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angelikatyborska's solution

to Robot Simulator in the Elixir Track

Published at Nov 15 2018 · 0 comments
Test suite


This exercise has changed since this solution was written.

Write a robot simulator.

A robot factory's test facility needs a program to verify robot movements.

The robots have three possible movements:

  • turn right
  • turn left
  • advance

Robots are placed on a hypothetical infinite grid, facing a particular direction (north, east, south, or west) at a set of {x,y} coordinates, e.g., {3,8}, with coordinates increasing to the north and east.

The robot then receives a number of instructions, at which point the testing facility verifies the robot's new position, and in which direction it is pointing.

  • The letter-string "RAALAL" means:
    • Turn right
    • Advance twice
    • Turn left
    • Advance once
    • Turn left yet again
  • Say a robot starts at {7, 3} facing north. Then running this stream of instructions should leave it at {9, 4} facing west.

Running tests

Execute the tests with:

$ elixir robot_simulator_test.exs

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

For more detailed information about the Elixir track, please see the help page.


Inspired by an interview question at a famous company.

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("robot_simulator.exs", __DIR__)

ExUnit.configure(exclude: :pending, trace: true)

defmodule RobotSimulatorTest do
  use ExUnit.Case

  test "create has sensible defaults" do
    robot = RobotSimulator.create()
    assert RobotSimulator.position(robot) == {0, 0}
    assert RobotSimulator.direction(robot) == :north

  @tag :pending
  test "create works with valid arguments" do
    robot = RobotSimulator.create(:north, {0, 0})
    assert RobotSimulator.position(robot) == {0, 0}
    assert RobotSimulator.direction(robot) == :north

    robot = RobotSimulator.create(:south, {-10, 0})
    assert RobotSimulator.position(robot) == {-10, 0}
    assert RobotSimulator.direction(robot) == :south

    robot = RobotSimulator.create(:east, {0, 10})
    assert RobotSimulator.position(robot) == {0, 10}
    assert RobotSimulator.direction(robot) == :east

    robot = RobotSimulator.create(:west, {100, -100})
    assert RobotSimulator.position(robot) == {100, -100}
    assert RobotSimulator.direction(robot) == :west

  @tag :pending
  test "create errors if invalid direction given" do
    position = {0, 0}
    invalid_direction = {:error, "invalid direction"}

    assert RobotSimulator.create(:invalid, position) == invalid_direction
    assert RobotSimulator.create(0, position) == invalid_direction
    assert RobotSimulator.create("east", position) == invalid_direction

  @tag :pending
  test "create errors if invalid position given" do
    direction = :north
    invalid_position = {:error, "invalid position"}

    assert RobotSimulator.create(direction, {0, 0, 0}) == invalid_position
    assert RobotSimulator.create(direction, {0, :invalid}) == invalid_position
    assert RobotSimulator.create(direction, {"0", 0}) == invalid_position

    assert RobotSimulator.create(direction, "invalid") == invalid_position
    assert RobotSimulator.create(direction, 0) == invalid_position
    assert RobotSimulator.create(direction, [0, 0]) == invalid_position
    assert RobotSimulator.create(direction, nil) == invalid_position

  @tag :pending
  test "simulate robots" do
    robot1 = RobotSimulator.create(:north, {0, 0}) |> RobotSimulator.simulate("LAAARALA")
    assert RobotSimulator.direction(robot1) == :west
    assert RobotSimulator.position(robot1) == {-4, 1}

    robot2 = RobotSimulator.create(:east, {2, -7}) |> RobotSimulator.simulate("RRAAAAALA")
    assert RobotSimulator.direction(robot2) == :south
    assert RobotSimulator.position(robot2) == {-3, -8}

    robot3 = RobotSimulator.create(:south, {8, 4}) |> RobotSimulator.simulate("LAAARRRALLLL")
    assert RobotSimulator.direction(robot3) == :north
    assert RobotSimulator.position(robot3) == {11, 5}

  @tag :pending
  test "simulate errors on invalid instructions" do
    assert RobotSimulator.create() |> RobotSimulator.simulate("UUDDLRLRBASTART") ==
             {:error, "invalid instruction"}
defmodule RobotSimulator do
  @enforce_keys [:direction, :position]
  defstruct direction: nil, position: nil

  @right "R"
  @left "L"
  @advance "A"
  @instructions [@right, @left, @advance]
  @directions [:north, :east, :south, :west]

  @doc """
  Create a Robot Simulator given an initial direction and position.

  Valid directions are: `:north`, `:east`, `:south`, `:west`
  @spec create(direction :: atom, position :: {integer, integer}) :: any
  def create(direction \\ :north, position \\ {0, 0}) do
    cond do
      !direction_valid?(direction) ->
        {:error, "invalid direction"}

      !position_valid?(position) ->
        {:error, "invalid position"}

      true ->
          direction: direction,
          position: position

  @doc """
  Simulate the robot's movement given a string of instructions.

  Valid instructions are: "R" (turn right), "L", (turn left), and "A" (advance)
  @spec simulate(robot :: any, instructions :: String.t()) :: any
  def simulate(robot, instructions) do
    instructions = String.split(instructions, "", trim: true)

    if Enum.all?(instructions, &instruction_valid?/1) do
      Enum.reduce(instructions, robot, &execute/2)
      {:error, "invalid instruction"}

  @doc """
  Return the robot's direction.

  Valid directions are: `:north`, `:east`, `:south`, `:west`
  @spec direction(robot :: any) :: atom
  def direction(robot) do

  @doc """
  Return the robot's position.
  @spec position(robot :: any) :: {integer, integer}
  def position(robot) do

  defp execute(@right, robot), do: turn(robot, 1)
  defp execute(@left, robot), do: turn(robot, -1)
  defp execute(@advance, robot), do: advance(robot)

  defp turn(robot, step) do
    new_direction = Loop.move(@directions, robot.direction, step)

    %__MODULE__{robot | direction: new_direction}

  defp advance(robot) do
    new_position = move(robot.position, robot.direction)

    %__MODULE__{robot | position: new_position}

  defp move({x, y}, :north), do: {x, y + 1}
  defp move({x, y}, :east), do: {x + 1, y}
  defp move({x, y}, :south), do: {x, y - 1}
  defp move({x, y}, :west), do: {x - 1, y}

  defp direction_valid?(direction) do
    Enum.member?(@directions, direction)

  defp position_valid?({x, y}) when is_integer(x) and is_integer(y), do: true
  defp position_valid?(_), do: false

  defp instruction_valid?(instruction) do
    Enum.member?(@instructions, instruction)

defmodule Loop do
  @moduledoc """
    Operations on lists that treat them like a loop.

  def move(list, from, step) when is_list(list) do
    new_index =
      |> Enum.find_index(&(&1 == from))
      |> Kernel.+(step)
      |> Kernel.rem(Enum.count(list))

    Enum.at(list, new_index)

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