Review Constant-Time Random Courier Selection and Removal

Review a delivery-assignment service that stores couriers in dasher_map under contiguous integer keys starting at 0. add_dasher inserts a courier. pick_dasher chooses a courier at random and removes it. adjust_map restores contiguous keys by moving the last entry into a removed slot.

Explain the invariants and constant-time operations needed for this design, the bugs you would look for, and the code-quality improvements you would request in a review.

Requirements and Constraints

• After each operation, the map keys must be exactly 0 through n - 1, where n is the number of stored entries.
• Selection should be uniform over the currently stored entries.
• Preserve the original operations' expected or amortized constant-time behavior; do not repair the map by scanning and renumbering every entry.
• An external delivery-recording service can be assumed to work correctly.
• No actual implementation is supplied here, so identify failure modes and required behavior rather than claiming a particular unseen line is wrong.

Clarifying Questions

• What should selection do when the map is empty?
• May the same courier identity be added more than once, or must insertion reject duplicates?
• Can calls occur concurrently, and must selecting and removing a courier be one atomic operation?
• Which method owns the size update, and when is the selected courier sent to the recording service?