Is it possible to test if a text file can be loaded with a NSStringEncoding without actually loading it?

Question

I'm working on an app that handles text files.

On the open dialog accessory view, when the user choses a file I want to display on a NSPopUpButton the supported encodings for the selected file, but so far the only way I could find to test if a file can be opened with a certain encoding is to actually trying to load it to a NSString.

Right now I'm doing this for each encoding:

NSString *fileContents = [NSString stringWithContentsOfURL:selectedURL encoding:encoding error:nil];

if (fileContents)
{
    /* Add encoding to the list of supported encodings */
}

This loads the whole file to the fileContents object.

Is it possible possible to only test if the file can be loaded without actually loading it?

Answer 1

This is not possible to predict with 100% accuracy unless you read everything. For instance many encodings are supersets of ASCII and you'd be fooled if you stopped reading partway through a file; maybe the first paragraph just happens to contain nothing but ASCII and then 4 paragraphs later the file suddenly contains a bunch of mathematical equations.

A good middle ground is to support the conventions for specifying encodings that already exist. For instance text editors like vim and emacs specify a certain syntax for embedding encoding hints in the first few lines of a file, and HTML and XML files can contain <meta> tags with encoding information. If you can read those, it's fairly safe to assume they're correct and not read the entire file; otherwise, you do have to read the whole file to know for sure.

(An exception: certain Unicode encodings start with byte-order marks that are not used by any other encoding so you can assume those are what they say they are.)