local Queue = {}

Queue.__index = Queue

function Queue.new ()

return setmetatable({ head = 1, tail = 1 }, Queue)

end

function Queue:push (v)

if self:length() == 0 then

self.tail = 1

self.head = 1

end

self[self.tail] = v

self.tail = self.tail + 1

end

function Queue:pop ()

if self:length() == 0 then return nil end

local v = self[self.head]

self[self.head] = nil

self.head = self.head + 1

return v

end

function Queue:front ()

if self:length() == 0 then return nil end

return self[self.head]

end

function Queue:back ()

if self:length() == 0 then return nil end

return self[self.tail - 1]

end

function Queue:map (f)

if self:length() == 0 then return end

for i = self.head, self.tail - 1 do

f(self[i])

end

end

function Queue:length ()

return self.tail - self.head

end

return Queue