diff --git a/data_structures/skip_list.cpp b/data_structures/skip_list.cpp new file mode 100644 index 000000000..cc679a31c --- /dev/null +++ b/data_structures/skip_list.cpp @@ -0,0 +1,221 @@ +/** + * @file skip_list.cpp + * @brief Data structure for fast searching and insertion in \f$O(\log n)\f$ + * time + * @details + * A skip list is a data structure that is used for storing a sorted list of + * items with a help of hierarchy of linked lists that connect increasingly + * sparse subsequences of the items + * + * References used: [GeeksForGeek](https://www.geeksforgeeks.org/skip-list/), + * [OpenGenus](https://iq.opengenus.org/skip-list) for PseudoCode and Code + * @author [enqidu](https://github.com/enqidu) + * @author [Krishna Vedala](https://github.com/kvedala) + */ + +#include +#include +#include +#include +#include +#include + +/** \namespace data_structure + * \brief Data-structure algorithms + */ +namespace data_structure { +constexpr int MAX_LEVEL = 2; ///< Maximum level of skip list +constexpr float PROBABILITY = 0.5; ///< Current probability for "coin toss" + +/** + * Node structure [Key][Node*, Node*...] + */ +struct Node { + int key; ///< key integer + void* value; ///< pointer of value + std::vector> + forward; ///< nodes of the given one in all levels + + /** + * Creates node with provided key, level and value + * @param key is number that is used for comparision + * @param level is the maximum level node's going to added + */ + Node(int key, int level, void* value = nullptr) : key(key), value(value) { + // Initialization of forward vector + for (int i = 0; i < (level + 1); i++) { + forward.push_back(nullptr); + } + } +}; + +/** + * SkipList class implementation with basic methods + */ +class SkipList { + int level; ///< Maximum level of the skiplist + std::shared_ptr header; ///< Pointer to the header node + + public: + /** + * Skip List constructor. Initializes header, start + * Node for searching in the list + */ + SkipList() { + level = 0; + // Header initialization + header = std::shared_ptr(new Node(-1, MAX_LEVEL)); + } + + /** + * Returns random level of the skip list. + * Every higher level is 2 times less likely. + * @return random level for skip list + */ + int randomLevel() { + int lvl = 0; + while (static_cast(std::rand()) / RAND_MAX < PROBABILITY && + lvl < MAX_LEVEL) + lvl++; + return lvl; + } + + /** + * Inserts elements with given key and value; + * It's level is computed by randomLevel() function. + * @param key is number that is used for comparision + * @param value pointer to a value, that can be any type + */ + void insertElement(int key, void* value) { + std::cout << "Inserting" << key << "..."; + std::shared_ptr x = header; + std::array, MAX_LEVEL + 1> update; + update.fill(nullptr); + + for (int i = level; i >= 0; i--) { + while (x->forward[i] != nullptr && x->forward[i]->key < key) + x = x->forward[i]; + update[i] = x; + } + + x = x->forward[0]; + + bool doesnt_exist = (x == nullptr || x->key != key); + if (doesnt_exist) { + int rlevel = randomLevel(); + + if (rlevel > level) { + for (int i = level + 1; i < rlevel + 1; i++) update[i] = header; + + // Update current level + level = rlevel; + } + + std::shared_ptr n = + std::shared_ptr(new Node(key, rlevel, value)); + for (int i = 0; i <= rlevel; i++) { + n->forward[i] = update[i]->forward[i]; + update[i]->forward[i] = n; + } + std::cout << "Inserted" << std::endl; + + } else { + std::cout << "Exists" << std::endl; + } + } + + /** + * Deletes an element by key and prints if has been removed successfully + * @param key is number that is used for comparision. + */ + void deleteElement(int key) { + std::shared_ptr x = header; + + std::array, MAX_LEVEL + 1> update; + update.fill(nullptr); + + for (int i = level; i >= 0; i--) { + while (x->forward[i] != nullptr && x->forward[i]->key < key) + x = x->forward[i]; + update[i] = x; + } + + x = x->forward[0]; + + bool doesnt_exist = (x == nullptr || x->key != key); + + if (!doesnt_exist) { + for (int i = 0; i <= level; i++) { + if (update[i]->forward[i] != x) + break; + update[i]->forward[i] = x->forward[i]; + } + /* Remove empty levels*/ + while (level > 0 && header->forward[level] == nullptr) level--; + std::cout << "Deleted" << std::endl; + } else { + std::cout << "Doesn't exist" << std::endl; + } + } + + /** + * Searching element in skip list structure + * @param key is number that is used for comparision + * @return pointer to the value of the node + */ + void* searchElement(int key) { + std::shared_ptr x = header; + std::cout << "Searching for " << key << std::endl; + + for (int i = level; i >= 0; i--) { + while (x->forward[i] && x->forward[i]->key < key) x = x->forward[i]; + } + + x = x->forward[0]; + if (x && x->key == key) { + std::cout << "Found" << std::endl; + return x->value; + } else { + std::cout << "Not Found" << std::endl; + return nullptr; + } + } + + /** + * Display skip list level + */ + void displayList() { + std::cout << "Displaying list:\n"; + for (int i = 0; i <= level; i++) { + std::shared_ptr node = header->forward[i]; + std::cout << "Level " << (i) << ": "; + while (node != nullptr) { + std::cout << node->key << " "; + node = node->forward[i]; + } + std::cout << std::endl; + } + } +}; + +} // namespace data_structure + +/** + * Main function: + * Creates and inserts random 2^[number of levels] + * elements into the skip lists and than displays it + */ +int main() { + std::srand(std::time(nullptr)); + + data_structure::SkipList lst; + + for (int j = 0; j < (1 << (data_structure::MAX_LEVEL + 1)); j++) { + int k = (std::rand() % (1 << (data_structure::MAX_LEVEL + 2)) + 1); + lst.insertElement(k, &j); + } + + lst.displayList(); + + return 0; +}