London | 26-SDC-March | Jamal Laqdiem| Sprint 2 | Implement an LRU cache

Sprint-2/implement_linked_list/linked_list.py

@@ -0,0 +1,62 @@
+from dataclasses import dataclass
+from typing import Any, Optional
+# to reduce memory usage we use slots true to tells Python not to create a dynamic __dict__ for every node,
+@dataclass(slots=True)
+class Node:
+    value: Any
+    next: Optional['Node'] = None
+    previous: Optional['Node'] = None
+
+
+class LinkedList:
+    def __init__(self) -> None:
+        self.head: Optional[Node] = None
+        self.tail: Optional[Node] = None
+
+    def push_head(self, value: Any) -> Node:
+        new_node = Node(value)
+
+        if not self.head:
+            self.head = new_node
+            self.tail = new_node
+        else:
+            new_node.next = self.head
+            self.head.previous = new_node
+            self.head = new_node
+
+        return new_node
+
+    def pop_tail(self) -> Optional[Any]:
+        if not self.tail:
+            return None
+
+        value_to_return = self.tail.value
+        self.remove(self.tail)
+        return value_to_return
+
+    def remove(self, node_handle: Optional[Node]) -> None:
+        if not node_handle:
+            return
+
+        if node_handle == self.head:
+            self.head = node_handle.next
+            if self.head:
+                self.head.previous = None
+            else:
+                self.tail = None
+
+        elif node_handle == self.tail:
+            self.tail = node_handle.previous
+            if self.tail:
+                self.tail.next = None
+            else:
+                self.head = None
+
+        else:
+                assert node_handle.previous is not None
+                assert node_handle.next is not None
+                node_handle.previous.next = node_handle.next
+                node_handle.next.previous = node_handle.previous
+
+        node_handle.next = None
+        node_handle.previous = None

Sprint-2/implement_lru_cache/lru_cache.py

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+import sys
+import os
+sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), "../implement_linked_list")))
+
+from linked_list import LinkedList, Node # type:ignore
+from typing import Any
+
+class LruCache:
+    def __init__(self, limit: int):
+        if limit <= 0:
+            raise ValueError("Limit must be greater than 0")
+        self.capacity = limit
+        # maps key
+        self.cache: dict[Any, Node] = {} 
+        # tracking the usage order head first tail last.
+        self.list = LinkedList() 
+
+    def get(self, key: Any) -> Any:
+        # If key not exist return None
+        if key not in self.cache:
+            return None
+
+        node_handle = self.cache[key]
+
+        # we get  the value to return
+        _, value = node_handle.value
+
+        # we use remove() and push_head() to remove it from the position to the head
+        self.list.remove(node_handle)
+        self.cache[key] = self.list.push_head((key, value))
+
+        return value
+
+    def set(self, key: Any, value: Any) -> None:
+        if key in self.cache:
+            self.list.remove(self.cache[key])
+            # we do this because the position and value about to change.
+            del self.cache[key]
+
+          # we call pop_tail() to cut off the tail
+        elif len(self.cache) >= self.capacity:
+            # return the value stored in the tail node
+            oldest_item = self.list.pop_tail()
+            if oldest_item:
+                oldest_key, _ = oldest_item
+                del self.cache[oldest_key]
+
+        ## place the new items at the head of stack and update and return the object Node.
+        new_node = self.list.push_head((key, value))
+        self.cache[key] = new_node