finding differences of two arrays

gistfile1.m

NSArray *a = [NSArray arrayWithObjects:@"a", @"c", @"d", @"b", @"k", nil];
NSArray *b = [NSArray arrayWithObjects:@"z", @"a", @"d", @"b", @"m", nil];

NSComparator comparator =  ^NSComparisonResult(id obj1, id obj2) {
  return [obj1 caseInsensitiveCompare:obj2];
};

NSArray *sortedA = [a sortedArrayUsingComparator:comparator];
NSArray *sortedB = [b sortedArrayUsingComparator:comparator];

NSLog(@"a=%@, b=%@", sortedA, sortedB);

NSMutableArray *added = [NSMutableArray array];
NSMutableArray *updated = [NSMutableArray array];
NSMutableArray *removed = [NSMutableArray array];

NSEnumerator *enumA = [sortedA objectEnumerator];
NSEnumerator *enumB = [sortedB objectEnumerator];

NSString *objA = [enumA nextObject];
NSString *objB = [enumB nextObject];

while (objA && objB) {
  NSComparisonResult result = comparator(objA, objB);
  switch (result) {
    case NSOrderedSame:
      [updated addObject:objA];
      objA = [enumA nextObject];
      objB = [enumB nextObject];
      break;
    case NSOrderedAscending:
      [removed addObject:objA];
      objA = [enumA nextObject];
      break;
    case NSOrderedDescending:
      [added addObject:objB];
      objB = [enumB nextObject];
      break;
    default:
      assert("This should not happen");
      break;
  }
}

// objA, enumA -> removed
while (objA) {
  [removed addObject:objA];
  objA = [enumA nextObject];
}
// objB, enumB -> added
while (objB) {
  [added addObject:objB];
  objB = [enumB nextObject];
}

NSLog(@"added=%@, updated=%@, removed=%@", added, updated, removed);