Table data structure that "garbage collects" not-recently-used items in O(1) time

gistfile1.md

I use this data structure all the time. Can someone leave a comment and tell me what it's called?

1. insert(k,v): add a new value
2. lookup(k): lookup an existing value
3. age(): delete old entries (that have not been used since previous call to age())

# Initialize 'old' and 'new' to empty tables
local old, new = {}, {}

function insert(k, v)
  new[k] = v
end

function lookup(k)
  if new[k] then
    # Found in new table
    return new[k]
  elseif old[k] then
    # Migrate from old table to new table
    new[k] = old[k]
    return old[k]
  else
    # Not found
    return nil
  end
end

function age()
  # Entries in 'old' are dropped, entries in 'new' become old.
  old = new
  new = {} # empty table
end

It looks like an LRU cache that has no hard limit on the number of elements.

Also, I ported this data structure to Python recently.