local History = {}
History.__index = History
function History.new(capacity)
  assert(type(capacity) == "number" and capacity % 1 == 0
      and capacity >= 1 and capacity <= 4096, "capacity must be 1..4096")
  return setmetatable({ capacity = capacity, count = 0, next = 1, rows = {} }, History)
end
function History:push(value)
  assert(value ~= nil, "history value cannot be nil")
  self.rows[self.next] = value
  self.next = self.next % self.capacity + 1
  self.count = math.min(self.count + 1, self.capacity)
end
function History:each(callback)
  local start = self.count == self.capacity and self.next or 1
  for offset = 0, self.count - 1 do
    callback(self.rows[(start + offset - 1) % self.capacity + 1])
  end
end
return History