Numbers in Words with recursion up to 1 trillion

numbers_in_words_recursion.rb

class Fixnum
  
  def in_words

    ten         =   10    
    hundred     =   100
    thousand    =   1_000
    million     =   1_000_000
    billion     =   1_000_000_000
    trillion    =   1_000_000_000_000
    
    digit_words = { 1  => "one",        2  => "two",       3 => "three",     4 => "four", 
                    5  => "five",       6  => "six",       7 => "seven",     8 => "eight",
                    9  => "nine",       10 => "ten",      11 => "eleven",   12 => "twelve", 
                    13 => "thirteen",   14 => "fourteen", 15 => "fifteen",  16 => "sixteen", 
                    17 => "seventeen",  18 => "eighteen", 19 => "nineteen", 20 => "twenty",
                    30 => "thirty",     40 => "forty",    50 => "fifty",    60 => "sixty",
                    70 => "seventy",    80 => "eighty",   90 => "ninety",   0  => "zero" } 
        
    if (self % trillion) < self
      "#{ (self / trillion).in_words } trillion"
      
    elsif (self % billion) < self
      "#{ (self / billion).in_words } billion #{ (self % billion).in_words }".strip
      
    elsif (self % million) < self
      "#{ (self / million).in_words } million #{ (self % million).in_words }".strip
      
    elsif (self % thousand) < self
      "#{ (self / thousand).in_words } thousand #{ (self % thousand).in_words }".strip
      
    elsif (self % hundred) < self
      "#{ (self / hundred).in_words } hundred #{ (self % hundred).in_words }".strip
        
    elsif (self < 99) && (self >= 20)
      "#{ digit_words[self / ten * ten] } #{ (self % ten).in_words }".strip
    
    elsif (self >=10) && (self < 20)
      "#{ digit_words[self] }"

    else
      "#{ digit_words[self] }"
    end
  end
end