random keygen

begin
Charset = ?A.upto(?Z).to_a.concat(?a.upto(?z).to_a).concat(?0.upto(?9).to_a);

def get_key
    i = 0;
    parts = [ ];
    while i < 4
        chars = Charset.sample(3).join;
        sum = chars.each_codepoint.reduce(:+);
        if Charset.include?((400+3*i - sum).chr)
            parts << chars + (400+3*i-sum).chr;
            i += 1;
        end
    end
    parts.join(?-);
end

puts get_key;
rescue Exception => e
p e;
p e.message;
p e.backtrace;
ensure
gets
end