class LibraryCollection{
    constructor(capacity, books) {
        this.capacity = capacity ? capacity : 0;
        this.books = books ? books : [];
      }

      addBook(bookName, bookAuthor) {
          if (this.capacity <= this.books.length) {
              throw 'Not enough space in the collection.';
          }

          this.books.push({
              bookName,
              bookAuthor,
              payed: false
          });
          return `The ${bookName}, with an author ${bookAuthor}, collect.`;
      }

    payBook( bookName ) {
        if(!this.books.some(x => x.bookName == bookName)){
            throw new Error(`${bookName} is not in the collection.`);
        }
        else if(this.books.some(x => x.payed == true)) {
            throw new Error(`${bookName} has already been paid.`);
        }
        else{
            let book = this.books.some(x => x.bookName == bookName);
            book.payed = true;
            return `${bookName} has been successfully paid.`;
        }
    }
    removeBook(bookName) {
        if(!this.books.some(x => x.bookName == bookName)){
            throw new Error(`The book, you're looking for, is not found.`);
        }
        if (this.books.some(x => x.payed == false)) {
            throw new Error(`${bookName} need to be paid before removing from the collection.`);
        }else{
            this.books=this.books.filter(x => x.bookName != bookName);
            return `${bookName} remove from the collection.`;
        }
    }
    getStatistics(bookAuthor) {
        let result=[];

        if (!bookAuthor || bookAuthor === '') {
            result.push(`The book collection has ${this.capacity - this.books.length} empty slots left.`);

            this.books.sort(function(a, b) {
                var bookA = a.bookName.toUpperCase();
                var bookB = b.bookName.toUpperCase();
                return (bookA < bookB) ? -1 : (bookA > bookB) ? 1 : 0;
            });

            this.books.forEach(book => {
                result.push(`${book.bookName} == ${book.bookAuthor} - ${book.payed ? 'Has Paid' : 'Not Paid'}`);
            });
        } else {
            let bookFound = false;
            this.books.forEach(book => {
                if (book.bookAuthor === bookAuthor) {
                    bookFound = true;
                    result.push(`${book.bookName} == ${book.bookAuthor} - ${book.payed ? 'Has Paid' : 'Not Paid'}`);
                }
            });

            if (!bookFound) {
                throw `${bookAuthor} is not in the collection.`;
            }

            return result.join('\n');
        }
    }
}