README.md

Test the Smart Contract

The smart contract can be tested using Remix IDE. After compiling & deploying, an example interaction with the deployed contract looks like this:

• Make sure account is the deployer's account
• addBook(1, 3, "KingRat") adds 3 copies of a Book to the libraryBooks mapping with id = 3 and title = "KingRat"
• addBook(1, 1, "KingRat") adds 1 more copy of the same book
• addBook(2, 1, "The Bitcoin Standard") adds a new book with id=2 and title "The Bitcoin Standard"
• libraryBooks(1) and libraryBooks(2) to see their count and title
• since the mapping is public, everyone can easy see which books are available
• Try switching to other account and see that only owner is allowed to add new books
• BorrowBook(1) borrows the book with id=1, check libraryBooks(1) to see that count of "KingRat" is decreased by 1
• BorrowBook(2) borrows book with id=2, -/-

Now, to see who borrowed which books:

• borrowedBooks(0xCA35b7d915458EF540aDe6068dFe2F44E8fa733c, 1) pass an address (this one is an example, you can copy a real one directly in Remix) and array index as argument and the mapping returns the book id that is borrowed
• Since the mapping is public anyone can easily see it
• returnBook(1) returns the book with id=1, see libraryBook(1)'s count is increased by 1
• returnBook(2) -/-
• returnBook(1) should revert now because the user has already returned it
• BorrowBook(3) should revert because book with id=3 doesn't exist

Library.sol

pragma solidity ^0.8.0;

import "./2_Owner.sol";

contract Library is Owner { // Owner contract provided by Remix IDE by default

    event bookAdded(uint id, uint count);
   
    event borrowedBook(uint id);
   
    event returnedBook(uint id);
   
    struct Book {
        string title;
        uint count;
    }

    mapping(address => uint[]) public borrowedBooks;
   
    mapping(uint => Book) public libraryBooks;
   
    function addBook(uint _id, uint _count, string memory title) public isOwner {
        require(bytes(title).length != 0, "Book's title cannot be empty");
        require(_id > 0 && _count > 0, "IDs and count of books should be greater than 0 "); // we cannot have id = 0, also can't add 0 books
        if(bytes(libraryBooks[_id].title).length == 0) { // only possible if the book is added for the first time
            libraryBooks[_id].title = title;
        }
        libraryBooks[_id].count += _count;
        emit bookAdded(_id, _count);
    }

    function borrowBook(uint id) public {
        require(id > 0, "ID cannot be 0");
        require(libraryBooks[id].count >= 1, "The book is not available"); // require there's at least 1 copy of the book available
        uint[] memory userBooks = borrowedBooks[msg.sender];
        for(uint i = 0;i<userBooks.length;i++) {
            uint bookId = userBooks[i];
            if(id == bookId) { // already borrowed
                revert("Book is already borrowed by the user");
            }
        }
        borrowedBooks[msg.sender].push(id); // if we are here, the book hasn't yet been borrowed
        libraryBooks[id].count--;
        emit borrowedBook(id);
    }
   
    function returnBook(uint id) public {
        bool hasBook = false;
        uint[] memory userBooks = borrowedBooks[msg.sender];
        for(uint i = 0;i < userBooks.length;i++){
            if(id == userBooks[i]) {
                userBooks = remove(i, userBooks);
                borrowedBooks[msg.sender] = userBooks;
                libraryBooks[id].count++;
                emit returnedBook(id);
                hasBook = true;
                break;
            }
        }
        if(!hasBook) {
            revert("The user doesn't have the book in order to return it!");
        }
    }
    
    function remove(uint index, uint[] memory array) private pure returns(uint[] memory) {
        if (index >= array.length) revert();

        for (uint i = index; i<array.length-1; i++){
            array[i] = array[i+1];
        }
        delete array[array.length-1];
        //array.length--;
        return array;
    }
   
}