Project Euler 17 in ruby

def number_under_1000_to_s(n)
  if n == 1000
    'onethousand'
  else
    number_from_999_to_100_to_s(n)
  end
end

def number_from_999_to_100_to_s(n)
  if n > 99
    appendix = number_under_9_to_s( n.first_digit )
    result = "#{appendix}hundred"

    if n % 100 != 0 # need to calculate and...
      result = result + 'and' + number_from_99_to_10_to_s( n.except_first_digit )
    end

    result
  else
    number_from_99_to_10_to_s(n)
  end
end

def number_from_99_to_10_to_s(n)
  if n > 9
    if n.first_digit == 1
      number_from_19_to_10(n)
    else
      result = first_decimal_digit( n.first_digit )
      if n % 10 != 0
        result = result + number_under_9_to_s( n.except_first_digit )
      end
      result
    end
  else
    number_under_9_to_s(n)
  end
end

def number_from_19_to_10(n)
  case n
  when 19: 'nineteen'
  when 18: 'eighteen'
  when 17: 'seventeen'
  when 16: 'sixteen'
  when 15: 'fifteen'
  when 14: 'fourteen'
  when 13: 'thirteen'
  when 12: 'twelve'
  when 11: 'eleven'
  when 10: 'ten'
  end
end

def first_decimal_digit(n)
  case n
  when 9: 'ninety'
  when 8: 'eigthy'
  when 7: 'seventy'
  when 6: 'sixty'
  when 5: 'fifty'
  when 4: 'forty'
  when 3: 'thirty'
  when 2: 'twenty'
  end
end

def number_under_9_to_s(n)
  case n
  when 9: 'nine'
  when 8: 'eigth'
  when 7: 'seven'
  when 6: 'six'
  when 5: 'five'
  when 4: 'four'
  when 3: 'three'
  when 2: 'two'
  when 1: 'one'
  end
end