class FixedSizeQueue
    def initialize(size)
        @max_size = size
        @items = []
    end

    def pop
        @items.shift        
    end

    def push(item)
        @items.push item
        if @items.size > @max_size
            pop            
        end
    end
end

##</code><code>##

2:16% irb -r ./fsq.rb
irb(main):001:0> q = FixedSizeQueue.new(3)
=> #<FixedSizeQueue:0x9317c6c @max_size=3, @items=[]>
irb(main):002:0> q.push 2
=> nil
irb(main):003:0> q.push 3
=> nil
irb(main):004:0> q.push 5
=> nil
irb(main):005:0> q
=> #<FixedSizeQueue:0x9317c6c @max_size=3, @items=[2, 3, 5]>
irb(main):006:0> q.push 7
=> 2
irb(main):007:0> q
=> #<FixedSizeQueue:0x9317c6c @max_size=3, @items=[3, 5, 7]>
irb(main):009:0> FixedSizeQueue.instance_methods(false)
=> [:pop, :push]