def _build_index(self):
        row0 = list(map(len, self._lists))
        if len(row0) == 1:
            self._index[:] = row0
            self._offset = 0
            return
        head = iter(row0)
        tail = iter(head)
        row1 = list(starmap(add, zip(head, tail)))
        if len(row0) & 1:
            row1.append(row0[-1])
        if len(row1) == 1:
            self._index[:] = row1 + row0
            self._offset = 1
            return
        size = 2 ** (int(log_e(len(row1) - 1, 2)) + 1)
        row1.extend(repeat(0, size - len(row1)))
        tree = [row0, row1]
        while len(tree[-1]) > 1:
            head = iter(tree[-1])
            tail = iter(head)
            row = list(starmap(add, zip(head, tail)))
            tree.append(row)
        reduce(iadd, reversed(tree), self._index)
        self._offset = size * 2 - 1