from __future__ import annotations
from collections.abc import Callable
from typing import Generic, TypeVar
T = TypeVar("T")
U = TypeVar("U")
class DoubleLinkedListNode(Generic[T, U]):
""" Double Linked List Node built specifically for LRU Cache >>> DoubleLinkedListNode(1,1) Node: key: 1, val: 1, has next: False, has prev: False """
def __init__(self, key: T | None, val: U | None):
self.key = key
self.val = val
self.next: DoubleLinkedListNode[T, U] | None = None
self.prev: DoubleLinkedListNode[T, U] | None = None
def __repr__(self) -> str:
return (
f"Node: key: {
self.key}, val: {
self.val}, "
f"has next: {
bool(self.next)}, has prev: {
bool(self.prev)}"
)
class DoubleLinkedList(Generic[T, U]):
def __init__(self) -> None:
self.head: DoubleLinkedListNode[T, U] = DoubleLinkedListNode(None, None)
self.rear: DoubleLinkedListNode[T, U] = DoubleLinkedListNode(None, None)
self.head.next, self.rear.prev = self.rear, self.head
def __repr__(self) -> str:
rep = ["DoubleLinkedList"]
node = self.head
while node.next is not None:
rep.append(str(node))
node = node.next
rep.append(str(self.rear))
return ",\n ".join(rep)
def add(self, node: DoubleLinkedListNode[T, U]) -> None:
""" Adds the given node to the end of the list (before rear) """
previous = self.rear.prev
# All nodes other than self.head are guaranteed to have non-None previous
assert previous is not None
previous.next = node
node.prev = previous
self.rear.prev = node
node.next = self.rear
def remove(
self, node: DoubleLinkedListNode[T, U]
) -> DoubleLinkedListNode[T, U] | None:
""" Removes and returns the given node from the list Returns None if node.prev or node.next is None """
if node.prev is None or node.next is None:
return None
node.prev.next = node.next
node.next.prev = node.prev
node.prev = None
node.next = None
return node
class LRUCache(Generic[T, U]):
# class variable to map the decorator functions to their respective instance
decorator_function_to_instance_map: dict[Callable[[T], U], LRUCache[T, U]] = {
}
def __init__(self, capacity: int):
self.list: DoubleLinkedList[T, U] = DoubleLinkedList()
self.capacity = capacity
self.num_keys = 0
self.hits = 0
self.miss = 0
self.cache: dict[T, DoubleLinkedListNode[T, U]] = {
}
def __repr__(self) -> str:
""" Return the details for the cache instance [hits, misses, capacity, current_size] """
return (
f"CacheInfo(hits={
self.hits}, misses={
self.miss}, "
f"capacity={
self.capacity}, current size={
self.num_keys})"
)
def __contains__(self, key: T) -> bool:
""" >>> cache = LRUCache(1) >>> 1 in cache False >>> cache.set(1, 1) >>> 1 in cache True """
return key in self.cache
def get(self, key: T) -> U | None:
""" Returns the value for the input key and updates the Double Linked List. Returns None if key is not present in cache """
# Note: pythonic interface would throw KeyError rather than return None
if key in self.cache:
self.hits += 1
value_node: DoubleLinkedListNode[T, U] = self.cache[key]
node = self.list.remove(self.cache[key])
assert node == value_node
# node is guaranteed not None because it is in self.cache
assert node is not None
self.list.add(node)
return node.val
self.miss += 1
return None
def set(self, key: T, value: U) -> None:
""" Sets the value for the input key and updates the Double Linked List """
if key not in self.cache:
if self.num_keys >= self.capacity:
# delete first node (oldest) when over capacity
first_node = self.list.head.next
# guaranteed to have a non-None first node when num_keys > 0
# explain to type checker via assertions
assert first_node is not None
assert first_node.key is not None
assert (
self.list.remove(first_node) is not None
) # node guaranteed to be in list assert node.key is not None
del self.cache[first_node.key]
self.num_keys -= 1
self.cache[key] = DoubleLinkedListNode(key, value)
self.list.add(self.cache[key])
self.num_keys += 1
else:
# bump node to the end of the list, update value
node = self.list.remove(self.cache[key])
assert node is not None # node guaranteed to be in list
node.val = value
self.list.add(node)
@classmethod
def decorator(
cls, size: int = 128
) -> Callable[[Callable[[T], U]], Callable[..., U]]:
""" Decorator version of LRU Cache Decorated function must be function of T -> U """
def cache_decorator_inner(func: Callable[[T], U]) -> Callable[..., U]:
def cache_decorator_wrapper(*args: T) -> U:
if func not in cls.decorator_function_to_instance_map:
cls.decorator_function_to_instance_map[func] = LRUCache(size)
result = cls.decorator_function_to_instance_map[func].get(args[0])
if result is None:
result = func(*args)
cls.decorator_function_to_instance_map[func].set(args[0], result)
return result
def cache_info() -> LRUCache[T, U]:
return cls.decorator_function_to_instance_map[func]
setattr(cache_decorator_wrapper, "cache_info", cache_info)
return cache_decorator_wrapper
return cache_decorator_inner
if __name__ == "__main__":
import doctest
doctest.testmod()
