Ziphq·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Ziphq software engineer interview where they handed you a partially built vending machine OOD problem and asked you to implement one specific slice of it. Felt more like a collaborative design exercise than a pure coding grind, which was a nice change of pace.

Questions Asked (1)

Given a vending machine system with starter code already providing the product inventory and change-making subsystem, implement the bill insertion module. It should validate accepted denominations, reject bad bills, track a running balance, and expose that balance to the rest of the system.

System DesignTechnical Trade-offsData Modeling

Author's notes

The starter code being pre-provided was both a gift and a trap.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and the existing interfaces for inventory and change-making, then design the bill insertion module as a cohesive component with clear responsibilities. Focus on validation, state management, and error handling, and explain how you would test it and integrate with the rest of the system.

Pro tip: Emphasize idempotency and atomicity: ensure that inserting a bill either fully succeeds or fails without leaving the system in an inconsistent state, and that repeated calls with the same bill don't double-count. This shows you think about real-world reliability beyond the happy path.

1. Clarify requirements and constraints

Ask about accepted denominations, whether bills can be inserted concurrently, and how the balance should be exposed (e.g., getter, event, or shared state). Confirm error handling expectations and any existing interfaces you must adhere to.

2. Define the module's interface and state

Design a clear API: methods like insertBill(denomination), getBalance(), and resetBalance(). Decide on internal state representation (e.g., integer cents to avoid floating-point issues) and how it will be accessed by other components.

3. Implement validation and error handling

Validate the bill against a whitelist of accepted denominations. Reject invalid bills with appropriate errors or return values, and ensure the balance is not modified on rejection. Consider edge cases like negative values or non-integer inputs.

4. Manage balance updates and concurrency

Update the running balance atomically when a valid bill is inserted. If concurrent insertions are possible, use synchronization or atomic operations to prevent race conditions. Expose the balance via a thread-safe getter.

5. Integrate and test

Show how the module integrates with the existing inventory and change-making subsystems, and outline unit tests for validation, balance tracking, and error scenarios. Mention any trade-offs made (e.g., simplicity vs. extensibility).

Key Points to Mention

Use of integer arithmetic (e.g., cents) to avoid floating-point precision issues.
Validation against a configurable set of accepted denominations, with clear rejection behavior.
Atomicity and idempotency: ensuring balance updates are consistent and not duplicated.
Thread-safety considerations if the vending machine can accept multiple bills concurrently.
Clear separation of concerns: the bill insertion module should not directly handle change-making or inventory.
Testability: designing the module to be easily unit-tested with mock dependencies.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.