From 5466ff0d577883e3fcb27b3feddad141d20b6355 Mon Sep 17 00:00:00 2001
From: varnaa <swersie723>
Date: Sun, 2 Aug 2020 22:20:39 +0530
Subject: [PATCH] Added LRU cache implementation.

---
 src/main/java/com/caching/LRUCache.java     | 93 +++++++++++++++++++++
 src/test/java/com/caching/LRUCacheTest.java | 25 ++++++
 2 files changed, 118 insertions(+)
 create mode 100644 src/main/java/com/caching/LRUCache.java
 create mode 100644 src/test/java/com/caching/LRUCacheTest.java

diff --git a/src/main/java/com/caching/LRUCache.java b/src/main/java/com/caching/LRUCache.java
new file mode 100644
index 00000000..e043e35d
--- /dev/null
+++ b/src/main/java/com/caching/LRUCache.java
@@ -0,0 +1,93 @@
+package com.caching;
+
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.NoSuchElementException;
+
+/**
+ * Your LRUCache can be instantiated and called as such:
+ * LRUCache lruCache = new LRUCache(capacity);
+ * lruCache.put(key,value);
+ * int param_1 = lruCache.get(key);
+ */
+
+public class LRUCache<T> {
+    /**
+     * The class LinkedHashMap,is a subclass of HashMap that preserves the insertion order.
+     * We can take advantage of this class to avoid having to implement the linked list.
+     * A special constructor is provided to create a linked hash map whose order of
+     * iteration is the order in which its entries were least-recently (access-order).
+     */
+    private final LinkedHashMap<Integer, T> cache;
+    private final int capacity;
+
+    /**
+     * @param capacity - Instantiates LRUCache with the given capacity.
+     */
+    public LRUCache(int capacity) {
+        this.capacity = capacity;
+
+        /*
+          @param loadFactor Load Factor is a measure, which decides when exactly to resize the
+         *                  HashMap. By default capacity = 16 and loadFactor = 0.75f. This means
+         *                  that reisze when HashMap reaches 75% of its capacity. For we will
+         *                  remove an element only if the cache reaches 100% capacity (1.0f).
+         *
+         * @param accessOrder - Set to true if ordering mode is specified (removeEldestEntry).
+         */
+        this.cache = new LinkedHashMap<>(capacity, 1.0f, true) {
+            /**
+             * @param eldest - The least recently accessed entry This is the entry that will
+             *               be removed if the method returns {@code true}.
+             *               returns {@code false} if it should be retained.
+             */
+            @Override
+            protected boolean removeEldestEntry(Map.Entry eldest) {
+                return this.size() > capacity;
+            }
+        };
+    }
+
+
+    /**
+     * To put a value in LRU cache with corresponding key
+     * We add the value for key only if the key is not present.
+     * We don't update existing values, only access-order is updated.
+     *
+     * @param key   The key (int)
+     * @param value The value to be cached
+     */
+    public void put(int key, T value) {
+        if (capacity == 0) {
+            System.out.println("Cache set to 0 capacity. No elements will be cached");
+        }
+
+        T currentValue = cache.get(key);
+        if (!cache.containsKey(key)) {
+            cache.put(key, value);
+            System.out.println("Adding new key:" + key + " to cache");
+        } else {
+            System.out.println("Key:" + key + " already present in cache. Access order will be updated.");
+        }
+    }
+
+
+    /**
+     * To get the cached value for given key
+     *
+     * @param key The key (int) of the expected value
+     * @return corresponding value for input key
+     * @throws NoSuchElementException if key is absent
+     */
+    public T get(int key) {
+        // cache hit condition
+        if (cache.containsKey(key)) {
+            T value = cache.get(key);
+            System.out.println("Returning value from cache:" + value);
+            return value;
+        }
+        // cache miss condition
+        throw new NoSuchElementException("No element found for key:" + key);
+    }
+}
diff --git a/src/test/java/com/caching/LRUCacheTest.java b/src/test/java/com/caching/LRUCacheTest.java
new file mode 100644
index 00000000..78c3f45f
--- /dev/null
+++ b/src/test/java/com/caching/LRUCacheTest.java
@@ -0,0 +1,25 @@
+package com.caching;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.NoSuchElementException;
+
+public class LRUCacheTest {
+
+    @Test
+    public void testLFUCache() {
+        LRUCache<Integer> cache = new LRUCache<Integer>(2);
+
+        cache.put(1, 5);
+        cache.put(2, 4);
+        Assertions.assertEquals(5, cache.get(1)); // returns 5
+        cache.put(3, 6); // evicts key 2
+        Assertions.assertThrows(NoSuchElementException.class, () -> cache.get(7));// throws exception
+        Assertions.assertEquals(6, cache.get(3)); // returns 6.
+        cache.put(4, 8); // evicts key 1.
+        Assertions.assertThrows(NoSuchElementException.class, () -> cache.get(1));// throws exception
+        Assertions.assertEquals(6, cache.get(3)); // returns 6
+        Assertions.assertEquals(8, cache.get(4)); // returns 8
+    }
+}
\ No newline at end of file