How exactly does Steam verify game files?

[Aetherius]

Surely it doesn't compare each line of code in each file with what's on the servers, otherwise it would take as long to do as redownloading the whole game. My best guess is that it compares the checksums of each file? Anyone know for certain? Just wondering.

[kitt]

like you said it probably checks the Checksums of the Files like you can do in Torrent Files or Pictures

[aiusepsi]

Each particular version of a game on Steam is defined by a manifest, which lists all the chunks of data in the game, along with the size of that chunk, the position of that chunk in each file, and the hash of that chunk.

To verify the files, it just has to check what's on disk against what's specified in the manifest and check that the hashes match.

Steam also uses the manifests to work out how to do patches; it compares the new manifest against the old manifest to work out which new chunks of data it needs to download, and which it already has.

[Aetherius]

Originally posted by aiusepsi:

Each particular version of a game on Steam is defined by a manifest, which lists all the chunks of data in the game, along with the size of that chunk, the position of that chunk in each file, and the hash of that chunk.

To verify the files, it just has to check what's on disk against what's specified in the manifest and check that the hashes match.

Steam also uses the manifests to work out how to do patches; it compares the new manifest against the old manifest to work out which new chunks of data it needs to download, and which it already has.

So it's more or less a combination of checksums AND actual data comparison?

[Floid]

Originally posted by aiusepsi:

Each particular version of a game on Steam is defined by a manifest, which lists all the chunks of data in the game, along with the size of that chunk, the position of that chunk in each file, and the hash of that chunk.

To verify the files, it just has to check what's on disk against what's specified in the manifest and check that the hashes match.

Steam also uses the manifests to work out how to do patches; it compares the new manifest against the old manifest to work out which new chunks of data it needs to download, and which it already has.

Informative, nice :)