convert.rb

class NumberToString
  SINGLE_MAPPING = {'1' => 'one', '2' => 'two', '3' => 'three', 
                    '4' => 'four', '5' => 'five', '6' => 'six', 
                    '7' => 'seven', '8' => 'eight', '9' => 'nine', '0' => 'zero'}
  DECI_MAPPING = {'1' => 'ten', '2' => 'twenty', '3' => 'thirty',
                  '4' => 'forty', '5' => 'fifty', '6' => 'sixty',
                  '7' => 'seventy', '8' => 'eighty', '9' => 'ninety'}
  HUNDRED_MAPPING = Proc.new {|n| n == '0' ? '' : SINGLE_MAPPING[n] + " hundred" }
  MAPPINGS = [SINGLE_MAPPING, DECI_MAPPING, HUNDRED_MAPPING]
  TRIPLET_MAPPING = {
    1 => "",
    2 => " thousand",
    3 => " million",
    4 => " billion",
    5 => " trillion"
  }
  
  def convert(number, triplet = '')
    number = number.to_s
    if number.size > 3
      enum = number.split(//).reverse.each_slice(3).map {|t| t.reverse }
      return enum.map.with_index do |t, i|
        convert(t.join, TRIPLET_MAPPING[i+1])
      end.compact.reverse.join(" ")
    end
  
    return nil if number =~ /^00+$/
    0.upto(number.size - 1).map do |index|
      MAPPINGS[index][number[number.size - 1 - index]]
    end.reverse.join(" ").strip + triplet
  end
end

puts NumberToString.new.convert('1000335')
# => "one million three hundred thirty five"