Design a module to fetch a unique User ID from a pool of available User IDs and return the ID to the pool for later use.

To design a module for managing a pool of unique User IDs, we can consider two primary approaches:

1. Linked List: Maintain a linked list where each node represents an available User ID. To fetch an ID, remove a node from the list. To return an ID, add it back as a new node. This offers efficient insertion and deletion.
2. Boolean Array/Bitset: Use an array of booleans (or a bitset for memory optimization) where each index corresponds to a User ID. true indicates the ID is in use, false indicates it's available. Fetching involves finding the first false index, marking it true. Returning involves marking the index false.

Optimization Considerations:

• Time Complexity: The boolean array/bitset approach typically offers O(1) average time complexity for fetching and returning if we maintain a pointer to the next available ID. A linked list also offers O(1) for these operations.
• Memory Complexity: A bitset is the most memory-efficient for large ID pools, as it uses only one bit per ID. A boolean array uses a byte per ID, and a linked list has overhead for pointers.
• Scalability: For very large ID ranges, a bitset or a combination of a bitset for dense ranges and a linked list for sparse ranges might be optimal. We should also consider thread-safety if multiple threads will access the pool concurrently, potentially using atomic operations or locks.