
Buffer_Length := Source.Read(Buffer_1, Block_Size);
Destination.Read(Buffer_2, Block_Size);
if not CompareMem(#Buffer_1, #Buffer_2, Buffer_Length) then
Exit;
end;
Result := True;
end;

Answer 2

The IsIdenticalStreams function posted by daemon_x is excellent - but needs one adjustment to work properly. (Uwe Raabe caught the issue already.) It is critical that you reset the stream positions before starting the loop - or this procedure will probably return an incorrect TRUE if the two streams were already accessed outside this function.
This is the final solution that works every time. I just renamed the function to suit my naming conventions. Thank you daemon_x for the elegant solution.
function StreamsAreIdentical(Stream1, Stream2: TStream): boolean;
const
Block_Size = 4096;
var
Buffer_1: array[0..Block_Size-1] of byte;
Buffer_2: array[0..Block_Size-1] of byte;
Buffer_Length: integer;
begin
Result := False;
if Stream1.Size <> Stream2.Size then exit;
// These two added lines are critical for proper operation
Stream1.Position := 0;
Stream2.Position := 0;
while Stream1.Position < Stream1.Size do
begin
Buffer_Length := Stream1.Read(Buffer_1, Block_Size);
Stream2.Read(Buffer_2, Block_Size);
if not CompareMem(#Buffer_1, #Buffer_2, Buffer_Length) then exit;
end;
Result := True;
end;

Answer 3

There is no such built-in function. Only one thing I can recommend - read not byte-to-byte, but using blocks of 16-64kbytes, that would be much faster.

Answer 4

Answers from user532231 and Mike are working in 99% cases, but there are additional checks to be made!
Descendants of TStream can be almost anything, so it's not guaranteed that Stream.Read will return same amount of data, even if streams are of the same length (stream descendant can also download data, so may return readed=0 bytes, while waiting for next chunk). Streams can be also on completelly different media and stream read error could occur on just one.
For 100% working code all these checks should be made. I modified the function from Mike.
If this function is used for example to rewrite stream 2 if not identical to Stream1, all errors should be checked. When function result is True, everthing is ok, but if it is False, it would be very smart to check if Streams are actually different or just some error occured.
Edited: Added some additional checks, FilesAreIdentical function based on StreamsAreIdentical and usage example.
// Usage example
var lError: Integer;
...
if FilesAreIdentical(lError, 'file1.ext', 'file2.ext')
then Memo1.Lines.Append('Files are identical.')
else case lError of
0: Memo1.Lines.Append('Files are NOT identical!');
1: Memo1.Lines.Append('Files opened, stream read exception raised!');
2: Memo1.Lines.Append('File does not exist!');
3: Memo1.Lines.Append('File open exception raised!');
end; // case
...
// StreamAreIdentical
function StreamsAreIdentical(var aError: Integer;
const aStream1, aStream2: TStream;
const aBlockSize: Integer = 4096): Boolean;
var
lBuffer1: array of byte;
lBuffer2: array of byte;
lBuffer1Readed,
lBuffer2Readed,
lBlockSize: integer;
begin
Result:=False;
aError:=0;
try
if aStream1.Size <> aStream2.Size
then Exit;
aStream1.Position:=0;
aStream2.Position:=0;
if aBlockSize>0
then lBlockSize:=aBlockSize
else lBlockSize:=4096;
SetLength(lBuffer1, lBlockSize);
SetLength(lBuffer2, lBlockSize);
lBuffer1Readed:=1; // just for entering while
while (lBuffer1Readed > 0) and (aStream1.Position < aStream1.Size) do
begin
lBuffer1Readed := aStream1.Read(lBuffer1[0], lBlockSize);
lBuffer2Readed := aStream2.Read(lBuffer2[0], lBlockSize);
if (lBuffer1Readed <> lBuffer2Readed) or ((lBuffer1Readed <> lBlockSize) and (aStream1.Position < aStream1.Size))
then Exit;
if not CompareMem(#lBuffer1[0], #lBuffer2[0], lBuffer1Readed)
then Exit;
end; // while
Result:=True;
except
aError:=1; // stream read exception
end;
end;
// FilesAreIdentical using function StreamsAreIdentical
function FilesAreIdentical(var aError: Integer;
const aFileName1, aFileName2: String;
const aBlockSize: Integer = 4096): Boolean;
var lFileStream1,
lFilestream2: TFileStream;
begin
Result:=False;
try
if not (FileExists(aFileName1) and FileExists(aFileName2))
then begin
aError:=2; // file not found
Exit;
end;
lFileStream1:=nil;
lFileStream2:=nil;
try
lFileStream1:=TfileStream.Create(aFileName1, fmOpenRead or fmShareDenyNone);
lFileStream2:=TFileStream.Create(aFileName2, fmOpenRead or fmShareDenyNone);
result:=StreamsAreIdentical(aError, lFileStream1, lFileStream2, aBlockSize);
finally
if lFileStream2<>nil
then lFileStream2.Free;
if lFileStream1<>nil
then lFileStream1.Free;
end; // finally
except
aError:=3; // file open exception
end; // except
end;