Question

What is the best method to build an NSDictionary from an NSArray of flattened paths? For example, I want to convert this array’s contents:

<array>
<string>packs/</string>
<string>packs/Children/</string>
<string>packs/Children/Letters</string>
<string>packs/Children/Letters/abc.pack</string>
<string>packs/Children/Numbers</string>
<string>packs/Children/Numbers/123.pack</string>                                    
<string>packs/Children/Numbers/10_2_30.pack</string>
<string>packs/General/</string>
</array>

…into an NSDictionary of the path segments and filenames, like so:

packs/
    Children/
    Letters/
        abc.pack
    Numbers/
        123.pack
        10_20_30.pack
    General/

Would it be best to first look for the array items with a file extension (.pack) and build the structure back from that point? Or try to build the structure line by line through the array’s contents?

Any help is greatly appreciated!

Answer 1

I will assume that all leaf nodes end with .pack and all branch nodes do not, for simplicity.

A dictionary is a set of key/value pairs. It’s not clear what you want the value for key abc.pack to be in the packs/Letters dictionary. I’ll just use the string @"leaf node!" as the value.

You can do this pretty easily with a helper function that inserts a path into the dictionary tree.

void insertPathIntoTree(NSString *path, NSMutableDictionary *tree) {
    NSArray *components = [path pathComponents];
    for (int i = 0, count = components.count; i < count; ++i) {
        NSString *component = [components objectAtIndex:i];

        if (!component.length) {
            // This ignores a trailing slash, and any double slashes mid-path.
            continue;
        }

        if (i == count - 1 && [component hasSuffix:@".pack"]) {
            [tree setObject:@"leaf node!" forKey:component];
        }

        else {
            NSMutableDictionary *nextBranch = [tree objectForKey:component];
            if (!nextBranch) {
                nextBranch = [NSMutableDictionary dictionary];
                [tree setObject:nextBranch forKey:component];
            }
            tree = nextBranch;
        }
    }
}

Then it’s just a matter of creating an initial, empty tree (NSMutableDictionary) and inserting each path into it:

NSMutableDictionary *treeWithPathArray(NSArray *paths) {
    NSMutableDictionary *tree = [NSMutableDictionary dictionary];
    for (NSString *path in paths)
        insertPathIntoTree(path, tree);
    return tree;
}