split_sum.rb

def split(input)
  remainders = []
  loop do
    quotient, remainder = input.divmod(10)
    remainders << remainder
    break if quocient.zero?
    input = quotient
  end
   
  remainders
end

def sum_of_splits(input)
  total = input

  loop do
    total = split(total).sum
    break if total < 10
  end

  total
end

# Second attempt

def sum_2(input)
  aux = input

  loop do
    aux = aux.to_s.chars.map(&:to_i).sum
    break if aux < 10
  end

  aux
end