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.
2. add_file(name, size) -> bool
   • Convenience method: add a file owned by admin .
   • Return false if the file already exists.
3. 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
4. get_file_size(name) -> int | null
   • Return the file size, or null if not found.
5. delete_file(name) -> int | null
   • Delete the file.
   • Return the deleted file’s size, or null if not found.
6. 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
7. 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.
8. 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.
9. 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.