There is something wrong with the random test cases for ruby. For all the random cases, the expected answer is [] regardless of the input.

The following is my code which should work, but is failing all the random test cases due to the bug.

def array_diff(a, b)
diff = a.select { |x| !b.include? x}.each {|x| a.delete(x)}
return diff
end

Thanks for this suggestion - I've edited the kata description and solution code to make it clearer.

Thanks - this issue has been resolved.