mask
An integer specifying the total number of allocated cache buckets (minus one). During method lookup, the Objective-C runtime uses this field to determine the index at which to begin a linear search of the
buckets
array. A pointer to a method’s selector is masked against this field using a logical AND operation (index = (mask & selector))
. This serves as a simple hashing algorithm.occupied
An integer specifying the total number of occupied cache buckets.