gistfile1.m

NSDictionary groupItems(NSArray *items) {
  int bucket = 0;
  NSDictionary *groupedItems = [NSDictionary alloc];
  id prevItem;
  const kItemTimeDistanceThreshold = 30;
  
  [groupedItems setObject:[NSArray alloc] forKey:bucket];
  
  //  I can't remember how to properly iterate in Objective C,
  //  so let's pretend you can use foreach.
  foreach(item in items) {
    if(prevItem && [item time] - [prevItem time] >= kItemTimeDistanceThreshold) {
      //  Start putting objects into the next bucket
      bucket++;
      [groupedItems setObject:[NSArray alloc] forKey:bucket];
    }
    
    [[groupedItems objectForKey:bucket] addObject:item];
    prevItem = items;
  }
  
  return groupedItems;
}