A BEAST for prowling in trees

When searching for convolutional codes and tailbiting codes of high complexity it is of vital importance to use fast algorithms for computing their weight spectra, which corresponds to finding low-weight paths in their code trellises. This can be efficiently done by a combined search in both forward and backward code trees. A bidirectional efficient algorithm for searching such code trees (BEAST) is presented. For large encoder memories, it is shown that BEAST is significantly more efficient than comparable algorithms. BEAST made it possible to find new convolutional and tailbiting codes that have larger free (minimum) distances than the previously best known codes with the same parameters. Tables of such codes are presented.