code: def get_spans(maparr, spans):
      
      ## start at 0, finds change at 1-index of map file
      bidx = 1
      spans = np.zeros((maparr[-1, 0], 2), np.uint64)
      ## read through marr and record when locus id changes
      for idx in xrange(1, maparr.shape[0]):
          cur = maparr[idx, 0]
          if cur != bidx:
              idy = idx + 1
              spans[cur-2, 1] = idx
              spans[cur-1, 0] = idx
              bidx = cur
      spans[-1, 1] = maparr[-1, -1]
      return spans
docs: get span distance for each locus in original seqarray