digital.erl

#!/usr/bin/env escript
-module(digital).

-import(lists, [sum/1]).
-export([main/1, root/1, dr/1]).


main([Number]) ->
    Answer = root(list_to_integer(Number)),
    io:format("~p~n", [Answer]);
main(_) ->
    halt(1).


root(N) when N >= 0, N =< 9 ->
    N;
root(N) ->
    Digits = integer_to_list(N),
    root(sum_digits(Digits)).

sum_digits(Digits) ->
    sum([X band 16#0F || X <- Digits]).


dr(N) -> 1 + ((N - 1) rem 9).