Clac CRC before downloading

[sdean]

Hi , i use indy 'idhttp' to download files , But what i want is :

Before begin downloading a file from server i should calc its CRC why :
because i must be sure if this file has not already been downloaded and saved .

please is there any way or other alternative to acheive that ? .

[wicht]

Hi,

this might not be possible because you can calculate the CRC only if you already downloaded the file I think. Is this over HTTP? I think there is no HTTP command to get the checksum of a remote file.

HTH

[sdean]

yes over HTTP , let me suppose the following scenario :

yesterday i downloaded x0.myfile which CRC is 3361685294 , and today i want to download other files among of them is x0.myfile so when the download process starts it should bypass x0.myfile because it's already downloaded yesterday .

That's what i want to acheive .

Yes i can use If FileExists(x0.myfile) then bypass , But here i want to work with CRC , MD5 or whatever


any idea wicht ?

[lbccaleb]

You can also check the Filesize before downloading the file!

[sdean]

Thank you lbccaleb :

But it can be possible 2 files have the same size !

I think the better way is to use the CRC or MD5

[lbccaleb]

I have found this. But its only german.

[HeikoAdams]

Is the file you want to download a file provided by you? If yes you could also provide a textfile containing the checksum of that file which will be downloaded first. After that you could download the main-file and compare its checksum with the checksum from your textfile.

IMHO there is no way to calc a checksum of a file placed on a webserver without downloading that file to your local computer.

[generic]

you cant build a crc on a url without downloading the file!

but you can request via HEAD the headers to a file.
with the header you can compare the size, date and most important the Etag.

[sdean]

Thank you HeikoAdams , that's a good idea , Ok then let me suppose this :

i place an ini file in my webserver :
update.ini

[File01]
Name=x0.myfile
checksum=3361685294
[File02]
Name=x1.myfile
checksum=3361685295
.
.
.

Then should i download x0.myfile to compare its checksum with the already downloaded one or what ?

[sdean]

Thank you generic, could you explain more , or give me a little exemple for that

[generic]

Look here:
http://en.wikipedia.org/wiki/HTTP_ETag
the german one is better:
http://de.wikipedia.org/wiki/HTTP_ETag
and
http://solariz.de/2044/etag_webseite...chleunigen.htm

Depending on the Webserver you have a strong Etag or not.
Strong ETags using checksums.
Weak ETags you can detect on "W/" before the Etag-Code.
last link uses php code for MD5-Etaging

Delphi-Quellcode:

uses
  MSXML2_TLB;

var
  req : IXMLHTTPRequest;
  str_link: string;
begin
  req := CoXMLHTTP40.Create;
  str_link:='http://linktofile'; 
  req.Open('HEAD', str_link, False, EmptyParam, EmptyParam);
  req.send(EmptyParam);
  // check for req.status=200 -> if true no error
  MessageDlg(req.getAllResponseHeaders(), mtWarning, [mbOK], 0);
  MessageDlg(req.getResponseHeader('Etag'), mtWarning, [mbOK], 0);

  req:=nil;
end;

the headers may look like:

Code:

Date   Mon, 19 Apr 2010 15:45:57 GMT
Server   Apache
Pragma   public
Expires   Mon, 19 Apr 2010 16:34:07 GMT
Last-Modified   Mon, 19 Apr 2010 15:34:07 GMT
Cache-Control   max-age=2890, public, must-revalidate, proxy-revalidate Vary   Accept-Encoding,Cookie,User-Agent
Etag   7e828ab2622e112d6a6bd3b7083a560b
Content-Encoding   gzip
Content-Type   text/html; charset=UTF-8

[SirThornberry]

@sdean: please dont push within 24 hours. If you want to add any information during this time, you can use the edit-button at the top of your last entry.

[sdean]

@SirThornberry : Sorry for that