From: Fabian Streitel <karottenreibe@...>
Date: 2009-09-19T05:10:19+09:00
Subject: Re: Roman numeral conversion

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how about that one (it's a little longish):

    class String
        def to_roman
            str = self.downcase
            raise SyntaxError, "Not a roman numeral" unless str =~
/^[mdclxvi]+$/
            raise SyntaxError, "Not a roman numeral" if
                %w{iiii vv xxxx ll cccc dd}.any? { |x| str.include?(x) }
            str.tr!("ivxlcdm", "0123456")  # translate into numbers
            level, last, deviated, ret = 7, 0, false, 0
            table = [1,5,10,50,100,500,1000]  # the translation table
            str.each_char do |char|
                num = char.to_i
                if num > level  # means a deviation
                    raise SyntaxError, "Not a roman numeral" if deviated or
not  # no double deviation
                        %w{01 02 23 24 45 46}.include?("#{last}#{num}")  #
only allowed deviations
                    ret -= table[last]*2 # remedy deviation
                    level = last-1 # don't allow IXI or IXV etc.
                    deviated = true
                else
                    deviated = false
                    level = num  # don't allow MLM etc.
                end
                ret += table[num]
                last = num
            end
            ret
        end
    end

fulfills all your examples, but i dunno if it's correct. would be great if
someone
could test this (let alone improve it).

What it checks for:
*   only [ivxlcdm] allowed
*   ixc may each only occur max. 3 times in a row
*   vld may each only occur max once in a row
*   when dropping to a certain level, only the following sequences may rise
    above that level:    cm cd xc xl ix iv
*   and not twice in a row, so  ixc is not legal
*   and after that, the level drops to the smaller char, so ixivi is not
legal

...and then you try to evaluate the string value XD (for example), it
> will convert this string to 510.  This should not happen; XD is not a
> legal roman numeral, DX would be 510, and CDXC would be 490.  The
> problem seems to be that in roman to arabic numeral conversion, the
> solutions just add the raw values together.  This means that converters
> using this kind of conversion will accept any combination of roman
> numeral values from the table above...regardless of order.
>
> A crazier example:
>
> If you evaluate XIV, you get 14, by finding X and then IV in the table
> above and then adding their corresponding values together.
>
> If you put in IXIVI, it will *also* evaluate to 14 by adding IX + IV + I
> and accept it as a valid roman numeral.
>
>

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