data class Author(val id: Int,
                  val books: List<Int>)

data class Book(val id: Int,
                val author: Int,
                val title: String)

interface HttpClient {
    // All methods here can throw an IOException.

    suspend fun getAuthor(id: Int): Author

    suspend fun getBook(id: Int): Book
}

data class AuthorBooks(val author: Int,
                       val books: List<Book>)

// Given a list of author ids, return a list of pairs (Author id, List<Book>). The list of authors should be processed
// sequentially (i.e. one by one), but fetching all books from a given author should be done in parallel. In case of
// error, return the sub list of (Author ID, List<Book>) that was computed successfully before the error occurred.
suspend fun getAuthorBooks(authors: List<Int>): List<AuthorBooks> {
    val httpClient: HttpClient = TODO() // get that from somewhere.

    val result = arrayListOf<AuthorBooks>()
    try {
        for (authorId in authors) {
            val author = httpClient.getAuthor(authorId)
            coroutineScope {
                val booksAsyncJobs: List<Deferred<Book>> = author.books.map { bookId ->
                    async { httpClient.getBook(bookId) }
                }
                val books = booksAsyncJobs.map { it.await() }
                result.add(AuthorBooks(authorId, books))
            }
        }
    } catch (e: Exception) {
        // log, handle better.
    } finally {
        return result
    }
}