Timed LLD mock rounds focused on patterns

A timed mock round focused on patterns is a 45-minute rehearsal of a low-level design interview, run on your own: you clarify a prompt, list what varies in it, sketch the classes, code the core against tests, and then answer two change requests that put your pattern choices under strain. Afterwards you score the answer with a rubric that rewards patterns justified by a variation, and alternatives rejected for a reason, more than patterns named. The format matters because the change requests, not the class diagram, show whether a design was right: a good choice makes the change small.

The 45-minute format

Each phase leaves something behind for the next one:

• Minutes 0 to 5, clarify: the questions asked, the assumptions written down, and what is out of scope.
• Minutes 5 to 10, what varies: a short list, each item with its pattern, or with none.
• Minutes 10 to 15, sketch: the classes and their public methods, no more.
• Minutes 15 to 35, code the core: the smallest working core, with the tests run as you go.
• Minutes 35 to 45, change requests: for each one, what changes, said first and then changed.

The rubric is MySmartCoPilot’s own and has five criteria, each scored from 0 to 4:

• Requirements and scope (weight 1): the questions you asked, and what you wrote down as assumed or out of scope.
• Pattern fit and rejections (weight 2): each pattern tied to a variation, with the alternative you rejected.
• Extensibility under changes (weight 2): how little of the code each change request had to touch.
• Correctness against tests (weight 2): how many of the provided tests pass.
• Communication of trade-offs (weight 1): decisions said aloud, with their costs, while you made them.

The weights favour justified choices over both pattern-collecting and plain code that passes. Here they are applied to three made-up answers to one prompt:

"""The self-mock rubric: five criteria scored 0 to 4, weighted, applied to three made-up answers to the same prompt."""

CRITERIA = [  # (criterion, weight)
    ("requirements and scope", 1),
    ("pattern fit and rejections", 2),
    ("extensibility under changes", 2),
    ("correctness against tests", 2),
    ("communication of trade-offs", 1),
]

ANSWERS = {  # three illustrative answers, scored 0-4 on each criterion, in the order above
    "collector": [2, 1, 2, 2, 2],  # six patterns named, three of them without a reason; 4 of 6 tests pass
    "plain": [3, 2, 1, 4, 2],  # no pattern, every test passes, the change request edits one large class
    "justified": [3, 4, 4, 4, 3],  # two patterns, each with its rejected alternative; both change requests absorbed
}

best = sum(4 * weight for _, weight in CRITERIA)
print(f"{'criterion (weight)':<34}" + "".join(f"{name:>11}" for name in ANSWERS))
for i, (criterion, weight) in enumerate(CRITERIA):
    print(f"{criterion + f' ({weight})':<34}" + "".join(f"{scores[i]:>11}" for scores in ANSWERS.values()))
totals = [sum(s * w for s, (_, w) in zip(scores, CRITERIA)) for scores in ANSWERS.values()]
print(f"{f'weighted total, out of {best}':<34}" + "".join(f"{t:>11}" for t in totals))

criterion (weight)                  collector      plain  justified
requirements and scope (1)                  2          3          3
pattern fit and rejections (2)              1          2          4
extensibility under changes (2)             2          1          4
correctness against tests (2)               2          4          4
communication of trade-offs (1)             2          2          3
weighted total, out of 32                  14         19         30

The collector named six patterns and scored 14 of 32; the plain answer passed every test and scored 19, but lost points when its change request edited one large class; the justified answer scored 30. This lesson’s sample prompt is the core of a drawing editor: shapes on a canvas that can be added, moved, recoloured and deleted, with undo and redo for every action and no user interface.

The sample mock, minute by minute Pro: opens with a Pro, Premium or Ultimate pass