London | 26-SDC-Mar | Ebrahim Beiati-Asl | Sprint 2 | implement_lru_cache

Sprint-2/implement_lru_cache/lru_cache.py

@@ -0,0 +1,84 @@
+class Node:
+    __slots__ = ("key", "value", "prev", "next")
+
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+        self.prev = None
+        self.next = None
+
+
+class LruCache:
+    def __init__(self, limit: int):
+        if limit < 1:
+            raise ValueError(f"limit must be at least 1, got {limit}")
+
+        self._limit = limit
+        self.map: dict = {}
+        self.head = None
+        self.tail = None
+
+    def _remove_node(self, node: Node):
+        """Remove a node from the linked list in O(1) time."""
+        if node.prev:
+            node.prev.next = node.next
+        else:
+            self.head = node.next
+
+        if node.next:
+            node.next.prev = node.prev
+        else:
+            self.tail = node.prev
+
+        node.prev = None
+        node.next = None
+
+    def _add_node_to_head(self, node: Node):
+        """Add a node to the head of the linked list in O(1) time."""
+        node.next = self.head
+        node.prev = None
+
+        if self.head:
+            self.head.prev = node
+        else:
+            self.tail = node
+
+        self.head = node
+
+    def get(self, key):
+        """Return the value for key, Non if not present"""
+        node = self.map.get(key)
+        if node is None:
+            return None
+
+        self._touch(node)
+        return node.value
+
+    def set(self, key, value) -> None:
+        """Combine value with key, evicting the LRU entry if necessary."""
+        node = self.map.get(key)
+
+        if node:
+            node.value = value
+            self._touch(node)
+        else:
+            if len(self.map) >= self._limit:
+                self._evict()
+
+            new_node = Node(key, value)
+            self._add_node_to_head(new_node)
+            self.map[key] = new_node
+
+    def _touch(self, node) -> None:
+        """Move an existing node to the head (most-recently-used position)."""
+        self._remove_node(node)
+        self._add_node_to_head(node)
+
+    def _evict(self) -> None:
+        """Remove the least-recently-used entry (tail of the list)."""
+        if self.tail is None:
+            return
+
+        lru = self.tail
+        self._remove_node(lru)
+        del self.map[lru.key]