Roman numeral conversion. Took about 25 minutes, with tests!

convert-roman.rb

def to_roman(i)
  if i > 999 or i < 1
    raise "Input not supported"
  end
  i_ones = i % 10
  i_tens = (i % 100)/10
  i_hundreds = i/100
 
  return HUNDREDS[i_hundreds] + TENS[i_tens] + ONES[i_ones]
end

def to_arabic(i)
  if i.match /[^ivxlcdm]+$/ # TODO : better check
    raise "Input not supported: #{i} contains invalid characters"
  end
  if !matches = i.scan(/^([mdc]*)?([xlc]*)([ivx]*)$/).first
    raise "Input not supported: #{i} is out of range"
  end

  i_hundreds, i_tens, i_ones = matches

  return 100 * HUNDREDS.index(i_hundreds) + 10 * TENS.index(i_tens) + ONES.index(i_ones)
end

ONES = [""] + %w{i ii iii iv v vi vii viii ix}
TENS = [""] + %w{x xx xxx xl l lx lxx lxxx xc}
HUNDREDS = [""] + %w{c cc ccc cd d dc dcc dccc cm}

TESTS = %w{8 viii  33 xxxiii  545 dxlv  1 i  49 xlix 929 cmxxix}
Hash[*TESTS].each do |k,v| 
  k = k.to_i
  if to_roman(k) != v
    puts "FAILURE: to_roman(#{k}) should be #{v}, got #{to_roman(k)}"
  end
  if to_arabic(v) != k
    puts "FAILURE: to_arabic(#{v}) should be #{k}, got #{to_arabic(v)}"
  end
end
# Was this useful? YES! It caught a type which caused to_roman(700) => cc.
(1..999).select{|i| i != to_arabic(to_roman(i)) }.each do |i|
  puts "FAILURE: to_arabic(to_roman(#{i})) is #{to_arabic(to_roman(i))}"
end