Decode

func roman2arabic(roman) {
 
    var arabic = 0
    var last_digit = 1000
 
    static m = Hash(
        I =>    1,
        V =>    5,
        X =>   10,
        L =>   50,
        C =>  100,
        D =>  500,
        M => 1000,
    )
 
    roman.uc.chars.map{m{_} \\ 0}.each { |digit|
        if (last_digit < digit) {
            arabic -= (2 * last_digit)
        }
        arabic += (last_digit = digit)
    }
 
    return arabic
}
 
%w(MCMXC MMVIII MDCLXVI).each { |roman_digit|
    "%-10s == %d\n".printf(roman_digit, roman2arabic(roman_digit))
}

Output:

MCMXC      == 1990
MMVIII     == 2008
MDCLXVI    == 1666

Simpler solution:

func roman2arabic(digit) {
    digit.uc.trans([
        :M:  '1000+',
        :CM:  '900+',
        :D:   '500+',
        :CD:  '400+',
        :C:   '100+',
        :XC:   '90+',
        :L:    '50+',
        :XL:   '40+',
        :X:    '10+',
        :IX:    '9+',
        :V:     '5+',
        :IV:    '4+',
        :I:     '1+',
    ]).split('+').map{.to_i}.sum;
}
 
%w(MCMXC MMVIII MDCLXVI).each { |roman_num|
    say "#{roman_num}\t-> #{roman2arabic(roman_num)}";
}

Output:

MCMXC   -> 1990
MMVIII  -> 2008
MDCLXVI -> 1666