) where C(x) and C(y) are the compressed lengths of the SMILES representations of molecules x and y, respectively. C(xy), C(xx), and C(yy) are the compressed length of the concatenated SMILES representations of the two molecules. The distance definition was adapted because the original NCD definition was not symmetric. We changed the implementation of the k nearest-neighbor classifier by weighting the class counts C i among the k nearest-neighbors using the formula