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What is the space complexity of a thread-safe observer pattern implementation that stores observers in concurrent collections per event type and creates a snapshot copy of observers during notification?

medium🪤 Complexity Trap Q6 of Q15
OOP & Design Patterns - Observer Pattern - Event System, Publish-Subscribe
What is the space complexity of a thread-safe observer pattern implementation that stores observers in concurrent collections per event type and creates a snapshot copy of observers during notification?
AO(n + k), where n is total observers and k is snapshot size during notify
BO(1), constant space due to snapshot optimization
CO(k), where k is number of observers for the event type
DO(n * k), product of total and subscribed observers
Step-by-Step Solution
Solution:

  1. Step 1: Understand storage

    Observers are stored per event type, total n observers across all events.

  2. Step 2: Snapshot during notify

    Notify creates a snapshot of k observers for the event type, adding O(k) space temporarily.

  3. Step 3: Combine space usage

    Total space includes O(n) for storage plus O(k) for snapshot; worst case k can approach n, so O(n + k) ≈ O(n).

  4. Final Answer:

    Option A -> Option A

  5. Quick Check:

    Snapshot plus storage cause additive space usage [OK]
Quick Trick: Snapshot plus storage cause additive space usage [OK]
Common Mistakes:
MISTAKES
  • Ignoring snapshot space
  • Assuming constant space
  • Confusing total and subscribed counts
Trap Explanation:
PITFALL
  • Candidates often forget snapshot space or assume it is negligible, underestimating total space complexity.
Interviewer Note:
CONTEXT
  • Tests understanding of auxiliary space in thread-safe observer implementations.
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