In-Memory Cloud Storage: Design and Implement

You are asked to design and implement an in-memory cloud storage system that maintains a mapping from file names to their metadata (size) and supports both single-user and multi-user operations.

Assumptions to make explicit:

• File names are unique per user. The early single-user functions operate in a special default user namespace.
• Sizes are non-negative integers (bytes). Capacity is the maximum total size (sum of file sizes) allowed per user.
• For get_n_largest(prefix, n), return the n largest files across all users whose names start with prefix. Each result should identify the user.
• merge_user(u1, u2) combines user2 into user1; user1 remains, user2 is deleted. Capacity becomes the sum of both users' capacities.
• If file name collisions occur during merge, rename the incoming conflicting file(s) from user2 by appending a suffix to ensure uniqueness (e.g., "name (merged 2)").
• backup_user and restore_user operate per user and do not affect other users' files or capacities.

Required API

Single-user (default namespace):

1. add_file(name, size)
2. get_file_size(name) → size or None
3. delete_file(name)
4. get_n_largest(prefix, n) → list of (user_id, name, size)

Multi-user:

1. add_user(user_id, capacity)
2. add_file_by(user_id, name, size) // enforce per-user capacity limits
3. merge_user(user_id1, user_id2) // combine users and their files; capacity sums; rename on conflicts
4. backup_user(user_id) → version_id // versioned backups per user
5. restore_user(user_id, version_id=None) // restore to a saved version (latest if None)

Deliverables

• Data structures and algorithms to support the operations.
• Clear semantics for edge cases (e.g., capacity enforcement, file collisions on merge, restore behavior).
• Time/space complexity discussion and potential optimizations for prefix queries.
• A working in-memory implementation (any mainstream language) with small usage examples/tests.