refactor: separate DoublyLinkedList and LruCache to adhere to SRP

Author: RomanSanaye

Sprint-2/implement_lru_cache/lru_cache.py

@@ -1,4 +1,6 @@
 class Node:
+    __slots__ = ("key", "value", "next", "previous")
+
     # Create a building block for the cache
     def __init__(self, key, value):
         self.key = key
@@ -7,68 +9,86 @@ def __init__(self, key, value):
         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.head = None  # Most recently used
-        self.tail = None  # Least recently used
+        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._move_to_head(node)
+        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._move_to_head(node)
+            self.order_list.move_to_head(node)
             return
 
         if len(self.cache) >= self.limit:
-            if self.tail is not None:
-                del self.cache[self.tail.key]
-                
-                if self.head == self.tail:
-                    self.head = None
-                    self.tail = None
-                else:
-                    self.tail = self.tail.previous
-                    self.tail.next = None
+            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
-
-        if self.head is None:
-            self.head = new_node
-            self.tail = new_node
-        else:
-            new_node.next = self.head
-            self.head.previous = new_node
-            self.head = new_node
-
-    # Internal helper to reposition a used node to the front
-    def _move_to_head(self, node):
-        if self.head == node:
-            return
-
-        if node == self.tail:
-            self.tail = node.previous
-            self.tail.next = None
-        else:
-            node.previous.next = node.next
-            node.next.previous = node.previous
-
-        node.next = self.head
-        self.head.previous = node
-        node.previous = None
-        self.head = node
+        self.order_list.push_head(new_node)