#ifndef LINKEDLIST_H
#define LINKEDLIST_H

#include <concepts>
#include <cstdlib>
#include <expected>
#include <format>
#include <new>
#include <print>

/**
 * @brief Concept that ensures a type supports equality comparison via operator==.
 *
 * @tparam T Type to be checked.
 */
template <typename T>
concept HasEqual = requires (T a, T b) {
    { a == b } -> std::convertible_to<bool>;
};

/**
 * @brief Concept defining the requirements for types supported by LinkedList.
 *
 * A valid type must:
 * - Be formattable (usable with std::print / std::format)
 * - Support equality comparison
 *
 * @tparam T Type to be checked.
 */
template <typename T>
concept LinkedListSupported =
    std::formattable<T, char> &&
    std::equality_comparable<T> &&
    HasEqual<T>;


/**
 * @brief Error codes for LinkedList operations.
 */
enum class LinkedListErr {
    LINKEDLIST_OK,             /**< Operation completed successfully */
    LINKEDLIST_BAD_ALLOC,      /**< Memory allocation failed */
    LINKEDLIST_OUT_OF_BOUNDS,  /**< Index is out of valid range */
    LINKEDLIST_INVALID_SIZE,   /**< Invalid size provided */
    LINKEDLIST_NOT_FOUND,      /**< Element not found */
    LINKEDLIST_EMPTY,          /**< The list is empty */
};

/**
 * @brief Node of a singly linked list.
 *
 * @tparam T Type of data stored in the node.
 */
template<LinkedListSupported T>
class Node {
public:
    T data;          /**< Data stored in the node */
    Node<T>* next;   /**< Pointer to the next node */
};

/**
 * @brief Implementation of a singly linked list.
 *
 * A dynamic data structure that stores elements of type T
 * using nodes connected via pointers.
 *
 * @tparam T Type of elements stored in the list.
 */
template<LinkedListSupported T>
class LinkedList {
private:
    size_t size;     /**< Number of elements in the list */
    Node<T>* head;   /**< Pointer to the first node */
    Node<T>* tail;   /**< Pointer to the last node */

public: 

    /**
     * @brief Constructs an empty linked list.
     */
    LinkedList();

    /**
     * @brief Destroys the linked list.
     *
     * Releases all dynamically allocated nodes.
     */
    ~LinkedList();

    /**
     * @brief Retrieves the element at a given index.
     *
     * @param index Position of the element.
     * @return std::expected<T, LinkedListErr> The element or an error code.
     */
    std::expected<T, LinkedListErr> get(size_t index);  

    /**
     * @brief Appends an element to the end of the list.
     *
     * @param value Value to insert.
     * @return LinkedListErr Status code of the operation.
     */
    LinkedListErr append(const T& value);

    /**
     * @brief Inserts an element at the beginning of the list.
     *
     * @param value Value to insert.
     * @return LinkedListErr Status code of the operation.
     */
    LinkedListErr prepend(const T& value);

    /**
     * @brief Removes and returns the last element of the list.
     *
     * @return std::expected<T, LinkedListErr> Removed element or an error.
     */
    std::expected<T, LinkedListErr> pop();

    /**
     * @brief Removes the element at a specific index.
     *
     * @param index Index of the element to remove.
     * @return LinkedListErr Status code.
     */
    LinkedListErr remove(size_t index);

    /**
     * @brief Updates the value of an element at a given index.
     *
     * @param index Index of the element.
     * @param value New value.
     * @return LinkedListErr Status code.
     */
    LinkedListErr set(size_t index, const T& value);

    /**
     * @brief Inserts a value at a specified index.
     *
     * @param index Target position.
     * @param value Value to insert.
     * @return LinkedListErr Status code.
     */
    LinkedListErr insert(size_t index, const T& value);

    /**
     * @brief Searches for a value in the list.
     *
     * @param value Value to search for.
     * @return std::expected<size_t, LinkedListErr> Index of the element or error.
     */
    std::expected<size_t, LinkedListErr> find(const T& value);

    /**
     * @brief Prints all elements in the list.
     *
     * @return LinkedListErr Status code.
     */
    LinkedListErr print();

    /**
     * @brief Checks whether the list is empty.
     *
     * @return true if the list contains no elements, false otherwise.
     */
    bool is_empty();

    /**
     * @brief Returns the number of elements in the list.
     *
     * @return size_t Number of stored elements.
     */
    size_t len();
};

template <LinkedListSupported T>
LinkedList<T>::LinkedList() {
    size = 0;
    head = nullptr;
    tail = nullptr;
}

template <LinkedListSupported T>
LinkedList<T>::~LinkedList() {
    Node<T> *current = this->head;

    while (current != nullptr) {
        Node<T> *temp = current;
        current = current->next;
        delete temp;
    }
}

template <LinkedListSupported T>
std::expected<T, LinkedListErr> LinkedList<T>::get(size_t index) {
    if (index >= this->size) {
        return std::unexpected(LinkedListErr::LINKEDLIST_OUT_OF_BOUNDS);
    }

    if (index == this->size - 1) {
        return tail->data;
    }

    Node<T> *current = this->head;
    for (size_t i = 0; i < index ; i++) {
        current = current->next;
    }

    return current->data;
}

template <LinkedListSupported T>
LinkedListErr LinkedList<T>::set(size_t index, const T& val) {
    if (index >= this->size) {
        return LinkedListErr::LINKEDLIST_OUT_OF_BOUNDS;
    }

    if (index == this->size - 1) {
        this->tail->data = val;
    }

    Node<T> *current = this->head;
    for (size_t i = 0; i < index; i++) {
        current = current->next;
    }

    current->data = val;
    return LinkedListErr::LINKEDLIST_OK;
}

template <LinkedListSupported T>
LinkedListErr LinkedList<T>::insert(size_t index, const T& value) {
    if (this->is_empty()) {
        return this->append(value);
    }

    if (index >= this->size) {
        return LinkedListErr::LINKEDLIST_OUT_OF_BOUNDS;
    }

    // If index is head
    if (index == 0) {
        return this->prepend(value);
    }

    // If index is tail
    if (index == this->size - 1) {
        return this->append(value);
    }

    Node<T> *current = this->head;
    for (size_t i = 1; i < index; i++) {
        current = current->next;
    }

    Node<T> *new_node = nullptr;

    try {
        new_node = new Node<T>;
    } catch(const std::bad_alloc&) {
        return LinkedListErr::LINKEDLIST_BAD_ALLOC;
    }

    new_node->data = value;
    new_node->next = current->next->next;
    current->next = new_node;

    return LinkedListErr::LINKEDLIST_OK;
}

template <LinkedListSupported T>
LinkedListErr LinkedList<T>::append(const T& value) {
    Node<T> *new_node = nullptr;

    try {
        new_node = new Node<T>;
    } catch(std::bad_alloc&) {
        return LinkedListErr::LINKEDLIST_BAD_ALLOC;
    }

    new_node->data = value;
    new_node->next = nullptr;

    if (this->head == nullptr && this->tail == nullptr) {
        this->head = new_node;
        this->tail = this->head;
    } else {
        this->tail->next = new_node;
        this->tail = this->tail->next;
    }

    this->size += 1;

    return LinkedListErr::LINKEDLIST_OK;
}

template <LinkedListSupported T>
LinkedListErr LinkedList<T>::prepend(const T& value) {
    Node<T> *new_node = nullptr;

    try {
        new_node = new Node<T>;
    } catch(std::bad_alloc&) {
        return LinkedListErr::LINKEDLIST_BAD_ALLOC;
    }

    new_node->data = value;
    new_node->next = nullptr;

    if (this->head == nullptr && this->tail == nullptr) {
        this->head = new_node;
        this->tail = this->head;
    } else {
        new_node->next = this->head;
        this->head = new_node;
    }

    this->size += 1;

    return LinkedListErr::LINKEDLIST_OK;
}

template <LinkedListSupported T>
std::expected<T, LinkedListErr> LinkedList<T>::pop() {
    if (this->size == 0) {
        return std::unexpected(LinkedListErr::LINKEDLIST_EMPTY);
    }

    if (this->head->next == nullptr) {
        T return_val = this->head->data;
        delete this->head;
        this->head = nullptr;
        this->size--;
        return return_val;
    }

    Node<T>* current = this->head;

    while (current->next->next != nullptr) {
        current = current->next;
    }

    T return_val = current->next->data;
    delete current->next;
    current->next = nullptr;

    this->size--;

    return return_val;
}

template <LinkedListSupported T>
LinkedListErr LinkedList<T>::remove(size_t index) {
    if (index >= this->size) {
        return LinkedListErr::LINKEDLIST_OUT_OF_BOUNDS;
    }

    if (index == 0) {
        Node<T>* tmp = this->head;
        this->head = this->head->next;
        delete tmp;
        this->size--;
        return LinkedListErr::LINKEDLIST_OK;
    }

    Node<T>* current = this->head;

    for (size_t i = 1; i < index; ++i) {
        current = current->next;
    }

    Node<T>* tmp = current->next;
    current->next = tmp->next;

    if(tmp == this->tail) {
        this->tail = current;
    }

    delete tmp;
    this->size--;

    return LinkedListErr::LINKEDLIST_OK;
}


template <LinkedListSupported T>
std::expected<size_t, LinkedListErr> LinkedList<T>::find(const T& value) {
    Node<T> *current = this->head;
    size_t count = 0;
    while (current != nullptr) {
        if (current->data == value) {
            return count;
        }

        count++;
        current = current->next;
    }

    return std::unexpected(LinkedListErr::LINKEDLIST_NOT_FOUND);
}

template <LinkedListSupported T>
LinkedListErr LinkedList<T>::print() {
    if (this->size == 0) {
        return LinkedListErr::LINKEDLIST_OK;
    }

    std::println("[LL]");
    std::println(" |\n v");
    Node<T> *current = this->head;
    while (current != nullptr) {
        std::println("Node({})", current->data);
        std::println(" |");
        std::println(" v");
        current = current->next;
    }

    std::println("NULL");
    return LinkedListErr::LINKEDLIST_OK;
}

template <LinkedListSupported T>
size_t LinkedList<T>::len() {
    return this->size;
}


#endif // !