London | 26-Jul-SDC | Roman Sanaye | Sprint 2 | Implement LRU Cache

Sprint-2/implement_lru_cache/lru_cache.py

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+class Node:
+    __slots__ = ("key", "value", "next", "previous")
+
+    # Create a building block for the cache
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+        self.next = None
+        self.previous = None
+
+
+class DoublyLinkedList:
+    # Manages node ordering (head, tail, pushing, removing, and moving to head)
+    def __init__(self):
+        self.head = None
+        self.tail = None
+
+    def push_head(self, node):
+        if self.head is None:
+            self.head = node
+            self.tail = node
+        else:
+            node.next = self.head
+            self.head.previous = node
+            self.head = node
+        return node
+
+    def remove(self, node):
+        if self.head == self.tail and node == self.head:
+            self.head = None
+            self.tail = None
+        elif node == self.head:
+            self.head = self.head.next
+            if self.head is not None:
+                self.head.previous = None
+        elif node == self.tail:
+            self.tail = self.tail.previous
+            if self.tail is not None:
+                self.tail.next = None
+        else:
+            node.previous.next = node.next
+            node.next.previous = node.previous
+
+        node.next = None
+        node.previous = None
+
+    def pop_tail(self):
+        if self.tail is None:
+            return None
+        tail_node = self.tail
+        self.remove(tail_node)
+        return tail_node
+
+    def move_to_head(self, node):
+        if self.head == node:
+            return
+        self.remove(node)
+        self.push_head(node)
+
+
+class LruCache:
+    # Initializes the cache with a maximum capacity
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError("Cache limit must be greater than zero.")
+        self.limit = limit
+        self.cache = {}  # Maps key -> Node for O(1) lookups
+        self.order_list = DoublyLinkedList()  # Manages LRU order
+
+    # Looks up a value and marks it as recently used
+    def get(self, key):
+        if key not in self.cache:
+            return None
+
+        node = self.cache[key]
+        self.order_list.move_to_head(node)
+        return node.value
+
+    # Add or update an item, evicting the oldest if full
+    def set(self, key, value):
+        if key in self.cache:
+            node = self.cache[key]
+            node.value = value
+            self.order_list.move_to_head(node)
+            return
+
+        if len(self.cache) >= self.limit:
+            evicted_node = self.order_list.pop_tail()
+            if evicted_node is not None:
+                del self.cache[evicted_node.key]
+
+        new_node = Node(key, value)
+        self.cache[key] = new_node
+        self.order_list.push_head(new_node)