Design versioned in-memory key-value store

You are asked to design an in-memory key–value store that supports versioning and rollback.

Requirements

• Store key–value pairs in memory ( string keys, arbitrary JSON/string values is fine).
• Support multiple versions of the store over time.
• Ability to rollback the entire store to a previous version .
• Reads should return values from the current active version .
• Assume single-process (no distributed requirements) unless you explicitly choose to extend it.

Suggested API (you may change it)

• put(key, value) -> version_id : writes and creates a new version
• get(key) -> value | null : reads from the current version
• delete(key) -> version_id
• snapshot() -> version_id : optional explicit snapshot
• rollback(version_id) -> void : make version_id the current version

Constraints / Discussion points

• Target good asymptotic performance for get , put , and rollback .
• Discuss time/space trade-offs (copy-on-write vs. log-based vs. persistent data structures).
• Consider edge cases: rolling back and then writing again (branching history), deleting keys, and memory growth.