WestMidlands | 26-SDC-Mar | Chioma Okeke | Sprint 2 | LRU cache

Sprint-2/implement_lru_cache/lru_cache.py

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+class Node:
+    """
+    A node in a double linked list.
+
+    Each node supports a data, previous, and next
+    - data: the value
+    - previous: pointer to the previous node
+    - next: pointer to the next node
+    """
+
+    _slots_ = ("data", "previous", "next")
+
+    def __init__(self, data):
+        self.data = data
+        self.previous = None
+        self.next = None
+
+class LinkedList:
+    """
+    Doubly Linked List
+
+    Supports O(1) operations on insertion at head, removal from tail, and removal from arbitrary node
+    """
+    def __init__(self):
+        # Head points to the first node in the list
+        self.head = None
+
+        # Tail points to the last node in the list
+        self.tail = None
+
+    def is_empty(self):
+        """Returns True if the list contains no nodes."""
+        return self.head is None
+
+    def push_head(self, data):
+        """
+        Insert a new node at the head of the list.
+
+        Time Complexity: O(1)
+
+        Returns:
+            Node: reference to the inserted node (handle for later removal)
+        """
+        node = Node(data)
+
+        if self.is_empty():
+            # If list is empty, head and tail both point to new node
+            self.head = node
+            self.tail = node
+        else:
+            # Link new node before current head
+            node.next = self.head
+            self.head.previous = node
+
+            # Update head pointer
+            self.head = node
+
+        return node
+
+    def pop_tail(self):
+        """
+        Remove and return the value at the tail of the list.
+
+        Time Complexity: O(1)
+
+        Raises:
+            ValueError: if the list is empty
+        """
+
+        if self.is_empty():
+            raise ValueError("List is empty.")
+
+        # store value before removing node
+        node = self.tail
+        value = node.data
+
+        self.remove(node)
+
+        return value
+
+    def remove(self, node):
+        """
+        Remove a node from the list using its reference.
+
+        Time Complexity: O(1)
+
+        Args:
+            node (Node): the node to remove
+        """
+        if node.previous:
+            node.previous.next = node.next
+        else:
+            self.head = node.next
+
+        if node.next:
+            node.next.previous = node.previous
+        else:
+            self.tail = node.previous
+
+        node.previous = None
+        node.next = None
+
+    def move_to_head(self, node):
+        """
+        Move an existing node to the head of the list
+
+        Time complexity: O(1)
+        """
+
+        if node is self.head:
+            return
+
+        self.remove(node)
+
+        if self.is_empty():
+            self.head = None
+            self.tail = None
+        else:
+            node.previous = None
+            node.next = self.head
+            self.head.previous = node
+            self.head = node
+
+class LruCache:
+    """
+    Least Recently Used (LRU) Cache.
+
+    Uses:
+    - dictionary for O(1) lookup
+    - doubly linked list for O(1) ordering
+
+    Head = most recently used.
+    Tail = least recently used.
+    """
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError("limit must be positive")
+
+        self.limit = limit
+        self.cache = {}
+        self.list = LinkedList()
+
+    def get(self, key):
+        """
+        Return value associated with key.
+
+        Access counts as use.
+        """
+        node = self.cache.get(key)
+
+        if node is None:
+            return None
+
+        self.list.move_to_head(node)
+
+        return node.data[1]
+
+    def set(self, key, value):
+        """
+        Insert or update a key.
+
+        Update counts as use.
+        """
+
+        node = self.cache.get(key)
+
+        # Existing key
+        if node is not None:
+            node.data = (key, value)
+            self.list.move_to_head(node)
+            return
+
+        # Evict least recently used if full
+        if len(self.cache) >= self.limit:
+            evicted_key, _ = self.list.pop_tail()
+            del self.cache[evicted_key]
+
+        # Insert new entry
+        node = self.list.push_head((key, value))
+        self.cache[key] = node
+
+    def __contains__(self, key):
+        return key in self.cache
+
+    def __len__(self):
+        return len(self.cache)