Library.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;
import "./Ownable.sol";


contract Library is Ownable{

    event NewBook(uint id, string name, uint copies);
    event CopiesAdded(uint id, string name, uint copies);
    event BookBorrowed(uint id, uint remainingCopies);
    event BookReturned(uint id, uint remainingCopies);

    struct Book{
        string name;
        uint256 copies;
    }

    //Mapping that helps to manage how many copies of each book are out.
    mapping(uint => uint) public bookIdToCopiesOut;
    
    // Mapping that saves a boolean if a book id is now borrowed by an address
    mapping(address => mapping(uint => bool)) public addressBookIdCopiesBorrowed; 
    
    // This keeps the order in which the books have been borrowed
    mapping(uint256 => address[]) public borrowers;
    
    // This is where the book info is stored. THE INDEX OF THE BOOK IN THE ARRAY IS TAKEN AS THE BOOK ID.
    Book[] public bookArray;

    // Owner Functions

    // Adds a new book to the library
    function addNewBook(string memory _name, uint256 _copies) public onlyOwner{
        Book memory _book = Book(_name, _copies);
        bookArray.push(_book);
        //bookIdToCopiesOut[bookArray.length-1]=0;
        emit NewBook(bookArray.length-1, _name, _copies);
    }

    // Adds more copies to a book in library by id
    function addCopies(uint256 _id, uint256 _copies) public onlyOwner{
        bookArray[_id].copies = bookArray[_id].copies + _copies;
        emit CopiesAdded(_id, bookArray[_id].name, bookArray[_id].copies);
    }

    // View functions

    //Returns the name of a book by id
    function getName(uint256 _id) public view returns(string memory){
        require(bookArray.length-1 >= _id, "No book with this index");
        return bookArray[_id].name;
    }

    // Returns boolean if book is available for borrowing
    function isAvailable(uint256 _id) public view returns(bool){
        require(bookArray.length-1 >= _id, "No book with this index");
        return bookArray[_id].copies - bookIdToCopiesOut[_id] > 0;
    }

    // returns the amount of available copies to borrow
    function availableUnits(uint256 _id) public view returns(uint256){
        require(bookArray.length-1 >= _id, "No book with this index");
        return bookArray[_id].copies - bookIdToCopiesOut[_id];
    }

    // returns array of book borrowers in chronological order
    function viewBookBorrowers(uint256 _id) public view returns(address [] memory){
        require(bookArray.length-1 >= _id, "No book with this index");
        return borrowers[_id];
    }

    // Book handleing functions

    // Lets user borrow a book
    function borrowBook(uint256 _id) public{
        require(bookArray.length-1 >= _id, "No book with this index");
        require(isAvailable(_id), "All copies are out");
        require(!addressBookIdCopiesBorrowed[msg.sender][_id], "You already have one of those");
        //borrow book
        borrowers[_id].push(msg.sender); 
        bookIdToCopiesOut[_id] = bookIdToCopiesOut[_id] + 1;
        addressBookIdCopiesBorrowed[msg.sender][_id] = true;
        emit BookBorrowed(_id, bookArray[_id].copies - bookIdToCopiesOut[_id]);
    }

    //Lets user return a book
    function returnBook(uint256 _id) public{
        require(bookArray.length-1 >= _id, "No book with this index");
        require(addressBookIdCopiesBorrowed[msg.sender][_id], "You dont have this book");
        //return book
        bookIdToCopiesOut[_id] = bookIdToCopiesOut[_id] - 1;
        addressBookIdCopiesBorrowed[msg.sender][_id] = false;
        emit BookBorrowed(_id, bookArray[_id].copies - bookIdToCopiesOut[_id]);
    }



}