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from __future__ import annotations
from collections.abc import Callable
from typing import TypeVar
T = TypeVar("T")
U = TypeVar("U")
class DoubleLinkedListNode[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[T, U]:
"""
Double Linked List built specifically for LRU Cache
>>> dll: DoubleLinkedList = DoubleLinkedList()
>>> dll
DoubleLinkedList,
Node: key: None, val: None, has next: True, has prev: False,
Node: key: None, val: None, has next: False, has prev: True
>>> first_node = DoubleLinkedListNode(1,10)
>>> first_node
Node: key: 1, val: 10, has next: False, has prev: False
>>> dll.add(first_node)
>>> dll
DoubleLinkedList,
Node: key: None, val: None, has next: True, has prev: False,
Node: key: 1, val: 10, has next: True, has prev: True,
Node: key: None, val: None, has next: False, has prev: True
>>> # node is mutated
>>> first_node
Node: key: 1, val: 10, has next: True, has prev: True
>>> second_node = DoubleLinkedListNode(2,20)
>>> second_node
Node: key: 2, val: 20, has next: False, has prev: False
>>> dll.add(second_node)
>>> dll
DoubleLinkedList,
Node: key: None, val: None, has next: True, has prev: False,
Node: key: 1, val: 10, has next: True, has prev: True,
Node: key: 2, val: 20, has next: True, has prev: True,
Node: key: None, val: None, has next: False, has prev: True
>>> removed_node = dll.remove(first_node)
>>> assert removed_node == first_node
>>> dll
DoubleLinkedList,
Node: key: None, val: None, has next: True, has prev: False,
Node: key: 2, val: 20, has next: True, has prev: True,
Node: key: None, val: None, has next: False, has prev: True
>>> # Attempt to remove node not on list
>>> removed_node = dll.remove(first_node)
>>> removed_node is None
True
>>> # Attempt to remove head or rear
>>> dll.head
Node: key: None, val: None, has next: True, has prev: False
>>> dll.remove(dll.head) is None
True
>>> # Attempt to remove head or rear
>>> dll.rear
Node: key: None, val: None, has next: False, has prev: True
>>> dll.remove(dll.rear) is None
True
"""
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[T, U]:
"""
LRU Cache to store a given capacity of data. Can be used as a stand-alone object
or as a function decorator.
>>> cache = LRUCache(2)
>>> cache.put(1, 1)
>>> cache.put(2, 2)
>>> cache.get(1)
1
>>> cache.list
DoubleLinkedList,
Node: key: None, val: None, has next: True, has prev: False,
Node: key: 2, val: 2, has next: True, has prev: True,
Node: key: 1, val: 1, has next: True, has prev: True,
Node: key: None, val: None, has next: False, has prev: True
>>> cache.cache # doctest: +NORMALIZE_WHITESPACE
{1: Node: key: 1, val: 1, has next: True, has prev: True, \
2: Node: key: 2, val: 2, has next: True, has prev: True}
>>> cache.put(3, 3)
>>> cache.list
DoubleLinkedList,
Node: key: None, val: None, has next: True, has prev: False,
Node: key: 1, val: 1, has next: True, has prev: True,
Node: key: 3, val: 3, has next: True, has prev: True,
Node: key: None, val: None, has next: False, has prev: True
>>> cache.cache # doctest: +NORMALIZE_WHITESPACE
{1: Node: key: 1, val: 1, has next: True, has prev: True, \
3: Node: key: 3, val: 3, has next: True, has prev: True}
>>> cache.get(2) is None
True
>>> cache.put(4, 4)
>>> cache.get(1) is None
True
>>> cache.get(3)
3
>>> cache.get(4)
4
>>> cache
CacheInfo(hits=3, misses=2, capacity=2, current size=2)
>>> @LRUCache.decorator(100)
... def fib(num):
... if num in (1, 2):
... return 1
... return fib(num - 1) + fib(num - 2)
>>> for i in range(1, 100):
... res = fib(i)
>>> fib.cache_info()
CacheInfo(hits=194, misses=99, capacity=100, current size=99)
"""
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.put(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 put(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]:
# variable to map the decorator functions to their respective instance
decorator_function_to_instance_map: dict[
Callable[[T], U], LRUCache[T, U]
] = {}
def cache_decorator_wrapper(*args: T) -> U:
if func not in decorator_function_to_instance_map:
decorator_function_to_instance_map[func] = LRUCache(size)
result = decorator_function_to_instance_map[func].get(args[0])
if result is None:
result = func(*args)
decorator_function_to_instance_map[func].put(args[0], result)
return result
def cache_info() -> LRUCache[T, U]:
return decorator_function_to_instance_map[func]
setattr(cache_decorator_wrapper, "cache_info", cache_info) # noqa: B010
return cache_decorator_wrapper
return cache_decorator_inner
if __name__ == "__main__":
import doctest
doctest.testmod()
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