feat: LFU (Least frequently used) cache (#2757)

* feat: add lfu cache

* docs: add comments and explanation to class LFUCache

* test: add tests for class lfu cache

* docs: document namespace and classes

* test: modify tests to check negative numbers

* docs: document template params and class data members

* test: make test func static and move tests in the same func

---------

Co-authored-by: realstealthninja <68815218+realstealthninja@users.noreply.github.com>

others/lfu_cache.cpp

@@ -0,0 +1,304 @@
+/**
+ * @file
+ * @brief Implementation for [LFU Cache]
+ * (https://en.wikipedia.org/wiki/Least_frequently_used)
+ *
+ * @details
+ * LFU discards the least frequently used value. if there are multiple items
+ * with the same minimum frequency then, the least recently used among them is
+ * discarded. Data structures used - doubly linked list and unordered_map(hash
+ * map).
+ *
+ * Hashmap maps the key to the address of the node of the linked list and its
+ * current usage frequency. If the element is accessed the element is removed
+ * from the linked list of the current frequency and added to the linked list of
+ * incremented frequency.
+ *
+ * When the cache is full, the last element in the minimum frequency linked list
+ * is popped.
+ *
+ * @author [Karan Sharma](https://github.com/deDSeC00720)
+ */
+
+#include <cassert>        // for assert
+#include <iostream>       // for std::cout
+#include <unordered_map>  // for std::unordered_map
+
+/**
+ * @namespace
+ * @brief Other algorithms
+ */
+namespace others {
+
+/**
+ * @namespace
+ * @brief Cache algorithm
+ */
+namespace Cache {
+
+/**
+ * @class
+ * @brief Node for a doubly linked list with data, prev and next pointers
+ * @tparam T type of the data of the node
+ */
+template <typename T>
+class D_Node {
+ public:
+    T data;           ///< data of the node
+    D_Node<T> *prev;  ///< previous node in the doubly linked list
+    D_Node<T> *next;  ///< next node in the doubly linked list
+
+    explicit D_Node(T data) : data(data), prev(nullptr), next(nullptr) {}
+};
+
+template <typename K, typename V>
+using CacheNode = D_Node<std::pair<K, V>>;
+
+/**
+ * @class
+ * @brief LFUCache
+ * @tparam K type of key in the LFU
+ * @tparam V type of value in the LFU
+ */
+template <typename K, typename V>
+class LFUCache {
+    std::unordered_map<K, std::pair<CacheNode<K, V> *, int>>
+        node_map;  ///< maps the key to the node address and frequency
+    std::unordered_map<int, std::pair<CacheNode<K, V> *, CacheNode<K, V> *>>
+        freq_map;  ///< maps the frequency to doubly linked list
+
+    int minFreq;    ///< minimum frequency in the cache
+    int _capacity;  ///< maximum capacity of the cache
+
+ public:
+    /**
+     * @brief Constructor, Initialize with minFreq and _capacity.
+     * @param _capacity Total capacity of the cache.
+     */
+    explicit LFUCache(int _capacity) : minFreq(0), _capacity(_capacity) {}
+
+ private:
+    /**
+     * @brief push the node at first position in the linked list of given
+     * frequency
+     * @param freq the frequency mapping to the linked list where node should be
+     * pushed.
+     * @param node node to be pushed to the linked list.
+     */
+    void push(int freq, CacheNode<K, V> *node) {
+        // if freq is not present, then make a new list with node as the head as
+        // well as tail.
+        if (!freq_map.count(freq)) {
+            freq_map[freq] = {node, node};
+            return;
+        }
+
+        std::pair<CacheNode<K, V> *, CacheNode<K, V> *> &p = freq_map[freq];
+
+        // insert the node at the beginning of the linked list and update the
+        // head.
+        p.first->prev = node;
+        node->next = p.first;
+        p.first = node;
+    }
+
+    /**
+     * @brief increase the frequency of node and push it in the respective list.
+     * @param p_node the node to be updated
+     */
+    void increase_frequency(std::pair<CacheNode<K, V> *, int> &p_node) {
+        CacheNode<K, V> *node = p_node.first;
+        int freq = p_node.second;
+
+        std::pair<CacheNode<K, V> *, CacheNode<K, V> *> &p = freq_map[freq];
+
+        // if the given node is the only node in the list,
+        // then erase the frequency from map
+        // and increase minFreq by 1.
+        if (p.first == node && p.second == node) {
+            freq_map.erase(freq);
+            if (minFreq == freq) {
+                minFreq = freq + 1;
+            }
+        } else {
+            // remove the given node from current freq linked list
+            CacheNode<K, V> *prev = node->prev;
+            CacheNode<K, V> *next = node->next;
+            node->prev = nullptr;
+            node->next = nullptr;
+
+            if (prev) {
+                prev->next = next;
+            } else {
+                p.first = next;
+            }
+
+            if (next) {
+                next->prev = prev;
+            } else {
+                p.second = prev;
+            }
+        }
+        push(freq + 1, node);
+        ++p_node.second;
+    }
+
+    /**
+     * @brief pop the last node in the least frequently used linked list
+     */
+    void pop() {
+        std::pair<CacheNode<K, V> *, CacheNode<K, V> *> &p = freq_map[minFreq];
+
+        // if there is only one node
+        // remove the node and erase
+        // the frequency from freq_map
+        if (p.first == p.second) {
+            delete p.first;
+            freq_map.erase(minFreq);
+            return;
+        }
+
+        // remove the last node in the linked list
+        CacheNode<K, V> *temp = p.second;
+        p.second = temp->prev;
+        p.second->next = nullptr;
+        delete temp;
+    }
+
+ public:
+    /**
+     * @brief upsert a key-value pair
+     * @param key key of the key-value pair
+     * @param value value of the key-value pair
+     */
+    void put(K key, V value) {
+        // update the value if key already exists
+        if (node_map.count(key)) {
+            node_map[key].first->data.second = value;
+            increase_frequency(node_map[key]);
+            return;
+        }
+
+        // if the cache is full
+        // remove the least frequently used item
+        if (node_map.size() == _capacity) {
+            node_map.erase(freq_map[minFreq].second->data.first);
+            pop();
+        }
+
+        // insert the new node and set minFreq to 1
+        CacheNode<K, V> *node = new CacheNode<K, V>({key, value});
+        node_map[key] = {node, 1};
+        minFreq = 1;
+        push(1, node);
+    }
+
+    /**
+     * @brief get the value of the key-value pair if exists
+     * @param key key of the key-value pair
+     * @return the value mapped to the given key
+     * @exception exception is thrown if the key is not present in the cache
+     */
+    V get(K key) {
+        if (!node_map.count(key)) {
+            throw std::runtime_error("key is not present in the cache");
+        }
+
+        // increase the frequency and return the value
+        V value = node_map[key].first->data.second;
+        increase_frequency(node_map[key]);
+        return value;
+    }
+
+    /**
+     * @brief Returns the number of items present in the cache.
+     * @return number of items in the cache
+     */
+    int size() const { return node_map.size(); }
+
+    /**
+     * @brief Returns the total capacity of the cache
+     * @return Total capacity of the cache
+     */
+    int capacity() const { return _capacity; }
+
+    /**
+     * @brief returns true if the cache is empty, false otherwise.
+     * @return true if the cache is empty, false otherwise.
+     */
+    bool empty() const { return node_map.empty(); }
+
+    /**
+     * @brief destructs the cache, iterates on the map and deletes every node
+     * present in the cache.
+     */
+    ~LFUCache() {
+        auto it = node_map.begin();
+        while (it != node_map.end()) {
+            delete it->second.first;
+            ++it;
+        }
+    }
+};
+}  // namespace Cache
+}  // namespace others
+
+/**
+ * @brief self test implementation
+ * @return void
+ */
+static void test() {
+    others::Cache::LFUCache<int, int> cache(5);
+
+    // test the initial state of the cache
+    assert(cache.size() == 0);
+    assert(cache.capacity() == 5);
+    assert(cache.empty());
+
+    // test insertion in the cache
+    cache.put(1, 10);
+    cache.put(-2, 20);
+
+    // test the state of cache after inserting some items
+    assert(cache.size() == 2);
+    assert(cache.capacity() == 5);
+    assert(!cache.empty());
+
+    // test getting items from the cache
+    assert(cache.get(1) == 10);
+    assert(cache.get(-2) == 20);
+
+    cache.put(-3, -30);
+    cache.put(4, 40);
+    cache.put(5, -50);
+    cache.put(6, 60);
+
+    // test the state after inserting more items than the capacity
+    assert(cache.size() == 5);
+    assert(cache.capacity() == 5);
+    assert(!cache.empty());
+
+    // test retrieval of all items in the cache
+    assert(cache.get(1) == 10);
+    assert(cache.get(-2) == 20);
+
+    // fetching -3 throws runtime_error
+    // as -3 was evicted being the least frequently used
+    // when 6 was added
+    // assert(cache.get(-3) == -30);
+
+    assert(cache.get(4) == 40);
+    assert(cache.get(5) == -50);
+    assert(cache.get(6) == 60);
+
+    std::cout << "test - passed\n";
+}
+
+/**
+ * @brief main function
+ * @return 0 on exit
+ */
+int main() {
+    test();  // run the self test implementation
+    return 0;
+}