Question

I have a bunch of files, either NSStrings or NSURLs (it doesn’t matter; they are, for the most part, interchangeable), and I need a way to find the common ancestor directory. Does anyone know how to do this?

Answer 1

I could have sworn there was a pathByJoiningPathComponents method somewhere, or at least one like it, but I must be thinking of something else. This does the trick for paths, it may work for URLs too if you’re on 10.6 (I only tested it with paths):

NSString *path1 = @"/path/to/file1.txt";
NSString *path2 = @"/path/to/file/number2.txt";

NSArray *path1Comps = [path1 pathComponents];
NSArray *path2Comps = [path2 pathComponents];

NSUInteger total = [path1Comps count];
if ([path2Comps count] < total)
    total = [path2Comps count];  // get the smaller of the two

NSUInteger i;
for (i = 0; i < total; i++)
    if (![[path1Comps objectAtIndex:i] isEqualToString:[path2Comps objectAtIndex:i]])
        break;

NSArray *commonComps = [path1Comps subarrayWithRange:NSMakeRange(0, i)];

// join commonComps together to get the common path as a string

I don’t think there is a “built-in” way to do it, unfortunately.

If you have an array of file paths that you want to find the common ancestor, you could do something like this:

NSArray *allPaths = [NSArray arrayWithObjects:@"/path/to/1.txt", @"/path/to/number/2.txt", @"/path/to/number/3/file.txt", nil];

// put some checks here to make sure there are enough paths in the array.

NSArray *currentCommonComps = [[allPaths objectAtIndex:0] pathComponents];

for (NSUInteger i = 1; i < [allPaths count]; i++)
{
    NSArray *thisPathComps = [[allPaths objectAtIndex:i] pathComponents];
    NSUInteger total = [currentCommonComps count];
    if ([thisPathComps count] < total)
        total = [thisPathComps count];

    NSUInteger j;
    for (j = 0; j < total; j++)
        if (![[currentCommonComps objectAtIndex:j] isEqualToString:[thisPathComps objectAtIndex:j]])
            break;

    if (j < [currentCommonComps count])
        currentCommonComps = [currentCommonComps subarrayWithRange:NSMakeRange(0, j)];

    if ([currentCommonComps count] == 0)
        break; // no point going on
}

// join currentCommonComps together

You may want to explicitly allocate and release some of these objects if you want to keep the autorelease pool clean, especially if you have a large array of paths.