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| 1 | +class DoubleLinkedListNode { |
| 2 | + // Double Linked List Node built specifically for LFU Cache |
| 3 | + constructor (key, val) { |
| 4 | + this.key = key |
| 5 | + this.val = val |
| 6 | + this.freq = 0 |
| 7 | + this.next = null |
| 8 | + this.prev = null |
| 9 | + } |
| 10 | +} |
| 11 | + |
| 12 | +class DoubleLinkedList { |
| 13 | + // Double Linked List built specifically for LFU Cache |
| 14 | + constructor () { |
| 15 | + this.head = new DoubleLinkedListNode(null, null) |
| 16 | + this.rear = new DoubleLinkedListNode(null, null) |
| 17 | + this.head.next = this.rear |
| 18 | + this.rear.prev = this.head |
| 19 | + } |
| 20 | + |
| 21 | + _positionNode (node) { |
| 22 | + // Helper function to position a node based on the frequency of the key |
| 23 | + while (node.prev.key && node.prev.freq > node.freq) { |
| 24 | + const node1 = node |
| 25 | + const node2 = node.prev |
| 26 | + node1.prev = node2.prev |
| 27 | + node2.next = node1.prev |
| 28 | + node1.next = node2 |
| 29 | + node2.prev = node1 |
| 30 | + } |
| 31 | + } |
| 32 | + |
| 33 | + add (node) { |
| 34 | + // Adds the given node to the end of the list (before rear) and positions it based on frequency |
| 35 | + const temp = this.rear.prev |
| 36 | + temp.next = node |
| 37 | + node.prev = temp |
| 38 | + this.rear.prev = node |
| 39 | + node.next = this.rear |
| 40 | + this._positionNode(node) |
| 41 | + } |
| 42 | + |
| 43 | + remove (node) { |
| 44 | + // Removes and returns the given node from the list |
| 45 | + const tempLast = node.prev |
| 46 | + const tempNext = node.next |
| 47 | + node.prev = null |
| 48 | + node.next = null |
| 49 | + tempLast.next = tempNext |
| 50 | + tempNext.prev = tempLast |
| 51 | + |
| 52 | + return node |
| 53 | + } |
| 54 | +} |
| 55 | + |
| 56 | +class LFUCache { |
| 57 | + // LFU Cache to store a given capacity of data |
| 58 | + // The Double Linked List is used to store the order of deletion from the cache |
| 59 | + // The rear.prev holds the most frequently used key and the head.next holds the least used key |
| 60 | + // When the number of elements reaches the capacity, the least frequently used item is removed before adding the next key |
| 61 | + constructor (capacity) { |
| 62 | + this.list = new DoubleLinkedList() |
| 63 | + this.capacity = capacity |
| 64 | + this.numKeys = 0 |
| 65 | + this.hits = 0 |
| 66 | + this.miss = 0 |
| 67 | + this.cache = {} |
| 68 | + } |
| 69 | + |
| 70 | + cacheInfo () { |
| 71 | + // Return the details for the cache instance [hits, misses, capacity, current_size] |
| 72 | + return `CacheInfo(hits=${this.hits}, misses=${this.miss}, capacity=${this.capacity}, current size=${this.numKeys})` |
| 73 | + } |
| 74 | + |
| 75 | + set (key, value) { |
| 76 | + // Sets the value for the input key and updates the Double Linked List |
| 77 | + if (!(key in this.cache)) { |
| 78 | + if (this.numKeys >= this.capacity) { |
| 79 | + const keyToDelete = this.list.head.next.key |
| 80 | + this.list.remove(this.cache[keyToDelete]) |
| 81 | + delete this.cache[keyToDelete] |
| 82 | + this.numKeys -= 1 |
| 83 | + } |
| 84 | + this.cache[key] = new DoubleLinkedListNode(key, value) |
| 85 | + this.list.add(this.cache[key]) |
| 86 | + this.numKeys += 1 |
| 87 | + } else { |
| 88 | + const node = this.list.remove(this.cache[key]) |
| 89 | + node.val = value |
| 90 | + this.list.add(node) |
| 91 | + } |
| 92 | + } |
| 93 | + |
| 94 | + get (key) { |
| 95 | + // Returns the value for the input key and updates the Double Linked List. Returns null if key is not present in cache |
| 96 | + if (key in this.cache) { |
| 97 | + this.hits += 1 |
| 98 | + this.list.add(this.list.remove(this.cache[key])) |
| 99 | + return this.cache[key].val |
| 100 | + } |
| 101 | + this.miss += 1 |
| 102 | + return null |
| 103 | + } |
| 104 | +} |
| 105 | + |
| 106 | +function main () { |
| 107 | + // Example 1 (Small Cache) |
| 108 | + const cache = new LFUCache(2) |
| 109 | + cache.set(1, 1) |
| 110 | + cache.set(2, 2) |
| 111 | + |
| 112 | + console.log(cache.get(1)) |
| 113 | + |
| 114 | + cache.set(3, 3) |
| 115 | + |
| 116 | + console.log(cache.get(2)) // cache miss |
| 117 | + |
| 118 | + cache.set(4, 4) |
| 119 | + |
| 120 | + console.log(cache.get(1)) // cache miss |
| 121 | + console.log(cache.get(3)) |
| 122 | + console.log(cache.get(4)) |
| 123 | + |
| 124 | + console.log('Example Cache: ', cache.cacheInfo(), '\n') |
| 125 | + |
| 126 | + // Example 2 (Computing Fibonacci Series - 100 terms) |
| 127 | + function fib (num, cache = null) { |
| 128 | + if (cache) { |
| 129 | + const value = cache.get(num) |
| 130 | + if (value) { return value } |
| 131 | + } |
| 132 | + if (num === 1 || num === 2) { return 1 } |
| 133 | + const result = fib(num - 1, cache) + fib(num - 2, cache) |
| 134 | + if (cache) { cache.set(num, result) } |
| 135 | + return result |
| 136 | + } |
| 137 | + |
| 138 | + const fibCache = new LFUCache(100) |
| 139 | + for (let i = 1; i <= 100; i++) { fib(i, fibCache) } |
| 140 | + console.log('Fibonacci Series Cache: ', fibCache.cacheInfo(), '\n') |
| 141 | +} |
| 142 | + |
| 143 | +main() |
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