Question

I need to sort a NSMutableArray containing NSURLs with localizedStandardCompare:

[array sortUsingComparator:^NSComparisonResult(id obj1, id obj2) {
    NSString *f1 = [(NSURL *)obj1 absoluteString];
    NSString *f2 = [(NSURL *)obj2 absoluteString];
    return [f1 localizedStandardCompare:f2];
}];

This works fine, but I worry a bit about the performance: the block will be evaluated n log n times during the sort, so I’d like it to be fast (the array might have up to 100,000 elements). Since localizedStandardCompare is only available on NSString, I need to convert the URLs to strings. Above, I use absoluteString, but there are other methods that return a NSString, for example relativeString. Reading the NSURL class reference, I get the impression that relativeString might be faster, since the URL does not need to be resolved, but this is my first time with Cocoa and OS-X, and thus just a wild guess.

Additional constraint: in this case, all URLs come from a NSDirectoryEnumerator on local storage, so all are file URLs. It would be a bonus if the method would work for all kinds of URL, though.

My question: which method should I use to convert NSURL to NSString for best performance?

Profiling all possible methods might be possible, but I have only one (rather fast) OS-X machine, and who knows – one day the code might end up on iOS.

I’m using Xcode 4.5.2 on OS-X 10.8.2, but the program should work on older version, too (within reasonable bounds).

Answer 1

I benchmarked the sort. It turned out that absoluteString and relativeString are much faster that path or relativePath.

Sorting about 26000 entries:

relativeString      550ms
absoluteString      580ms
path                920ms
relativePath        960ms
field access        480ms

For field access, I put the value of absoluteString into a field prior to the sort and access that. So, the ...String accessors are almost as fast as field access, and thus a good choice for my use case.