seperate class

Author: pathywang

Sprint-2/implement_lru_cache/linked_list.py

@@ -0,0 +1,55 @@
+class Node:
+    __slots__ = ("value", "next", "previous")
+
+    def __init__(self, value):
+        self.value = value
+        self.next = None
+        self.previous = None
+
+class LinkedList:
+    def __init__(self):
+        self.head = None
+        self.tail = None
+
+    def push_head(self, value):
+        node = Node(value)
+
+        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 pop_tail(self):
+        node = self.tail
+
+        if node is None:
+            return None
+
+        self.remove(node)
+
+        return node.value
+
+    def remove(self, node):
+        if node == self.head and node == self.tail:
+            self.head = None
+            self.tail = None
+
+        elif node == self.head:
+            self.head = node.next
+            self.head.previous = None
+
+        elif node == self.tail:
+            self.tail = node.previous
+            self.tail.next = None
+
+        else:
+            node.previous.next = node.next
+            node.next.previous = node.previous
+
+        node.next = None
+        node.previous = None

Sprint-2/implement_lru_cache/lru_cache.py

@@ -1,73 +1,39 @@
-class Node:
-    def __init__(self, key, value):
-        self.key = key
-        self.value = value
-        self.next = None
-        self.previous = None
+from linked_list import LinkedList
+
 
 class LruCache:
     def __init__(self, limit):
         if limit <= 0:
-            raise ValueError
+            raise ValueError("Limit must be greater than zero")
 
         self.limit = limit
         self.cache = {}
-        self.head = None
-        self.tail = None
-
-    def _remove_node(self, node):
-        if node.previous is not None:
-            node.previous.next = node.next
-        else:
-            self.head = node.next
-
-        if node.next is not None:
-            node.next.previous = node.previous
-        else:
-            self.tail = node.previous
-
-    def _add_to_tail(self, node):
-        node.next = None
-        node.previous = self.tail
-
-        if self.tail is not None:
-            self.tail.next = node
-        else:
-            self.head = node
-
-        self.tail = node
+        self.order = LinkedList()
 
     def get(self, key):
         if key not in self.cache:
             return None
 
         node = self.cache[key]
+        value = node.value[1]
 
-        # Move node to the most-recently-used position
-        if node is not self.tail:
-            self._remove_node(node)
-            self._add_to_tail(node)
+        # Move the accessed item to the head.
+        self.order.remove(node)
+        new_node = self.order.push_head(node.value)
+        self.cache[key] = new_node
 
-        return node.value
+        return value
 
     def set(self, key, value):
         if key in self.cache:
             node = self.cache[key]
-            node.value = value
-
-            # Updating an item counts as using it
-            if node is not self.tail:
-                self._remove_node(node)
-                self._add_to_tail(node)
-
-            return
-
-        node = Node(key, value)
-        self.cache[key] = node
-        self._add_to_tail(node)
+            self.order.remove(node)
 
-        if len(self.cache) > self.limit:
-            lru_node = self.head
+        elif len(self.cache) >= self.limit:
+            # Remove the least recently used item.
+            lru_item = self.order.pop_tail()
+            del self.cache[lru_item[0]]
 
-            self._remove_node(lru_node)
-            del self.cache[lru_node.key]
+        # Add the item to the head as most recently used.
+        node = self.order.push_head((key, value))
+        self.cache[key] = node