From e646b454608c6fede5a52291b9695661813e79aa Mon Sep 17 00:00:00 2001
From: Tapajyoti Bose <44058757+ruppysuppy@users.noreply.github.com>
Date: Thu, 2 Jul 2020 16:06:02 +0530
Subject: [PATCH] Added LRU Cache (#219)

---
 Cache/LRUCache.js | 126 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 126 insertions(+)
 create mode 100644 Cache/LRUCache.js

diff --git a/Cache/LRUCache.js b/Cache/LRUCache.js
new file mode 100644
index 000000000..5106331df
--- /dev/null
+++ b/Cache/LRUCache.js
@@ -0,0 +1,126 @@
+class DoubleLinkedListNode {
+  // Double Linked List Node built specifically for LRU Cache
+  constructor (key, val) {
+    this.key = key
+    this.val = val
+    this.next = null
+    this.prev = null
+  }
+}
+
+class DoubleLinkedList {
+  // Double Linked List built specifically for LRU Cache
+  constructor () {
+    this.head = new DoubleLinkedListNode(null, null)
+    this.rear = new DoubleLinkedListNode(null, null)
+    this.head.next = this.rear
+    this.rear.prev = this.head
+  }
+
+  add (node) {
+    // Adds the given node to the end of the list (before rear)
+    const temp = this.rear.prev
+    temp.next = node
+    node.prev = temp
+    this.rear.prev = node
+    node.next = this.rear
+  }
+
+  remove (node) {
+    // Removes and returns the given node from the list
+    const tempLast = node.prev
+    const tempNext = node.next
+    node.prev = null
+    node.next = null
+    tempLast.next = tempNext
+    tempNext.prev = tempLast
+
+    return node
+  }
+}
+
+class LRUCache {
+  // LRU Cache to store a given capacity of data
+  constructor (capacity) {
+    this.list = new DoubleLinkedList()
+    this.capacity = capacity
+    this.numKeys = 0
+    this.hits = 0
+    this.miss = 0
+    this.cache = {}
+  }
+
+  cacheInfo () {
+    // Return the details for the cache instance [hits, misses, capacity, current_size]
+    return `CacheInfo(hits=${this.hits}, misses=${this.miss}, capacity=${this.capacity}, current size=${this.numKeys})`
+  }
+
+  set (key, value) {
+    // Sets the value for the input key and updates the Double Linked List
+    if (!(key in this.cache)) {
+      if (this.numKeys >= this.capacity) {
+        const keyToDelete = this.list.head.next.key
+        this.list.remove(this.cache[keyToDelete])
+        delete this.cache[keyToDelete]
+        this.numKeys -= 1
+      }
+      this.cache[key] = new DoubleLinkedListNode(key, value)
+      this.list.add(this.cache[key])
+      this.numKeys += 1
+    } else {
+      const node = this.list.remove(this.cache[key])
+      node.val = value
+      this.list.add(node)
+    }
+  }
+
+  get (key) {
+    // Returns the value for the input key and updates the Double Linked List. Returns null if key is not present in cache
+    if (key in this.cache) {
+      this.hits += 1
+      this.list.add(this.list.remove(this.cache[key]))
+      return this.cache[key].val
+    }
+    this.miss += 1
+    return null
+  }
+}
+
+function main () {
+  // Example 1 (Small Cache)
+  const cache = new LRUCache(2)
+  cache.set(1, 1)
+  cache.set(2, 2)
+
+  console.log(cache.get(1))
+
+  cache.set(3, 3)
+
+  console.log(cache.get(2)) // cache miss
+
+  cache.set(4, 4)
+
+  console.log(cache.get(1)) // cache miss
+  console.log(cache.get(3))
+  console.log(cache.get(4))
+
+  console.log('Example Cache: ', cache.cacheInfo(), '\n')
+
+  // Example 2 (Computing Fibonacci Series - 100 terms)
+  function fib (num, cache = null) {
+    if (cache) {
+      const value = cache.get(num)
+      if (value) { return value }
+    }
+    if (num === 1 || num === 2) { return 1 }
+    const result = fib(num - 1, cache) + fib(num - 2, cache)
+    if (cache) { cache.set(num, result) }
+    return result
+  }
+
+  const fibCache = new LRUCache(100)
+  for (let i = 1; i <= 100; i++) { fib(i, fibCache) }
+  console.log('Fibonacci Series Cache: ', fibCache.cacheInfo(), '\n')
+}
+
+main()