Page replacement algorithm, LRU (#871)

* Page replacement algorithm, LRU

* small rectifications

* Rename paging/LRU.py to other/least_recently_used.py

other/least_recently_used.py

@@ -0,0 +1,62 @@
+from abc import abstractmethod
+import sys
+from collections import deque
+
+class LRUCache:
+    """ Page Replacement Algorithm, Least Recently Used (LRU) Caching."""
+
+    dq_store = object() # Cache store of keys
+    key_reference_map = object() # References of the keys in cache
+    _MAX_CAPACITY: int = 10 # Maximum capacity of cache
+
+    @abstractmethod
+    def __init__(self, n: int):
+        """ Creates an empty store and map for the keys.
+        The LRUCache is set the size n.
+        """
+        self.dq_store = deque()
+        self.key_reference_map = set()
+        if not n:
+            LRUCache._MAX_CAPACITY = sys.maxsize
+        elif n < 0:
+            raise ValueError('n should be an integer greater than 0.')
+        else:
+            LRUCache._MAX_CAPACITY = n
+
+    def refer(self, x):
+        """
+            Looks for a page in the cache store and adds reference to the set.
+            Remove the least recently used key if the store is full.
+            Update store to reflect recent access.
+        """
+        if x not in self.key_reference_map:
+            if len(self.dq_store) == LRUCache._MAX_CAPACITY:
+                last_element = self.dq_store.pop()
+                self.key_reference_map.remove(last_element)
+        else:
+            index_remove = 0
+            for idx, key in enumerate(self.dq_store):
+                if key == x:
+                    index_remove = idx
+                    break
+            self.dq_store.remove(index_remove)
+
+        self.dq_store.appendleft(x)
+        self.key_reference_map.add(x)
+
+    def display(self):
+        """
+            Prints all the elements in the store.
+        """
+        for k in self.dq_store:
+            print(k)
+
+if __name__ == "__main__":
+    lru_cache = LRUCache(4)
+    lru_cache.refer(1)
+    lru_cache.refer(2)
+    lru_cache.refer(3)
+    lru_cache.refer(1)
+    lru_cache.refer(4)
+    lru_cache.refer(5)
+    lru_cache.display()