Birmingham | 26-Jul-SDC | Merve Reis | Sprint 2 | Implement LRU Cache in Phyton

Sprint-2/implement_lru_cache/linked_list.py

@@ -0,0 +1,45 @@
+class Node:
+    def __init__(self, value):
+        self.value = value
+        self.previous = None
+        self.next = None
+
+
+class LinkedList:
+    def __init__(self):
+        self.head = None
+        self.tail = None
+
+    def push_head(self, value):
+        node = Node(value)
+        node.next = self.head
+
+        if self.head is None:
+            self.tail = node
+        else:
+            self.head.previous = node
+
+        self.head = node
+        return node
+
+    def pop_tail(self):
+        if self.tail is None:
+            return None
+
+        node = self.tail
+        self.remove(node)
+        return node.value
+
+    def remove(self, node):
+        if node.previous is None:
+            self.head = node.next
+        else:
+            node.previous.next = node.next
+
+        if node.next is None:
+            self.tail = node.previous
+        else:
+            node.next.previous = node.previous
+
+        node.previous = None
+        node.next = None

Sprint-2/implement_lru_cache/lru_cache.py

@@ -0,0 +1,41 @@
+from linked_list import LinkedList
+
+
+class _CacheEntry:
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+
+
+class LruCache:
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError("Limit must be greater than 0")
+
+        self.limit = limit
+        self.cache = {}
+        self.order = LinkedList()
+
+    def get(self, key):
+        node = self.cache.get(key)
+        if node is None:
+            return None
+
+        self.order.remove(node)
+        self.cache[key] = self.order.push_head(node.value)
+        return node.value.value
+
+    def set(self, key, value):
+        node = self.cache.get(key)
+        if node is not None:
+            node.value.value = value
+            self.order.remove(node)
+            self.cache[key] = self.order.push_head(node.value)
+            return
+
+        entry = _CacheEntry(key, value)
+        self.cache[key] = self.order.push_head(entry)
+
+        if len(self.cache) > self.limit:
+            least_recently_used = self.order.pop_tail()
+            del self.cache[least_recently_used.key]

Sprint-2/implement_lru_cache/lru_cache_test.py

@@ -54,6 +54,18 @@ def test_eviction_order_after_gets(self):
         self.assertEqual(cache.get("a"), 1)
         self.assertEqual(cache.get("c"), 3)
 
+    def test_setting_existing_key_updates_value_and_recency(self):
+        cache = LruCache(limit=2)
+
+        cache.set("a", 1)
+        cache.set("b", 2)
+        cache.set("a", 3)
+        cache.set("c", 4)
+
+        self.assertIsNone(cache.get("b"))
+        self.assertEqual(cache.get("a"), 3)
+        self.assertEqual(cache.get("c"), 4)
+
 
 if __name__ == "__main__":
     unittest.main()