lfu_cache.cpp

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#include <cassert>        // for assert
#include <iostream>       // for std::cout
#include <unordered_map>  // for std::unordered_map

namespace others {

namespace Cache {

template <typename T>
class D_Node {
 public:
    T data;           
    D_Node<T> *prev;  
    D_Node<T> *next;  
 
    explicit D_Node(T data) : data(data), prev(nullptr), next(nullptr) {}
};
 
template <typename K, typename V>
using CacheNode = D_Node<std::pair<K, V>>;

template <typename K, typename V>
class LFUCache {
    std::unordered_map<K, std::pair<CacheNode<K, V> *, int>>
        node_map;  
    std::unordered_map<int, std::pair<CacheNode<K, V> *, CacheNode<K, V> *>>
        freq_map;  
 
    int minFreq;    
    int _capacity;  
 
 public:
    explicit LFUCache(int _capacity) : minFreq(0), _capacity(_capacity) {}
 
 private:
    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;
    }

    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;
    }

    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:
    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);
    }

    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;
    }

    int size() const { return node_map.size(); }

    int capacity() const { return _capacity; }

    bool empty() const { return node_map.empty(); }

    ~LFUCache() {
        auto it = node_map.begin();
        while (it != node_map.end()) {
            delete it->second.first;
            ++it;
        }
    }
};
}  // namespace Cache
}  // namespace others

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";
}

int main() {
    test();  // run the self test implementation
    return 0;
}