Online Book Reader

Problem

Design OnlineBookReader to manage a copy-counted library and registered users. One logged-in user at a time can borrow books, read one unfinished book, and rate completed books.

Requirements

Library setup. The constructor creates each distinct book with its supplied page count and copy count. Every copy is initially available.
Registration. registerUser creates a new user and returns true. A duplicate user ID returns false without changing state.
Single login. login makes a registered user the sole logged-in user and returns true, replacing any previous login. An unknown user returns false and keeps the current login.
Borrowing. borrowBook returns true, adds the book to the logged-in user's holdings and borrowing history, and consumes one available copy when every borrowing rule is satisfied. Otherwise it returns false without changing state.
Holding limit. A user may hold at most two books at once.
Distinct holdings. A user cannot hold two copies of the same book.
One current book. The first readPage call for a held book starts it when no book is current. A call for a different held book returns 0 without changing either book while another book is unfinished.
Page navigation. The movement string is "current", "next", or "previous". "current" reports the page, "next" advances one page, and "previous" moves back one page without going below page 1. The method returns the resulting page.
Completion. When "next" reaches the last page, the book is completed for that user, removed from holdings and the current slot, and its copy becomes available again.
Rating. rateBook stores one integer rating from 1 through 5 only after the logged-in user completes that book. It returns true when the rating is stored and false otherwise.
User details. getCurrentUserDetails returns the logged-in user ID, held book IDs in borrow order, current book ID and page, and all successfully borrowed book IDs in chronological order. With no current book, its ID is "" and its page is 0.
Available books. listAvailableBooks returns an object keyed by each book ID with at least one available copy. Each value contains availableCopies and averageRating.
Average rating. averageRating is the arithmetic mean of all accepted ratings for that book, or 0.0 when the book has no ratings.
No rereading. borrowBook returns false without changing state when the logged-in user has already completed that book.

API

Signature	Returns	Behavior
`OnlineBookReader(books: object[])`	Not applicable	Creates the library. Each book object has `bookId`, `pageCount`, and `copyCount`.
`registerUser(userId: string)`	`boolean`	Registers a distinct user.
`login(userId: string)`	`boolean`	Selects the sole logged-in user.
`borrowBook(bookId: string)`	`boolean`	Borrows one copy when inventory and the user's holding rules permit it.
`readPage(bookId: string, movement: string)`	`integer`	Starts or navigates a held book and returns its resulting page, or `0` when another book is current.
`rateBook(bookId: string, rating: integer)`	`boolean`	Stores the user's one valid rating for a completed book.
`getCurrentUserDetails()`	`object`	Returns `{userId, heldBookIds, currentBookId, currentPage, borrowingHistory}`.
`listAvailableBooks()`	`object`	Returns available books as `{bookId: {availableCopies, averageRating}}`.

Examples

Given books [{bookId: "book-a", pageCount: 3, copyCount: 1}, {bookId: "book-b", pageCount: 2, copyCount: 1}]:

Step	Operation	Result
1	`registerUser("u1")`	`true`
2	`login("u1")`	`true`
3	`borrowBook("book-a")`	`true`
4	`borrowBook("book-b")`	`true`
5	`readPage("book-a", "current")`	`1`
6	`readPage("book-b", "current")`	`0`
7	`rateBook("book-a", 5)`	`false`
8	`readPage("book-a", "next")`	`2`
9	`readPage("book-a", "next")`	`3`
10	`rateBook("book-a", 5)`	`true`
11	`readPage("book-b", "current")`	`1`

Constraints

The constructor receives 1 to 100 distinct books. Each bookId is non-empty, each pageCount is from 2 to 10,000, and each copyCount is from 1 to 1,000.
User IDs are non-empty strings, and at most 10,000 users register.
At most 100,000 public method calls occur in one testcase.
Calls other than the documented boolean rejections use existing book IDs, a logged-in user where required, a held unfinished book for readPage, and at most one rating per user and completed book.

Notes

Calls are sequential and take effect in invocation order. Counts and pages never become negative. Floating-point results are compared with an absolute tolerance of $10^{-9}$ ; object key order is not significant.