def _sw_start_offsets(target_kmer_index, read_kmers):
  """Returns target start offsets for Smith-Waterman alignment.

  Args:
    target_kmer_index: {str: [int]}, target kmer index.
    read_kmers: [str], read kmers.

  Returns:
    A sorted list of start offsets.
  """
  return sorted({(s - i)
                 for i, kmer in enumerate(read_kmers)
                 for s in target_kmer_index[kmer]})