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davearonson's solution

to Robot Simulator in the Elixir Track

Published at Jul 13 2018 · 0 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code 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
  @type t :: %RobotSimulator{ direction: atom, position: {integer, integer} }
  defstruct direction: :north, position: {0,0}
  @directions [:north, :east, :south, :west]
  @directions_to_numbers %{north: 0, east: 1, south: 2, west: 3}
  @numbers_to_directions %{0 => :north, 1 => :east, 2 => :south, 3 => :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 }) :: RobotSimulator.t()

  def create(direction \\ :north, position \\ {0,0}) do
    cond do
      ! Enum.member?(@directions, direction) ->
        { :error, "invalid direction" }
      ! (is_tuple(position) &&
         tuple_size(position) == 2 &&
         is_integer(elem(position, 0)) &&
         is_integer(elem(position, 1))) ->
        { :error, "invalid position" } 
      true ->
        %RobotSimulator{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
    do_simulate(robot, instructions |> String.graphemes)

  defp do_simulate(robot, [instruction|more]) do
    case instruction do
      "A" -> do_simulate(advance(robot) , more)
      "L" -> do_simulate(turn(robot, -1), more)
      "R" -> do_simulate(turn(robot,  1), more)
      _   -> { :error, "invalid instruction" }
  defp do_simulate(robot, []), do: robot

  defp advance(robot) do
    # TODO MAYBE: we could probably do some funny math
    # on the direction index, to replace case....
    case robot.direction do
      :north -> move(robot, 1,  1)
      :east  -> move(robot, 0,  1)
      :south -> move(robot, 1, -1)
      :west  -> move(robot, 0, -1)

  defp move(robot, axis, amount) do
                    elem(robot.position, axis) + amount))

  defp turn(robot, way) do
    # add 4 just in case it's a left turn from north
    new_dir_num = rem(@directions_to_numbers[robot.direction] + way + 4, 4)
    create(@numbers_to_directions[new_dir_num], robot.position)

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

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

  @doc """
  Return the robot's position, as a tuple of x,y coordinates.
  @spec position(robot :: any) :: { integer, integer }
  def position(robot), do: robot.position

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