Design unit tests for grid navigation

Q: Design unit tests for grid navigation

This question evaluates unit test design, mocking/stubbing of external APIs, deterministic simulation, and correctness of grid-navigation controller logic under edge cases, loops, performance limits, and API error behaviors.

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This is a medium difficulty Software Engineering Fundamentals question, commonly asked during Technical Screen rounds at Meta.

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Question

Mouse-Maze Controller: Design a Comprehensive Unit Test Suite

Context (assumptions to make the task self-contained)

Assume we are testing a controller that navigates a grid-based maze to find cheese. The controller drives an abstract Maze API and must return the first cheese it finds according to a deterministic neighbor-order policy. Multiple cheeses may exist; some may be unreachable.

Grid representation: rectangular grid with walls and open cells.
Start position: a single cell where the mouse begins.
Cheese: one or more target cells; reaching any one ends the search successfully.
Deterministic neighbor exploration order: Up, Right, Down, Left (URDL), unless otherwise specified.
The Maze API can signal walls/valid moves and may throw errors on move attempts.
Controller output: either a path (sequence of directions) to a found cheese, or a NotFound/Failure result.

You may minimally adjust the exact API shape but keep behavior semantically equivalent to the above.

Task

Propose a comprehensive set of unit tests for the mouse-maze controller. Include tests for:

Single-cell maze
Unreachable cheese
Narrow corridors and cul-de-sacs
Loops/cycles
Large open spaces
Obstacle directly ahead
Repeated visits and backtracking correctness
Multiple cheeses (ensure the first found is returned)
Performance and stack-depth limits
API error behavior (e.g., move() throwing)

Also describe how to stub/simulate the Maze API deterministically to support these tests.

Design unit tests for grid navigation

Quick Overview

Design unit tests for grid navigation

Mouse-Maze Controller: Design a Comprehensive Unit Test Suite

Context (assumptions to make the task self-contained)

Task

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