Design an in-memory cloud file system

In-Memory Cloud File System (OOD)

Design an in-memory cloud file system that supports files owned by users with storage quotas.

Data model / rules

• Each file has a unique name (string) and a size (positive integer).
• Each file is owned by exactly one user.
• Each user has a remaining capacity (how many bytes they can still store).
• Special user "admin" exists and has infinite capacity.
• Adding a file consumes the owner’s remaining capacity by size.
• Deleting a file refunds its size back to the owner’s remaining capacity.

API to implement

Implement a class with the following methods (exact names not required, behavior is):

1. add_user(user_id, capacity) -> bool

• Create a new user with the given remaining capacity.
• Return false if the user already exists.

1. add_file(name, size) -> bool

• Convenience method: add a file owned by admin.
• Return false if the file already exists.

1. add_file_by(user_id, name, size) -> int | null

• Add a file owned by user_id.
• Return the user’s remaining capacity after adding.
• Return null if:
• user_id does not exist
• name already exists
• size exceeds the user’s remaining capacity

1. get_file_size(name) -> int | null

• Return the file size, or null if not found.

1. delete_file(name) -> int | null

• Delete the file.
• Return the deleted file’s size, or null if not found.

1. get_n_largest(prefix, n) -> list<string>

• Consider only file names starting with prefix.
• Return up to n files formatted as: "<name>(<size>)".
• Sort by:

1. size descending
2. name ascending (lexicographic) for ties

1. merge_user(user_id_1, user_id_2) -> int | null

• Merge user_id_2 into user_id_1.
• All files owned by user_id_2 become owned by user_id_1.
• user_id_1’s remaining capacity increases by user_id_2’s remaining capacity.
• Delete user_id_2.
• Return user_id_1’s new remaining capacity.
• Return null if either user doesn’t exist or the IDs are the same.

1. backup_user(user_id) -> int | null

• Store a snapshot of the current set of files owned by user_id (name -> size).
• Return the number of files included in this backup.
• Return null if the user doesn’t exist.

1. restore_user(user_id) -> int | null

• Restore user_id’s files from the most recent backup for that user.
• First, remove all files currently owned by user_id.
• Then, for each file in the backup, restore it only if its name is not currently used by some other user.
• Return the number of files actually restored.
• If the user exists but has no backup, return 0.
• If the user doesn’t exist, return null.

Notes

• Assume up to ~1e5 operations.
• You may choose any internal data structures, but must satisfy the behavior above.