Objective C fragment from Add Folder Icons which removes duplicate folders

remove_duplicates.m

/******************************************************************************\
 * -removeDuplicatesFromIndices:comparedAgainst:
 *
 * Pass two pointers to NSIndexSet instances. The first describes indices in
 * 'tableContents' which are to be searched. The second describes indices in
 * 'tableContents' which are to be compared. If any items from the second set
 * of indices are found to have a match in the first set of indices, then the
 * matching item in the *first set* is deleted.
 *
 * A 'match' is defined as 'same value for the @"path" key'. Other keys in the
 * dictionaries are ignored. The "-compare:" message is used to match the path
 * strings.
 *
 * The returned autoreleased index set may be useful if you were maintaining
 * one or more indices which may have been altered by the deletions.
 *
 * In:       ( NSIndexSet * ) sourceBlock
 *           Indices describing items in 'tableContents' which are to be
 *           searched through and possibly removed;
 *
 *           ( NSIndexSet * ) matchBlock
 *           Indices describing items in 'tableContents' which are each
 *           compared to all the items in the source block (see above) and, if
 *           any matches are found, the matched item in the *source block* will
 *           be deleted.
 *
 * Out:      Autoreleased index set describing the indices that were deleted.
 *
 * See also: -insertFolder:atIndex:
 *           -insertFolder:atIndex:WithStyle:
 *           -removeDuplicatesFromIndices:comparedAgainst:
 *           -addSubfoldersOf:
\******************************************************************************/

- ( NSIndexSet * ) removeDuplicatesFromIndices: ( NSIndexSet * ) sourceBlock
                               comparedAgainst: ( NSIndexSet * ) matchBlock
{
    NSMutableIndexSet * duplicates = [ NSMutableIndexSet indexSet ];

    /* Match against the lower and upper blocks concurrently, adding the index
     * of any duplicate found into the 'duplicates' set. For each item in the
     * match block we have to scan the whole source block.
     */

    [
        matchBlock enumerateIndexesUsingBlock: ^ ( NSUInteger matchIndex, BOOL * stop )
        {
            NSDictionary * matchRecord = [ tableContents objectAtIndex: matchIndex ];
            NSString     * matchPath   = [ matchRecord valueForKey: @"path" ];

            NSUInteger found =
            [
                tableContents indexOfObjectWithOptions: NSEnumerationConcurrent
                                           passingTest: ^ ( id obj, NSUInteger index, BOOL * stop )
                {
                    /* Proceed on the basis that checking for an index in a set
                     * is comparatively fast, while extracting a string from a
                     * dictionary and comparing it to another string is
                     * comparatively slow. Only do the comparision if the index
                     * is within the source block range and has not been marked
                     * as a duplicate already.
                     */

                    if (
                           [ sourceBlock containsIndex: index ] == YES &&
                           [ duplicates  containsIndex: index ] == NO
                       )
                    {
                        NSDictionary * sourceRecord = ( NSDictionary * ) obj;
                        NSString     * sourcePath   = [ sourceRecord valueForKey: @"path" ];

                        if ( [ sourcePath compare: matchPath ] == NSOrderedSame )
                        {
                            *stop = YES;
                            return YES;
                        }
                    }

                    return NO;
                }
            ];

            if ( found != NSNotFound ) [ duplicates addIndex: found ];
        }
    ];

    /* Now remove all found duplicates in one go */

    [ tableContents removeObjectsAtIndexes: duplicates ];
    return duplicates;
}

Available here too, with better and configurable syntax colouring: http://pastie.org/1560400