Library.java

package LibraryManagementSystem;
import java.io.*;

public class Library {
    // Data structures for different management features [cite: 3]
    private DynamicArray catalog = new DynamicArray(); 
    private BST titleIndex = new BST();               
    private Queue waitList = new Queue();               
    private Stack history = new Stack();                

   
    public void addBook(Book book) {
        catalog.add(book);
        titleIndex.insert(book);
    }

    // Worst Case Time Complexity is O(n)
  
    public void borrowBook(int bookId) {
        for (int i = 0; i < catalog.size(); i++) {
            Book b = catalog.get(i);
            if (b.getId() == bookId && b.isAvailable()) {
                b.setAvailable(false);
                history.push("BORROW:" + bookId); 
                System.out.println("The book was lent out.");
                return;
            }
        }
        System.out.println("The book was either unavailable or already on loan.");
    }

    //Time complexity is O(n) to write n amount book in alphabetical order
    public void listAlphabetically() {
        System.out.println("\n--- Alphabetical Book List (BST) ---");
        titleIndex.listAlphabetically();
    }

    //Time complexity is O(n) because of for loop
    public void removeBookById(int id) {
        for (int i = 0; i < catalog.size(); i++) {
            if (catalog.get(i).getId() == id) {
                catalog.remove(i); 
                System.out.println("Book has been removed.");
                return;
            }
        }
        System.out.println("Book couldn't be found.");
    }
    
   
    //Worst Case Time Complexity: O(n) check in for loop 

    public Book searchById(int id) {
        for (int i = 0; i < catalog.size(); i++) {
            if (catalog.get(i).getId() == id) {
                return catalog.get(i);
            }
        }
        return null;
    }

    //O(n) is O(n) time complexity
    public Book searchByTitle(String title) {
        return titleIndex.search(title);
    }
    
    
   
    public void requestBorrow(String userName, int bookId) {
        waitList.enqueue(userName, bookId);
        System.out.println(userName + " added to the waiting list.");
    }

    //O(n) is O(1)
    public void processNextRequest() {
        if (waitList.isEmpty()) {
            System.out.println("There's no one on the waiting list.");
            return;
        }
        String request = waitList.dequeue();
        System.out.println("Processing: " + request);
    }
    
    
      //Worst Case Time Complexity: O(n)
 
    public void saveBooksToFile(String fileName) {
        try (BufferedWriter bw = new BufferedWriter(new FileWriter(fileName))) {
            for (int i = 0; i < catalog.size(); i++) {
                Book b = catalog.get(i);
                bw.write(b.getId() + "," + b.getTitle() + "," + b.getAuthor() + "," + b.isAvailable());
                bw.newLine();
            }
            System.out.println("Data saved successfully.");
        } catch (IOException e) {
            System.out.println("File writing error: " + e.getMessage());
        }
    }
    
    /**
     * Worst Case Time Complexity: O(n)
     * Searches for the book by ID and updates its availability status.
     */
    public void returnBook(int id) {
        Book b = searchById(id);
        if (b != null && !b.isAvailable()) {
            b.setAvailable(true);
            history.push("RETURN:" + id); 
            System.out.println(b.getTitle() + " has been returned.");
        } else {
            System.out.println("Error: The book is either already available or not found.");
        }
    }

  
    // Worst Case Time Complexity: O(n)
    // Pops from Stack (O(1)) and searches the catalog (O(n)) to revert the status
   
    public void undoLastAction() {
        String lastAction = history.pop();
        if (lastAction == null) {
            System.out.println("There is no action to undo.");
            return;
        }
        
        String[] parts = lastAction.split(":");
        String type = parts[0];
        int id = Integer.parseInt(parts[1]);

        for (int i = 0; i < catalog.size(); i++) {
            Book b = catalog.get(i);
            if (b.getId() == id) {
                if (type.equals("BORROW")) b.setAvailable(true);
                else if (type.equals("RETURN")) b.setAvailable(false);
                System.out.println("Action reversed: " + type + " for ID:" + id);
                break;
            }
        }
    }


     //Worst Case Time Complexity: O(n) print to all books

    public void listAllBooks() {
        for (int i = 0; i < catalog.size(); i++) {
            System.out.println(catalog.get(i));
        }
    }
}