Written numbers

written_numbers.rb

ONE = 0
TEN = 1
HUNDRED = 2

def german_words_for(number, triple_index)
  return 'null' if number == 0
  return 'eins' if number == 1 && triple_index == 0
  return 'minus eins' if number == -1 && triple_index == 0

  words = [
      [ :und, :ein, :zwei, :drei, :vier, :fünf, :sechs, :sieben, :acht, :neun ],
      [ :eine, :zehn, :zwanzig, :dreißig, :vierzig, :fünfzig, :sechzig, :siebzig, :achzig, :neunzig ],
      [ 
        :hundert, :tausend, :million, :milliarden, :billion, :billiarden, :trillion, :trilliarden,
        :quadrillionen, :quadrilliarden, :quintillion, :sextillion, :sextilliarden
      ]
    ]

  temp_number = number.abs
  number_word = ""
  temp_tens = ""

  while  temp_number > 0
    decimal_power = Math.log10(temp_number).floor
    num = temp_number - (temp_number % (10 ** decimal_power))
    index = num / (10 ** decimal_power)
    copula = ((index > 1) ? words[0][0].to_s : "")
    leading_single = (triple_index >= 2 && index == 1) ? words[1][0].to_s : words[0][index].to_s

    number_word << words[0][index].to_s << words[2][0].to_s if decimal_power == HUNDRED
    number_word << words[0][index].to_s if decimal_power == ONE && !temp_tens.empty?
    number_word << copula << leading_single if decimal_power == ONE && temp_tens.empty?
    temp_tens << copula << words[1][index].to_s if decimal_power == TEN

    temp_number = temp_number - num
  end

  number_word << temp_tens
  number_word << words[2][triple_index].to_s if triple_index > 0
  number_word = number_word.gsub('einzehn', 'elf')
  number_word = number_word.gsub('zweizehn', 'zwölf')
  number_word = number_word.gsub('sechszehn', 'sechzehn')
  number_word = number_word.gsub('siebenzehn', 'siebzehn')
  number_word = number_word.gsub(/^(und|null)/, '')
  number_word = "minus #{number_word}" if number < 0
  number_word
end

puts "Which number should be written in German words?"

number = gets.chomp.to_i
number_triples = number.to_s.reverse.scan(/.{1,3}/).map{ |triple| triple.reverse.to_i }
puts number_triples.map.with_index { |part, index| german_words_for(part.to_i, index) }.reverse.join('').gsub(/^und/, '')