portuguese_numbers.rb

class PortugueseNumbers
  class OutOfRange < StandardError; end

  SMALLER_POWERS = {
    10**0 => ['', 'um',  'dois',  'tres',  'quatro',  'cinco',  'seis',  'sete',  'oito',  'nove'],
    10**1 => ['', 'dez',  'vinte',  'trinta',  'quarenta',  'cinquenta',  'sessenta',  'setenta',  'oitenta',  'noventa'],
    10**2 => ['', 'cento', 'duzentos', 'trezentos', 'quatrocentos', 'quinhentos', 'seiscentos', 'setecentos', 'oitocentos', 'novecentos']
  }

  BIGGER_POWERS = {
    10**3 => ['mil', 'mil'],
    10**6 => ['milhao', 'milhoes'],
    10**9 => ['bilhao', 'bilhoes'],
    10**12 => ['trilhao', 'trilhoes'],
    # ... dá pra adicionar mais aqui seguindo o mesmo padrão
  }

  TEENS = ['', 'onze', 'doze', 'treze', 'quatorze', 'quinze', 'dezesseis', 'dezesete', 'dezoito', 'dezenove']

  def self.to_s(number)
    write_number(number)
  end

  private
  def self.write_number(number)
    number < 1000 ? below_thousand(number) : above_thousand(number)
  end

  def self.pluralize(labels, number)
    number == 1 ? labels.first : labels.last
  end

  def self.above_thousand(number)
    power = find_big_power(number)
    div, rest = number / power, number % power
    power_label = pluralize(BIGGER_POWERS[power], div)
    "#{below_thousand(div)} #{power_label} #{write_number(rest)}"
  end

  def self.below_thousand(number)
    raise OutOfRange if number > 999
    case number
    when 100 then 'cem'
    when 11..19 then TEENS[number-10]
    else
      power = find_small_power(number)
      div, rest = number / power, number % power
      labels = SMALLER_POWERS[power]
      "#{labels[div]}#{' e ' + below_thousand(rest) if rest > 0}"
    end
  end

  def self.find_power(number, powers)
    powers.find { |power| number / power > 0 }
  end

  def self.find_big_power(number)
    find_power(number, big_powers)
  end

  def self.find_small_power(number)
    find_power(number, small_powers)
  end

  def self.big_powers
    @_big_powers ||= BIGGER_POWERS.keys.sort.reverse
  end

  def self.small_powers
    @_small_powers ||= SMALLER_POWERS.keys.sort.reverse
  end
end