feat: add lru caching algorithm (TheAlgorithms#1557)

* updating DIRECTORY.md

* feat: add lru caching algorithm

* removing clang tidy warnings

* updating DIRECTORY.md

* clang-format and clang-tidy fixes for fcb50fb

* docs updated

* clang-format and clang-tidy fixes for ee18e3a

* using list instead of bits/stdc++.h

* clang-format and clang-tidy fixes for 19d09e8

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* clang-format and clang-tidy fixes for 4e223bc

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* clang-format and clang-tidy fixes for 4bea23e

* using uint for non negative numbers

* clang-format and clang-tidy fixes for 87e7df5

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* clang-format and clang-tidy fixes for b423fd4

* suggestions applied

* clang-format and clang-tidy fixes for e3ba29e

* Update others/lru_cache.cpp

Co-authored-by: David Leal <[email protected]>

* clang-format and clang-tidy fixes for bdeee3d

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <[email protected]>
Co-authored-by: Abhinn Mishra <[email protected]>

DIRECTORY.md

@@ -242,6 +242,7 @@
   * [Fast Integer Input](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/fast_integer_input.cpp)
   * [Happy Number](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/happy_number.cpp)
   * [Iterative Tree Traversals](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/iterative_tree_traversals.cpp)
+  * [Lru Cache](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/lru_cache.cpp)
   * [Matrix Exponentiation](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/matrix_exponentiation.cpp)
   * [Palindrome Of Number](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/palindrome_of_number.cpp)
   * [Paranthesis Matching](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/others/paranthesis_matching.cpp)

hashing/chaining.cpp

@@ -24,7 +24,7 @@ class hash_chain {
     };
 
     std::vector<std::shared_ptr<Node>> head;  ///< array of nodes
-    int _mod;                                        ///< modulus of the class
+    int _mod;                                 ///< modulus of the class
 
  public:
     /**
@@ -164,7 +164,8 @@ int main() {
             case 3:
                 std::cout << "Enter element to generate hash = " << std::endl;
                 std::cin >> x;
-                std::cout << "Hash of " << x << " is = " << mychain.hash(x) << std::endl;
+                std::cout << "Hash of " << x << " is = " << mychain.hash(x)
+                          << std::endl;
                 break;
             case 4:
                 mychain.display();

others/lru_cache.cpp

@@ -0,0 +1,268 @@
+/**
+ * @file
+ * @brief An implementation of
+ * [LRU
+ * Cache](https://en.wikipedia.org/wiki/Cache_replacement_policies#Least_recently_used_(LRU)).
+ * Lru is a part of cache algorithms (also frequently called cache replacement
+ * algorithms or cache replacement policies).
+ *
+ * ### Logic
+ *      * Discards the least recently used items first.
+ *      * This algorithm requires keeping track of what was used when, which is
+ * expensive if one wants to make sure the algorithm always discards the least
+ * recently used item.
+ *      * General implementations of this technique require keeping "age bits"
+ * for cache-lines and track the "Least Recently Used" cache-line based on
+ * age-bits.
+ *      * In such an implementation, every time a cache-line is used, the age of
+ * all other cache-lines changes
+ *
+ * ### Algorithm explanation
+ *      For a cache of page frame x:
+ *          * Check if the page is present in cache.
+ *          * If not present, then check is the cache is full or not:
+ *          * If the cache is full, REMOVE the last element from the cache.
+ *          * If the element is present in cache, then shift that element to
+ * first position in cache from its original position.
+ *              * This way you can keep the least recently used elements in the
+ * last and most recently used in front of the cache.
+ *
+ *  Every time a requested page is  not found in cache, that is a miss or page
+ * fault, and if the page is present in cache, then its a hit.
+ *
+ * ## Data Structure used
+ *  * In the algorithm below we used two different data structure, one is linked
+ * list and other one is a hash map
+ *  * The linked list is used to contain the pages and the hash map contains the
+ * pages and their address.
+ *  * Every time a new page is requested, we first check in the hash map if the
+ * page is present or not.
+ *  * If not present, and the cache is full, we simply delete the last entry in
+ * the cache.
+ *  * If present, we shift that page from its current location to beginning of
+ * the cache and update the address in hash map for that page.
+ *
+ * @author [Nitin Sharma](https://github.com/foo290)
+ * */
+
+#include <cassert>        /// for assert
+#include <iostream>       /// for IO Operations
+#include <list>           /// for std::list
+#include <unordered_map>  /// for std::unordered_map
+
+/**
+ * @namespace others
+ * @brief Other algorithms
+ */
+namespace others {
+/**
+ * @namespace lru_cache
+ * @brief Implementation of the [LRU caching
+ * algorithm](https://en.wikipedia.org/wiki/Cache_replacement_policies#Least_recently_used_(LRU))
+ */
+namespace lru_cache {
+/**
+ * @brief LRU cache class
+ */
+class LRUCache {
+    uint64_t pageFrame;         ///< Page frame, or total size of the cache.
+    std::list<uint64_t> cache;  ///< Cache linked list (using the STL)
+    std::unordered_map<uint64_t, std::list<uint64_t>::iterator>
+        pageMap;  ///< Hash map containing pages and their addresses
+
+    uint64_t hits =
+        0;  ///< Total number of hits, or total number of times a page
+    ///< was found in cache.
+    uint64_t pageFault = 0;  ///< Total number of miss/page fault, or total
+                             ///< number of times a page was not found in cache
+
+ public:
+    /**
+     * @brief Constructor, Initialize thee LRU class with page frame.
+     * @param pf Page frame or total size of cache.
+     * */
+    explicit LRUCache(uint64_t pf) { pageFrame = pf; }
+
+    /**
+     * @brief Refer to a page, or request a page from memory.
+     * @param page The page that you are referring to.
+     * @returns void
+     * */
+    void refer(uint64_t page) {
+        // If the page requested not in cache.
+        if (pageMap.find(page) == pageMap.end()) {
+            pageFault++;  ///< Increase the page fault by one.
+
+            // Check if the cache is full
+            if (cache.size() == pageFrame) {
+                // delete the last page from cache
+                uint64_t lastPage = cache.back();
+                cache.pop_back();
+                pageMap.erase(lastPage);
+            }
+        }
+        // The requested page is in the cache
+        else {
+            hits++;
+            // present in cache, erase from current position to bring in front
+            cache.erase(pageMap[page]);
+        }
+        // Push it in the front of the cache and update the page reference in
+        // page map.
+        cache.push_front(page);
+        pageMap[page] = cache.begin();
+    }
+
+    /**
+     * @brief A function to display the current cache
+     * @returns Void
+     * */
+    void display() {
+        for (uint64_t &it : cache) {
+            std::cout << it << " ";
+        }
+        std::cout << std::endl;
+    }
+    /**
+     * @brief A function to get page hits
+     * @returns int
+     * */
+    uint64_t getHits() const { return hits; }
+    /**
+     * @brief A function to get page fault
+     * @returns int
+     * */
+    uint64_t getPageFault() const { return pageFault; }
+};
+
+}  // namespace lru_cache
+}  // namespace others
+
+namespace lru_tests {
+/**
+ * @brief A function to print given message on console.
+ * @tparam T Type of the given message.
+ * @returns void
+ * */
+template <typename T>
+void log(T msg) {
+    // It's just to avoid writing cout and endl
+    std::cout << "[TESTS] : ---> " << msg << std::endl;
+}
+
+/**
+ * @brief A simple test case
+ * The assert statement will check expected hist and miss to resultant hits and
+ * miss
+ * @returns void
+ * */
+static void test_1() {
+    uint64_t expected_hits = 2;
+    uint64_t expected_pageFault = 4;
+
+    log("Running Test-1...");
+
+    others::lru_cache::LRUCache cache(4);
+    cache.refer(1);
+    cache.refer(2);
+    cache.refer(5);
+    cache.refer(1);
+    cache.refer(4);
+    cache.refer(5);
+
+    log("Checking assert statement...");
+    assert(cache.getHits() == expected_hits &&
+           cache.getPageFault() == expected_pageFault);
+    log("Assert successful!");
+    log("Test-1 complete!");
+}
+
+/**
+ * @brief A test case contains hits more than cache size
+ * The assert statement will check expected hist and miss to resultant hits and
+ * miss
+ * @returns void
+ * */
+static void test_2() {
+    uint64_t expected_hits = 4;
+    uint64_t expected_pageFault = 2;
+
+    log("Running Test-2...");
+
+    others::lru_cache::LRUCache cache(4);
+    cache.refer(1);
+    cache.refer(1);
+    cache.refer(1);
+    cache.refer(1);
+    cache.refer(1);
+    cache.refer(5);
+
+    log("Checking assert statement...");
+    assert(cache.getHits() == expected_hits &&
+           cache.getPageFault() == expected_pageFault);
+    log("Assert successful!");
+    log("Test-2 complete!");
+}
+
+/**
+ * @brief A simple test case
+ * The assert statement will check expected hist and miss to resultant hits and
+ * miss
+ * @returns void
+ * */
+static void test_3() {
+    uint64_t expected_hits = 1;
+    uint64_t expected_pageFault = 5;
+
+    log("Running Test-3...");
+
+    others::lru_cache::LRUCache cache(4);
+    cache.refer(1);
+    cache.refer(2);
+    cache.refer(3);
+    cache.refer(4);
+    cache.refer(5);
+    cache.refer(5);
+
+    log("Checking assert statement...");
+    assert(cache.getHits() == expected_hits &&
+           cache.getPageFault() == expected_pageFault);
+    log("Assert successful!");
+    log("Test-3 complete!");
+}
+
+/**
+ * @brief A function to invoke all test cases
+ * @returns void
+ * */
+static void run_tests() {
+    test_1();
+    test_2();
+    test_3();
+    log("");
+    log("TESTS COMPLETED!");
+}
+}  // namespace lru_tests
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    lru_tests::run_tests();
+
+    // Usage
+    others::lru_cache::LRUCache cache(4);
+    cache.refer(1);
+    cache.refer(2);
+    cache.refer(3);
+    cache.refer(4);
+    cache.refer(5);
+    cache.refer(5);
+
+    cache.display();
+
+    std::cout << "Hits: " << cache.getHits()
+              << " Miss: " << cache.getPageFault() << std::endl;
+    return 0;
+}