converting integers to english

integertoenglish.rb

#!/usr/bin/env ruby

NTW = {
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",
100 => "hundred",
1000 => "thousand",
1000000 => "million",
1000000000 => "billion",
1000000000000 => "trillion"
}

class IntegerToEnglish
  class << self
    def convert(number)
      return "zero" if number == 0
      word_representation_accumulator = []
      number_digits_reversed = number.to_s.reverse
      digit_count = 0
      number_digits_reversed.chars.each_with_index do |digit, index|
        digit_as_number = Integer(digit)
        skip_zero(digit_as_number) do 
          if digit_count == 0
            word_representation_accumulator << "#{NTW[digit_as_number]}"
          elsif ten_to_twenty?(digit_as_number, digit_count)
            backtrack word_representation_accumulator
            actual_number = Integer("#{digit}#{number_digits_reversed[index - 1]}")
            multiplier = (digit_count > 1 ? 10**(digit_count - 1) : nil)
            word_representation = "#{NTW[actual_number]}"
            word_representation += " #{NTW[multiplier]}" if multiplier
            word_representation += " and" if word_representation_accumulator.size == 1
            word_representation_accumulator << word_representation
          elsif twenty_to_one_hundred?(digit_count)
            backtrack word_representation_accumulator 
            multiplier = (digit_count > 1 ? 10**(digit_count - 1) : nil)
            lookup_number = digit_as_number * 10
            word_representation = "#{NTW[lookup_number]}"
            word_representation += " #{NTW[Integer(number_digits_reversed[index - 1])]}"
            word_representation += " #{NTW[multiplier]}" if multiplier
            word_representation += " and" if word_representation_accumulator.size == 1
            word_representation_accumulator << word_representation
          elsif digit_count == 2 || digit_count % 3 == 2
            multiplier = 10**2
            word_representation = "#{NTW[digit_as_number]} #{NTW[multiplier]}"
            word_representation += " and" if word_representation_accumulator.size != 0
            word_representation_accumulator << word_representation
          else
            multiplier = 10**digit_count
            word_representation = "#{NTW[digit_as_number]} #{NTW[multiplier]}"
            word_representation += " and" if word_representation_accumulator.size == 1
            word_representation_accumulator << word_representation
          end
        end
        digit_count += 1
      end
      word_representation_accumulator.reverse.join(" ")
    end

    def skip_zero(digit)
      if digit != 0
        yield
      end
    end

    def backtrack(word_list)
      word_list.pop
    end

    def ten_to_twenty?(digit_as_number, digit_count)
      (digit_count - 1) % 3 == 0 && digit_as_number == 1
    end

    def twenty_to_one_hundred?(digit_count)
      (digit_count - 1) % 3 == 0
    end
  end
end

if __FILE__ == $0
  puts IntegerToEnglish.convert(0)
  puts IntegerToEnglish.convert(1)
  puts IntegerToEnglish.convert(3)
  puts IntegerToEnglish.convert(5)
  puts IntegerToEnglish.convert(11)
  puts IntegerToEnglish.convert(15)
  puts IntegerToEnglish.convert(25)
  puts IntegerToEnglish.convert(71)
  puts IntegerToEnglish.convert(40)
  puts IntegerToEnglish.convert(100)
  puts IntegerToEnglish.convert(101)
  puts IntegerToEnglish.convert(112)
  puts IntegerToEnglish.convert(123)
  puts IntegerToEnglish.convert(457)
  puts IntegerToEnglish.convert(999)
  puts IntegerToEnglish.convert(1000)
  puts IntegerToEnglish.convert(1001)
  puts IntegerToEnglish.convert(1010)
  puts IntegerToEnglish.convert(1011)
  puts IntegerToEnglish.convert(2117)
  puts IntegerToEnglish.convert(3001)
  puts IntegerToEnglish.convert(13101)
  puts IntegerToEnglish.convert(14001)
  puts IntegerToEnglish.convert(16000)
  puts IntegerToEnglish.convert(25119)
  puts IntegerToEnglish.convert(65009)
  puts IntegerToEnglish.convert(315119)
  puts IntegerToEnglish.convert(1000001)
  puts IntegerToEnglish.convert(1315119)
  puts IntegerToEnglish.convert(11315119)
  puts IntegerToEnglish.convert(74315119)
  puts IntegerToEnglish.convert(174315119)
  puts IntegerToEnglish.convert(1174315119)
  puts IntegerToEnglish.convert(15174315119)
  puts IntegerToEnglish.convert(35174315119)
  puts IntegerToEnglish.convert(935174315119)
  puts IntegerToEnglish.convert(2935174315119)
end