complete lru-cache

Author: pathywang

Sprint-2/implement_lru_cache/lru_cache.py

@@ -0,0 +1,73 @@
+class Node:
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+        self.next = None
+        self.previous = None
+
+class LruCache:
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError
+
+        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
+
+    def get(self, key):
+        if key not in self.cache:
+            return None
+
+        node = self.cache[key]
+
+        # Move node to the most-recently-used position
+        if node is not self.tail:
+            self._remove_node(node)
+            self._add_to_tail(node)
+
+        return node.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)
+
+        if len(self.cache) > self.limit:
+            lru_node = self.head
+
+            self._remove_node(lru_node)
+            del self.cache[lru_node.key]