RATS sequence generator in TWIG

rats.twig

{#
http://mathworld.wolfram.com/RATSSequence.html
A sequence produced by the instructions "reverse, add to the original, then sort the digits." For example, after 668, the next iteration is given by

 668+866=1534, 
so the next term is 1345.

Applied to 1, the sequence gives 1, 2, 4, 8, 16, 77, 145, 668, 1345, 6677, 13444, 55778, 133345, 666677, 1333444, 5567777, 12333445, 66666677, 133333444, 556667777, 1233334444, 5566667777, 12333334444, 55666667777, 123333334444, 556666667777, 1233333334444,
#}

{% macro rats(start) %}
    {% import _self as macros %}
    {{-start-}}
    
    {# prevent it looping forever #}
    {% if start < 99999 %}
    {{-macros.rats(((start + (start|reverse))|split('')|sort)|join(''))-}}
    {% endif %}
{% endmacro %}


{% import _self as macros %}

{# and away we go #}

{{ macros.rats(1) }}

{# 
result is

1        2        4        8        16        77        145        668        1345        6677        13444        55778        133345    
#}