english_numbers_kata.rb

class EnglishNumbers
  ZERO = "zero"
  DIGITS = [nil, "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"]
  TEN_TO_NINETEEN = ["ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"]
  TENTS = [nil, nil, "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"]
  HUNDRED = "hundred"
  GROUPS = [nil, "thousand", "million", "billions", "trillions"]
  MINUS = "minus"
  POINT = "point"

  def initialize nr
    @number = nr.abs.to_i
    @decimals = decimals_from(nr.to_f)
    @negative = nr.to_f < 0
  end

  def to_english
    english_nr = []

    english_nr += number_to_english(@number)

    if @decimals > 0
      english_nr << EnglishNumbers::POINT
      english_nr += number_to_english(@decimals)
    end

    english_nr.unshift(EnglishNumbers::MINUS) if @negative
    english_nr.compact.join(" ")
  end

private

  def number_to_english nr
    return [EnglishNumbers::ZERO] if nr == 0

    english_nr_ar = []
    groups_of_3_digits_for(nr) do |digits_group, index|
      next if digits_group == 0
      english_nr_ar.unshift(group_name(index))
      english_nr_ar.unshift(*group_of_3_digits_to_english(digits_group))
    end
    english_nr_ar
  end

  def groups_of_3_digits_for nr
    index = 0
    loop do
      yield nr % 1000, index
      nr = (nr / 1000).to_i
      break if nr == 0
      index += 1
    end
  end

  def group_name group = 0
    EnglishNumbers::GROUPS[group]
  end

  def group_of_3_digits_to_english nr
    nr_en = []

    tents_en = tents_to_english(nr % 100)
    nr_en.unshift(tents_en)
    hundreds_en = hundreds_to_english(nr / 100)
    nr_en.unshift(hundreds_en)

    nr_en
  end

  def hundreds_to_english nr
    return nil if nr == 0
    digit_to_english(nr) + " " + EnglishNumbers::HUNDRED
  end

  def tents_to_english nr
    return digit_to_english(nr) if (nr >= 0 && nr < 10)
    return ten_19_to_english(nr) if (nr >= 10 && nr < 20)
    return EnglishNumbers::TENTS[nr / 10] if (nr % 10 == 0)
    EnglishNumbers::TENTS[nr / 10] + "-" + digit_to_english(nr % 10)
  end

  def ten_19_to_english nr
    EnglishNumbers::TEN_TO_NINETEEN[nr % 10]
  end

  def digit_to_english digit
    EnglishNumbers::DIGITS[digit]
  end

  def decimals_from nr
    if nr.integer?
      0
    else
      nr.to_s.split(".").last.to_i
    end
  end
end