class LRU_Cache:

    def __init__(self, user_function, maxsize=1024):
        # Link layout:     [PREV, NEXT, KEY, RESULT]
        self.root = root = [None, None, None, None]
        self.user_function = user_function
        self.cache = cache = {}

        last = root
        for i in range(maxsize):
            key = object()
            cache[key] = last[1] = last = [last, root, key, None]
        root[0] = last

    def __call__(self, *key):
        cache = self.cache
        root = self.root
        link = cache.get(key)
        if link is not None:
            link_prev, link_next, _, result = link
            link_prev[1] = link_next
            link_next[0] = link_prev
            last = root[0]
            last[1] = root[0] = link
            link[0] = last
            link[1] = root
            return result
        result = self.user_function(*key)
        root[2] = key
        root[3] = result
        oldroot = root
        root = self.root = root[1]
        root[2], oldkey = None, root[2]
        root[3], oldvalue = None, root[3]
        del cache[oldkey]
        cache[key] = oldroot
        return result


if __name__ == '__main__':
    p = LRU_Cache(ord, maxsize=3)
    for c in 'abcdecaeaa':
        print(c, p(c))