xs = []
threads = []

def mkthread(f)
    Thread.new do
        (0...500).each do |i|
            xs << f(i)
        end
    end
end

threads << mkthread(-> (i) { 2 * i })
threads << mkthread(-> (i) { 2 * i + 1 })

threads.each(&:join)

puts xs.join("\n")