Library Checkout in Go

This is a design round. You write the classes, and the interviewer checks that they behave.

Here you build Library. It lends books to members and takes them back.

Each book has a number, its id. The library can own several copies of the same book.

Implement the class:

• NewLibrary() makes a library with no books.
• addBook(bookId, copies) puts that many more copies of the book on the shelf.
• checkout(member, bookId) lends the member one copy. It returns true if it worked.
• giveBack(member, bookId) takes the member's copy back. It returns true if it worked.
• available(bookId) returns how many copies are on the shelf.
• borrowed(member) returns the ids of the books the member holds, from smallest to largest.

Go has no classes. Library is a struct, made by NewLibrary. Its methods have a pointer receiver, *Library, because they change it.

A member holds at most 3 books at a time. A member never holds two copies of the same book.

checkout returns false when no copy is on the shelf. It also returns false when the loan would break a member rule.

giveBack returns false when the member doesn't hold that book.

A call that returns false changes nothing.

A book the library never got has 0 copies. A member who never borrowed holds no books.

The calls run in order on one new Library. Every call except addBook writes one line of output. Booleans are written as True and False. A list of ids is written with spaces between them. An empty list is an empty line.

Example 1:

Input:
NewLibrary()
1. addBook(7, 2)
2. checkout("Ana", 7)
3. checkout("Ana", 7)
4. checkout("Ben", 7)
5. available(7)
6. checkout("Cy", 7)
7. giveBack("Ana", 7)
8. checkout("Cy", 7)
9. borrowed("Cy")
Output:
True
False
True
0
False
True
True
7
Explanation:
Call 2 lends Ana one of the two copies.
Call 3 fails, because Ana already holds book 7.
Call 4 lends Ben the other copy. So call 5 returns 0.
Call 6 fails, because no copy is on the shelf.
Ana brings her copy back in call 7, and Cy gets it in call 8.

Example 2:

Input:
NewLibrary()
1. addBook(30, 1)
2. addBook(10, 1)
3. addBook(20, 1)
4. addBook(40, 1)
5. checkout("Dev", 30)
6. checkout("Dev", 10)
7. checkout("Dev", 20)
8. checkout("Dev", 40)
9. borrowed("Dev")
10. giveBack("Dev", 40)
11. giveBack("Dev", 10)
12. checkout("Dev", 40)
13. borrowed("Dev")
Output:
True
True
True
False
10 20 30
False
True
True
20 30 40
Explanation:
Dev borrows three books in calls 5, 6 and 7.
Call 8 fails, because Dev already holds 3 books.
Call 9 lists the ids from smallest to largest.
Call 10 fails, because Dev never got book 40.
Call 11 gives book 10 back. Now Dev can borrow book 40.

Example 3:

Input:
NewLibrary()
1. borrowed("Eve")
2. available(5)
3. checkout("Eve", 5)
4. addBook(5, 1)
5. addBook(5, 2)
6. available(5)
7. checkout("Eve", 5)
8. giveBack("Eve", 5)
9. available(5)
Output:

0
False
3
True
True
3
Explanation:
The first output line is empty, because Eve holds no books yet.
The library has no book 5 at first.
So call 2 returns 0, and call 3 fails.
Calls 4 and 5 add 1 copy, then 2 more.
Eve borrows one copy and gives it back. So 3 copies are on the shelf again.

Constraints:

• Each book id is a whole number from 1 to 1,000,000.
• copies is a whole number from 1 to 100.
• A member's name has 1 to 10 English letters.
• Names are case-sensitive: Ana and ANA are two members.
• At most 100,000 calls follow the constructor.