Added LFU Cache (#2151)

* Added LFU Cache

* Update lfu_cache.py

* None is returned

* Add type hints

Co-authored-by: Christian Clauss <cclauss@me.com>

other/lfu_cache.py

@@ -0,0 +1,186 @@
+from typing import Callable, Optional
+
+
+class DoubleLinkedListNode:
+    '''
+    Double Linked List Node built specifically for LFU Cache
+    '''
+
+    def __init__(self, key: int, val: int):
+        self.key = key
+        self.val = val
+        self.freq = 0
+        self.next = None
+        self.prev = None
+
+
+class DoubleLinkedList:
+    '''
+    Double Linked List built specifically for LFU Cache
+    '''
+
+    def __init__(self):
+        self.head = DoubleLinkedListNode(None, None)
+        self.rear = DoubleLinkedListNode(None, None)
+        self.head.next, self.rear.prev = self.rear, self.head
+
+    def add(self, node: DoubleLinkedListNode) -> None:
+        '''
+        Adds the given node at the head of the list and shifting it to proper position
+        '''
+
+        temp = self.rear.prev
+
+        self.rear.prev, node.next = node, self.rear
+        temp.next, node.prev = node, temp
+        node.freq += 1
+        self._position_node(node)
+
+    def _position_node(self, node: DoubleLinkedListNode) -> None:
+        while node.prev.key and node.prev.freq > node.freq:
+            node1, node2 = node, node.prev
+            node1.prev, node2.next = node2.prev, node1.prev
+            node1.next, node2.prev = node2, node1
+
+    def remove(self, node: DoubleLinkedListNode) -> DoubleLinkedListNode:
+        '''
+        Removes and returns the given node from the list
+        '''
+
+        temp_last, temp_next = node.prev, node.next
+        node.prev, node.next = None, None
+        temp_last.next, temp_next.prev = temp_next, temp_last
+        return node
+
+
+class LFUCache:
+    '''
+    LFU Cache to store a given capacity of data. Can be used as a stand-alone object
+    or as a function decorator.
+
+    >>> cache = LFUCache(2)
+    >>> cache.set(1, 1)
+    >>> cache.set(2, 2)
+    >>> cache.get(1)
+    1
+    >>> cache.set(3, 3)
+    >>> cache.get(2)  # None is returned
+    >>> cache.set(4, 4)
+    >>> cache.get(1)  # None is returned
+    >>> cache.get(3)
+    3
+    >>> cache.get(4)
+    4
+    >>> cache
+    CacheInfo(hits=3, misses=2, capacity=2, current size=2)
+    >>> @LFUCache.decorator(100)
+    ... def fib(num):
+    ...     if num in (1, 2):
+    ...         return 1
+    ...     return fib(num - 1) + fib(num - 2)
+
+    >>> for i in range(1, 101):
+    ...     res = fib(i)
+
+    >>> fib.cache_info()
+    CacheInfo(hits=196, misses=100, capacity=100, current size=100)
+    '''
+
+    # class variable to map the decorator functions to their respective instance
+    decorator_function_to_instance_map = {}
+
+    def __init__(self, capacity: int):
+        self.list = DoubleLinkedList()
+        self.capacity = capacity
+        self.num_keys = 0
+        self.hits = 0
+        self.miss = 0
+        self.cache = {}
+
+    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: int) -> bool:
+        '''
+        >>> cache = LFUCache(1)
+        >>> 1 in cache
+        False
+        >>> cache.set(1, 1)
+        >>> 1 in cache
+        True
+        '''
+        return key in self.cache
+
+    def get(self, key: int) -> Optional[int]:
+        '''
+        Returns the value for the input key and updates the Double Linked List. Returns
+        None if key is not present in cache
+        '''
+
+        if key in self.cache:
+            self.hits += 1
+            self.list.add(self.list.remove(self.cache[key]))
+            return self.cache[key].val
+        self.miss += 1
+        return None
+
+    def set(self, key: int, value: int) -> 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:
+                key_to_delete = self.list.head.next.key
+                self.list.remove(self.cache[key_to_delete])
+                del self.cache[key_to_delete]
+                self.num_keys -= 1
+            self.cache[key] = DoubleLinkedListNode(key, value)
+            self.list.add(self.cache[key])
+            self.num_keys += 1
+
+        else:
+            node = self.list.remove(self.cache[key])
+            node.val = value
+            self.list.add(node)
+
+    @staticmethod
+    def decorator(size: int = 128):
+        '''
+        Decorator version of LFU Cache
+        '''
+
+        def cache_decorator_inner(func: Callable):
+
+            def cache_decorator_wrapper(*args, **kwargs):
+                if func not in LFUCache.decorator_function_to_instance_map:
+                    LFUCache.decorator_function_to_instance_map[func] = LFUCache(size)
+
+                result = LFUCache.decorator_function_to_instance_map[func].get(args[0])
+                if result is None:
+                    result = func(*args, **kwargs)
+                    LFUCache.decorator_function_to_instance_map[func].set(
+                        args[0], result
+                    )
+                return result
+
+            def cache_info():
+                return LFUCache.decorator_function_to_instance_map[func]
+
+            cache_decorator_wrapper.cache_info = cache_info
+
+            return cache_decorator_wrapper
+
+        return cache_decorator_inner
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()