Library Management

Library Management

Model a small lending library: stock copies of a title, lend them out one-per-reader, take them back, and answer how many are on the shelf.

Requirements

• A title is identified by its isbn; addBook may be called several times for the same ISBN and the copies accumulate.
• A user may hold at most one copy of a given ISBN at a time — a second borrow of the same ISBN by the same user fails while they still hold it (borrowing a different ISBN is fine).
• borrow succeeds only when a copy is available and the user isn't already holding that ISBN; it then decrements availability by one.
• returnBook succeeds only when the user currently holds a copy of that ISBN; it then increments availability by one.
• available reports the copies currently on the shelf and returns 0 for an unknown ISBN — querying never creates a title.

API to implement

Implement the class Library. The I/O driver is already written in the starter code — it parses the command script below and calls your methods. You only implement the class.

| Command | Returns | Behavior | |---|---|---| | addBook <isbn> <copies> | — | Add copies copies of the title isbn to the shelf. May be called repeatedly for the same ISBN — the copies accumulate. | | borrow <user> <isbn> | boolean | user borrows one copy of isbn. Returns true (and decrements availability) only if a copy is free and this user isn't already holding a copy of that ISBN; otherwise false. | | returnBook <user> <isbn> | boolean | user returns a copy of isbn. Returns true (and increments availability) only if this user currently holds a copy of that ISBN; otherwise false. | | available <isbn> | int | Copies of isbn currently on the shelf. Returns 0 for an ISBN that was never added. |

Input format

The first line contains Q, the number of operations. Each of the next Q lines is one command as listed above. String arguments contain no spaces.

Output format

For each command with a non-void return, print its result on its own line, in order. The driver does this for you — your methods just return values.

This is a design problem: the hidden tests exercise edge cases and interaction sequences, not performance tricks. Model the state cleanly and the tests will pass.

6
addBook 111 2
available 111
borrow alice 111
available 111
returnBook alice 111
available 111

2
true
1
true
2

6
addBook 222 3
borrow bob 222
borrow bob 222
available 222
returnBook bob 222
returnBook bob 222

true
false
2
true
false