RUT Chile: obtener dígito verificador, validar y formatear

rut.rb

def obtener_dv rut
    rut = rut.to_s.tr(".-", "").reverse
    suma = 0
    for index in 0..rut.size
        suma+= rut[index].to_i * (2+index%6)
    end
    res = 11 - suma % 11
    res = (res == 11)? 0 : (res == 10)? "K" : res
    res.to_s
end

def validar rut
    rut.to_s.tr(".-", "")[-1] == (obtener_dv rut.to_s[0..-2])
end

def formatear rut
    rut = rut.to_s.tr(".-", "")
    num = rut[0..-2]
    dv = rut[-1]
    num = num.to_s.reverse.gsub(/...(?=.)/,'\&.').reverse
    "#{num}-#{dv}"
end