London|25-SDC-November|Donara Blanc |Sprint 2| Lru-cache

Sprint-2/implement_lru_cache/lru_cache.py

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+class _Node:
+    def __init__(self, value):
+        self.value = value  # stores as (key, value) tuple
+        self.next = None
+        self.previous = None
+
+
+class DoublyLinkedList:
+    """responsible only for managing a doubly linked list.
+    stores values as (key, value) tuples but knows nothing about cachin
+    """
+    def __init__(self):
+        self.head = None  # most recently used end
+        self.tail = None  # least recently used end
+
+    def add_to_head(self, value):
+        """sreates a new node with value, add to head, returns the node."""
+        node = _Node(value)
+        node.next = self.head
+        node.previous = None
+        if self.head is not None:
+            self.head.previous = node
+        self.head = node
+        if self.tail is None:
+            self.tail = node
+        return node
+
+    def remove_node(self, node):
+        """detaches any node from the list."""
+        prev = node.previous
+        nxt = node.next
+        if prev is not None:
+            prev.next = nxt
+        else:
+            self.head = nxt
+        if nxt is not None:
+            nxt.previous = prev
+        else:
+            self.tail = prev
+        node.next = None
+        node.previous = None
+
+    def remove_tail(self):
+        """removes and returns the tail node """
+        lru = self.tail
+        if lru is not None:
+            self.remove_node(lru)
+        return lru
+
+
+class LruCache:
+    """responsible only for cache logic (get, set, eviction).
+   gives all all ordering to DoublyLinkedList.
+    """
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError("limit must be positive")
+        self.limit = limit
+        self.map = {}                    
+        self.list = DoublyLinkedList()   
+
+    def get(self, key):
+        node = self.map.get(key)
+        if node is None:
+            return None
+        # re-inserts at head to mark as most recently used
+        self.list.remove_node(node)
+        new_node = self.list.add_to_head(node.value)
+        self.map[key] = new_node         # updates map to point to new node
+        return new_node.value[1]         
+
+    def set(self, key, value):
+        node = self.map.get(key)
+        if node is not None:
+            # key exists then remove old, adds updated to head
+            self.list.remove_node(node)
+            new_node = self.list.add_to_head((key, value))
+            self.map[key] = new_node     # update map to new node
+            return
+        # new key gets rid of LRU 
+        if len(self.map) >= self.limit:
+            lru = self.list.remove_tail()
+            if lru is not None:
+                del self.map[lru.value[0]]  
+        new_node = self.list.add_to_head((key, value))
+        self.map[key] = new_node        
+        new_node = _Node(key, value)
+        self._add_to_head(new_node)
+        self.map[key] = new_node