Sheffield | 25-SDC-Nov | Sheida Shabankari | Sprint 2 | Implement LRU cache in python

Sprint-2/implement_lru_cache/lru_cache.py

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+class Node:
+    """
+    Represents a node in a doubly linked list.
+    Each node holds a key-value pair and pointers to the previous and next nodes.
+    """
+    def __init__(self,key,value):
+        self.key=key
+        self.value=value
+        self.next=None
+        self.previous=None
+
+class LinkedList:
+    """
+    A doubly linked list supporting:
+    - push_head: add a node to the head (most recently used position)
+    - pop_tail: remove and return the tail node (least recently used)
+    - remove: remove a specific node from the list
+    Maintains references to both head and tail nodes for O(1) operations.
+    """
+    def __init__(self):
+        self.head=None
+        self.tail=None    
+
+    def push_head(self,node=None,key=None,value=None) -> Node:
+        """
+        Adds a node to the head of the list.
+        If 'node' is provided, it is moved to the head.
+        If 'node' is None, a new node is created from key and value.
+        """
+        if node is None:
+           new_node=Node(key,value)
+        else:
+            self.remove(node)
+            new_node=node 
+
+        if self.head is None:
+            self.head=new_node
+            self.tail=new_node
+        else:
+           old_head=self.head
+           self.head=new_node
+           new_node.next=old_head
+           old_head.previous=new_node
+        new_node.previous=None   
+        return new_node
+
+    def pop_tail(self) -> any:
+        """
+        Removes the tail node (least recently used) from the list
+        and returns its key and value as a tuple.
+        """
+        if self.head is None:
+            return None
+        elif self.head==self.tail:
+            node_value=self.head.value
+            node_key=self.head.key
+            self.head=None
+            self.tail=None
+        else:
+            node_value=self.tail.value
+            node_key=self.tail.key
+            previous_node=self.tail.previous   
+            self.tail=previous_node 
+            self.tail.next=None
+        return (node_key,node_value)   
+
+    def remove(self,node) -> None:
+        """
+        Removes a specific node from the list.
+        Handles nodes at the head, tail, or middle of the list.
+        """
+        node_to_remove=node
+        current_head=self.head
+        current_tail=self.tail
+        if current_head==current_tail and node_to_remove==current_head:
+            #list has only one node
+            self.head=None
+            self.tail=None
+        elif node_to_remove.next is None:
+            #Node is the tail
+            self.tail=node_to_remove.previous  
+            self.tail.next=None  
+        elif node_to_remove.previous is None:
+            #Node is the head   
+            self.head=node_to_remove.next
+            self.head.previous=None
+        else:
+            #Node is in the middle of list
+            previous_node=node_to_remove.previous        
+            next_node=node_to_remove.next
+            previous_node.next=next_node
+            next_node.previous=previous_node
+
+        node_to_remove.next=None
+        node_to_remove.previous=None   
+
+class LruCache:
+    def __init__(self,limit):
+        """
+        Initialize the LRU Cache.
+        Args: limit (int): Maximum number of items the cache can hold.
+        """
+        if limit<=0:
+            raise ValueError("limit must be greater than zero")
+        self.limit=limit
+        self.map={}
+        self.List=LinkedList()
+        self.count=0
+
+    def get(self,key) ->any:
+        """
+        Retrieve value by key and mark node as most recently used.
+        Args: key: Key to look up in cache.
+        Returns: The value associated with the key, or None if not found.
+        """
+        if key not in self.map:
+            return None
+        node=self.map[key]   
+        self.List.push_head(node=node)
+        return node.value  
+
+
+    def set(self,key,value):
+        """
+        Add a new key-value pair or update an existing key.
+        Args:
+            key: Key to insert or update.
+            value: Value to associate with the key.
+        Behavior:
+            - Updates value if key exists and moves node to head.
+            - Inserts new node at head if key does not exist.
+            - Evicts least recently used node if cache exceeds limit.
+        """
+        if key in self.map:
+            node=self.map[key] 
+            node.value=value  
+            self.List.push_head(node=node)
+        else:
+            if self.count>=self.limit:
+                old_key,old_value=self.List.pop_tail()
+                del self.map[old_key]
+                self.count -=1
+
+            new_node=self.List.push_head(key=key,value=value)
+            self.count +=1
+            self.map[key]=new_node
+
+
+
+