gistfile1.rb

class Integer
    def prime?
        if self == 1
            return false
        else
            (2...self).each { |n|
                return false if self % n == 0
            }
            return true
        end
    end

    def factor?(divisor)
        return self % divisor == 0
    end

    def largest_prime_factor
        (2..Math.sqrt(self)).reverse_each { |n|
            if self.factor?(n) && n.prime?
                return n
            end
        }
    end
end

puts 600_851_475_143.largest_prime_factor