London | 26-SDC-Mar | beko | Sprint 2 | implement an LRU cache in python

Author: Abubakar-Meigag

Sprint-2/implement_lru_cache/lru_cache.py

@@ -0,0 +1,61 @@
+from typing import Optional
+
+class _Node:
+    def __init__(self, key=None, value=None):
+        self.key   = key
+        self.value = value
+        self.prev: Optional["_Node"] = None
+        self.next: Optional["_Node"] = None
+
+class LruCache:
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError("limit must be positive")
+
+        self.limit = limit
+        self.cache = {}
+
+        self.head = _Node()
+        self.tail = _Node()
+        self.head.next = self.tail
+        self.tail.prev = self.head
+
+    def remove(self, node):
+        node.prev.next = node.next
+        node.next.prev = node.prev
+
+    def add_to_front(self, node):
+        next_node = self.head.next
+        assert next_node is not None
+
+        node.next = next_node
+        node.prev = self.head
+        next_node.prev = node
+        self.head.next = node
+
+    def get(self, key):
+        if key not in self.cache:
+            return None
+
+        node = self.cache[key]
+        self.remove(node)
+        self.add_to_front(node)
+        return node.value
+
+    def set(self, key, value):
+        if key in self.cache:
+            node = self.cache[key]
+            node.value = value
+            self.remove(node)
+            self.add_to_front(node)
+            return
+
+        node = _Node(key, value)
+        self.cache[key] = node
+        self.add_to_front(node)
+
+        if len(self.cache) > self.limit:
+            lru = self.tail.prev
+            assert lru is not None
+            self.remove(lru)
+            del self.cache[lru.key]