main.lua

local sliding = require("sliding")
local avg = sliding.new('taps', 'dt', 10)

avg:add({ dt = 5, taps = 5})

print(tostring(avg))

avg:add({ dt = 3, taps = 1})

print(tostring(avg))

avg:add({ dt = 1, taps = 1})

print(tostring(avg))

avg:add({ dt = 3, taps = 1})

print(tostring(avg))

sliding.lua

local sliding = {}

local slidingMt = {
  __index = sliding,
  __tostring = function(self)
    return "Avg: " .. self.avg .. 
           "  Total: " .. self.total .. 
           "  LimTotal: " .. self.limTotal ..
           "  Items: " .. #self.items
  end
}

function sliding.new(avgField, limField, limit)
  local slide = setmetatable({
    items    = {},
    limField = limField,
    avgField = avgField,
    limit    = limit,
    avg      = nil,
    total    = 0,
    limTotal = 0,
  }, slidingMt)

  return slide
end

function sliding:add(val)
  -- Check which values this new one pushes off the end, if any
  local diff = self.limTotal + val[self.limField] - self.limit

  if diff > 0 then
    print("Over limit! ", diff)

    -- Pop off the right number of things
    -- Although table.remove is expensive :(
    
    -- have to remove at least one
    local limRemoved = self.items[1][self.limField]
    local valRemoved = self.items[1][self.avgField]
    table.remove(self.items, 1)

    local numRemoved = 1
    while limRemoved < diff do
      numRemoved = numRemoved + 1
      
      limRemoved = limRemoved + self.items[i][self.limField]
      valRemoved = valRemoved + self.items[i][self.avgField]
      table.remove(self.items, 1) -- Remove the first element again. Slow?
    end
    print("Done! Removed ", numRemoved, " with total ", valRemoved)

    self.total    = self.total    - valRemoved
    self.limTotal = self.limTotal - limRemoved
  end

  print("Adding item with val ", val[self.avgField], " and limit ", val[self.limField])
  self.items[#self.items+1] = val
  self.total    = self.total    + val[self.avgField]
  self.limTotal = self.limTotal + val[self.limField]

  self.avg = self.total / #self.items
end

return sliding