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Implement a multi-rover Mars controller | Shopify Interview Question

Q: Implement a multi-rover Mars controller

This question evaluates a candidate's ability to implement stateful command parsing, entity management for multiple rovers, and collision-avoidance on an unbounded 2D grid, emphasizing deterministic simulation, input/output specification, and robust handling of invalid operations.

Problem

Build a small command-line “Mars Rover Controller” program.

Single rover (v1)

A rover starts at coordinate (0, 0) on an infinite 2D grid and initially faces North.

The controller must support these commands:

L : turn left 90° (N→W→S→E→N)
R : turn right 90° (N→E→S→W→N)
M : move forward 1 cell in the direction the rover is currently facing

After each command is processed, the program must report the rover’s current position and facing direction.

Multiple rovers (v2)

Extend the controller to manage multiple rovers and support:

CREATE <id> : create a new rover with the default starting state (0,0,N)
DELETE <id> : delete a rover
SELECT <id> : make the rover with <id> the active rover
Movement/turn commands ( L , R , M ) apply to the currently selected rover

Define behavior for invalid operations, e.g. selecting or deleting a non-existent rover, or issuing L/R/M with no rover selected.

Collision avoidance (v3)

Add collision prevention:

If the selected rover executes M and the target cell is already occupied by another rover , the move must fail and the rover must remain in its current position/direction.
The program must still report state after the command; you may choose an error/status message format.

Input / Output

You may design the exact CLI format (interactive REPL or reading lines from stdin), but it must be unambiguous and testable.

At minimum, specify:

Accepted command syntax
What is printed after each command (position + direction, and optionally status on failures)

Constraints / Notes

Grid is unbounded; coordinates may be negative.
Rover IDs are unique strings.
Aim for clean, testable design; include unit tests.

Example (one possible interaction)

CREATE a
SELECT a
M
R
M
CREATE b
SELECT b
M
SELECT a
M   # fails if b is at the target cell

(Your exact printed output format may differ, but it must include position + facing after each command.)

Problem

Build a small command-line “Mars Rover Controller” program.

Single rover (v1)

A rover starts at coordinate (0, 0) on an infinite 2D grid and initially faces North.

The controller must support these commands:

L : turn left 90° (N→W→S→E→N)
R : turn right 90° (N→E→S→W→N)
M : move forward 1 cell in the direction the rover is currently facing

After each command is processed, the program must report the rover’s current position and facing direction.

Multiple rovers (v2)

Extend the controller to manage multiple rovers and support:

CREATE <id> : create a new rover with the default starting state (0,0,N)
DELETE <id> : delete a rover
SELECT <id> : make the rover with <id> the active rover
Movement/turn commands ( L , R , M ) apply to the currently selected rover

Define behavior for invalid operations, e.g. selecting or deleting a non-existent rover, or issuing L/R/M with no rover selected.

Collision avoidance (v3)

Add collision prevention:

If the selected rover executes M and the target cell is already occupied by another rover , the move must fail and the rover must remain in its current position/direction.
The program must still report state after the command; you may choose an error/status message format.

Input / Output

You may design the exact CLI format (interactive REPL or reading lines from stdin), but it must be unambiguous and testable.

At minimum, specify:

Accepted command syntax
What is printed after each command (position + direction, and optionally status on failures)

Constraints / Notes

Grid is unbounded; coordinates may be negative.
Rover IDs are unique strings.
Aim for clean, testable design; include unit tests.

Example (one possible interaction)

CREATE a
SELECT a
M
R
M
CREATE b
SELECT b
M
SELECT a
M   # fails if b is at the target cell

(Your exact printed output format may differ, but it must include position + facing after each command.)

Implement a multi-rover Mars controller

Quick Overview

Problem

Single rover (v1)

Multiple rovers (v2)

Collision avoidance (v3)

Input / Output

Constraints / Notes

Example (one possible interaction)

Comments (0)

Implement a multi-rover Mars controller

Quick Overview

Problem

Single rover (v1)

Multiple rovers (v2)

Collision avoidance (v3)

Input / Output

Constraints / Notes

Example (one possible interaction)

Comments (0)