The cage encoding was jointly described by the concatenation of the BB1 skeleton, BB2 skeleton and reaction type. To maximise the efficiency of cage encoding, components of cages are processed using different encoding methods. BB2 skeletons were represented by SMILES strings. As shown in Table , the maximum length of tri-topic BB1 skeletons is nearly 50 characters longer than that of di-topic BB2 skeletons, a 100% increase. In addition, a large amount of BB2 skeletons lie in the low and medium-length regions (less than 50 characters long), resembling a more natural distribution that leads to potential generations with higher validity. The character-level vocabulary of SMILES tokens (see Table ) was based on the combined set of the original and augmented BB2 skeletons constructed in Section 2.3.