Design a rating aggregator that returns totals and averages

Design and implement a simple rating system (like movie/book reviews) that supports adding ratings and querying summary statistics.

Requirements

Implement a data structure/class that supports the following operations:

• addRating(itemId, userId, score) : user gives an item a numeric rating.
• updateRating(itemId, userId, newScore) : user updates their rating (if it exists).
• removeRating(itemId, userId) : user deletes their rating (if it exists).
• getItemStats(itemId) : return at least:
  • total number of ratings
  • sum of scores
  • average score

Notes

• score is an integer in [1, 5] .
• Multiple users can rate the same item, but each (itemId, userId) pair should have at most one active rating.
• Aim for efficient updates and queries.

You may assume item and user IDs are strings or integers.

Provide the time/space complexity of each operation.