implemented lru_cache

Author: donarbl

Sprint-2/implement_lru_cache/lru_cache.py

@@ -0,0 +1,91 @@
+class _Node:
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+        self.next = None
+        self.previous = None
+
+
+class LruCache:
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError("limit must be positive")
+
+        self.limit = limit
+        self.map = {}          # key -> _Node
+        self.head = None       # most recently used
+        self.tail = None       # least recently used
+
+    # ---- internal helpers for the linked list ----
+
+    def _add_to_head(self, node):
+        """Put node at the front (most recently used)."""
+        node.previous = None
+        node.next = self.head
+
+        if self.head is not None:
+            self.head.previous = node
+        self.head = node
+
+        if self.tail is None:
+            # First node in the list
+            self.tail = node
+
+    def _remove_node(self, node):
+        """Detach node from the linked list."""
+        prev = node.previous
+        nxt = node.next
+
+        if prev is not None:
+            prev.next = nxt
+        else:
+            # node was head
+            self.head = nxt
+
+        if nxt is not None:
+            nxt.previous = prev
+        else:
+            # node was tail
+            self.tail = prev
+
+        node.next = None
+        node.previous = None
+
+    def _move_to_head(self, node):
+        """Mark node as most recently used."""
+        if node is self.head:
+            return
+        self._remove_node(node)
+        self._add_to_head(node)
+
+    # ---- public API ----
+
+    def get(self, key):
+        node = self.map.get(key)
+        if node is None:
+            return None
+
+        # mark as recently used
+        self._move_to_head(node)
+        return node.value
+
+    def set(self, key, value):
+        node = self.map.get(key)
+
+        if node is not None:
+            # update existing
+            node.value = value
+            self._move_to_head(node)
+            return
+
+        # new key
+        if len(self.map) >= self.limit:
+            # evict least recently used (tail)
+            lru = self.tail
+            if lru is not None:
+                self._remove_node(lru)
+                del self.map[lru.key]
+
+        new_node = _Node(key, value)
+        self._add_to_head(new_node)
+        self.map[key] = new_node