Thread-Safe Singleton Pattern | Dataford Interview Questions

Implement a thread-safe Singleton design pattern.

Asked in the Round 3 - Hiring Manager stage. This was one of several fundamentals evaluated, along with API design, abstract classes vs interfaces, and exception handling.

Implement singleton_ids(thread_count). It must lazily create one shared Singleton instance, start thread_count threads, obtain the instance once per thread, and return the number of distinct instance identities observed.

Contract

Input: positive integer thread_count.
Output: integer, which must be 1 for every valid input.
Initialization must be safe when multiple threads call the accessor concurrently.

Constraints

1 <= thread_count <= 1000
The Singleton is initialized lazily on the first access
All calls must return the same object identity

Function Signature

def singleton_ids(thread_count):

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