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