Code snippet to find the number of pairs given by the difference in given array of unique numbers

gistfile1.m

int numberOfPairs(NSSet *uniqueNumbers, int difference) {
  if (difference <= 0 || [uniqueNumbers count] <= 0) {
    return 0;
  }

  //creating set of decremented numbers to do intersection
  NSMutableSet *setToCompare = [[NSMutableSet alloc] init];
  for (NSNumber *number in uniqueNumbers) {
      [setToCompare addObject:@([number intValue] - difference)];
  }
  //intersection will cause that only pairs will stay within given set
  [setToCompare intersectSet:uniqueNumbers];
  
  return (int)[[setToCompare allObjects] count];
}