A timed machine-coding mock, step by step

A timed machine-coding mock is a 90-minute rehearsal of a machine-coding round under the round’s own conditions: a written prompt with a sample input and its exact expected output, your own editor, a timer that does not stop, a change requested near the end, and a scored review afterwards. Its value comes from the clock. Untimed practice shows whether you can solve a problem; a timed machine-coding mock shows where your minutes go and which habits break under pressure. This lesson replays one mock step by step, on an original prompt.

The prompt, the plan and the three checkpoints

The prompt: a housing society’s security desk needs a program for visitors, and every input line starts with the time. A resident approves a visitor for a flat, and the visitor gets a pass code that works once. The guard lets the visitor in and out with that code. Someone without a pass waits at the gate until the flat allows or denies them. A delivery enters at once and must leave within 15 minutes. A resident may cancel a pass that has not been used, and a report lists everyone inside, with late deliveries flagged. Every refused command gets one answer line, and the program carries on. The sample has 17 commands and 21 expected answer lines.

The plan is six stages, and three of them end with a checkpoint: a run of the sample and of every test written so far.

Step by step, the checkpoints are the point of the exercise. At minute 30 the sample matched 8 of its 21 expected lines; at minute 60 it matched all 21, with 9 tests passing; at minute 90, after the follow-up, it still matched all 21, with 12 tests. Each number tells you whether the plan is on time while there is still time to react.

The finished program is scored on six areas, each from 1 to 4:

• Requirements coverage: every command and refusal of the prompt works.
• Model quality: each rule has one owner, and parsing stays apart from the rules.
• Correctness: tests prove the rules, starting with the prompt’s sample.
• Extensibility: the follow-up lands in few places, with the old tests green.
• Code quality: short functions, names from the domain and one way to refuse.
• Communication: decisions, assumptions and gaps are said out loud.

The scores are not a verdict on you; they show which habit to practise in the next mock.

The program at minutes 30, 60 and 90, replayed Pro: opens with a Pro, Premium or Ultimate pass

In this part:

• Why the line count and the command count differ
• Minutes 0 to 10: assumptions and one Visit class
• Minutes 10 to 30: the happy path and the first run
• Minutes 30 to 60: guarded transitions, then the tests
• Minutes 60 to 90: tidying, the follow-up and the demo
• Two mistakes the first run shows, with their cost
• The rubric: six areas, four levels each
• This mock scored, area by area
• Four weeks of timed mocks
• Practise: visitor parking in 25 minutes
  • Add visitor parking to the gate pass
• Check yourself
• Interview questions
  • Why does a timed mock stop to run the sample and the tests at fixed checkpoints, such as minutes 30, 60 and 90?
  • At minute 60 of a 90-minute round your happy path works, three refusals are missing, there are no tests, and the interviewer will ask for a change at minute 75. How do you spend the last 30 minutes?
  • After your demo of a visitor gate-pass program, the interviewer asks: "What would you do with two more hours?" What makes a strong answer, and what makes a weak one?
  • You finished a practice mock: every sample line matches, nine tests pass, and the follow-up took edits in four places. How do you score it on six areas from 1 to 4, and what evidence would move a 2 to a 3?
• Key takeaways

1 runnable example (expected.txt, replay.py, sample.txt, t30/gate.py, t60/gate.py, t60/test_gate.py, t90/gate.py, t90/test_gate.py) · 1 exercise with sample tests · 3 interview answers · about 26 minutes

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Read a complete sample first: Low-level design: parking lot (the model)