Mars Rover
Tags: kata, challenge, javascript, codurance
Published: 2020-01-16 23:54:52

In this post, we will be taking on [the Mars Rover Kata](https://codurance.com/2019/01/22/mars-rover-kata-refactoring-to-patterns/). This challenge entails implementing a `Rover` which can drive from grid cell to grid cell on a grid based on commands. Commands are passed as a string of individual instructions, these instructions can be to move (M), turn left (L) or turn right (R).

## Task solution

### Tests

```javascript
const {
  Rover
} = require("./rover");
let rover;

beforeEach(() => {
  rover = new Rover();
});

describe("rover", () => {
  it("Has an initial position of 0", () => {
    expect(rover.direction).toEqual(0);
  });

  it("Can move given instructions", () => {
    expect(rover.currentPosition).toEqual([0, 0]);
    rover.move("LMRMM");
    expect(rover.currentPosition).toEqual([0, 2]);
  });

  it("Translates direction correctly", () => {
    rover.translateInstructions(["L", "L", "R"]);
    expect(rover.direction).toEqual(-1);
  });

  it("Can move postion correctly", () => {
    rover.move("M");
    expect(rover.currentPosition).toEqual([0, 1]);
    rover.move("RM");
    expect(rover.currentPosition).toEqual([1, 1]);
    rover.move("RM");
    expect(rover.currentPosition).toEqual([1, 0]);
    rover.move("RM");
    expect(rover.currentPosition).toEqual([0, 0]);
    rover.move("M");
    expect(rover.currentPosition).toEqual([9, 0]);
    rover.move("LM");
    expect(rover.currentPosition).toEqual([9, 9]);
    rover.move("RRM");
    expect(rover.currentPosition).toEqual([9, 0]);
    rover.move("RM");
    expect(rover.currentPosition).toEqual([0, 0]);
  });

  it("throws when an invalid move is provided", () => {
    expect(() => rover.move("X")).toThrowErrorMatchingSnapshot();
  });
});
```

Each test uses a new `Rover` instance and cover the following cases:

1. Initial state
2. Instruction execution
3. Movement of the rover
4. Error handling

We can also see that we are working with an `x` and `y` coordinate system for the rovers current position. You may also have noticed the integer based direction of the rover. It will make more sense as to why I chose to do directionality in this way once the implementation is seen but in short, we will have an array of potential directions, each of these will represent the points of a compass (North, South, East, West). 

When we wish to see which direction we should move, we can use [the `%` (modulo) operator which I explained in an earlier article](https://dev.to/jamesrweb/kata-odd-or-even-58i8#sidenote-1-the-raw-endraw-operator) to access the relevant direction. Since we are using 4 compass points we can only ever receive values between -4 and 4 when using any number modulo the count of compass points. I chose to only allow moves on positive values but we could use `Math.abs` to convert the negatives to positives and use them but the programme behaviour would change from how it is currently setup in the tests. Just as a side note, here are some examples of potential actions based on a direction modulod by the 4 compass points:

| Direction | Compass point      | Action                     |
|-----------|--------------------|----------------------------|
| -1        | -1 % 4 = -1 = None | Don't move                 |
| 2         | 2 % 4 = 2 = South  | Move down                  |
| 5         | 5 % 4 = 1 = East   | Move right                 |

### Implementation

```javascript
class Rover {
  constructor(gridDimension = 10) {
    this.currentPosition = [0, 0];
    this.direction = 0;
    this.compassPoints = ["N", "E", "S", "W"];
    this.gridDimension = gridDimension;
  }

  move(instructions) {
    const individualInstructions = instructions.split("");
    this.translateInstructions(individualInstructions);
  }

  shiftUp() {
    let [x, y] = this.currentPosition;
    if (y === this.gridDimension - 1) y = 0;
    else y = ++y;
    this.currentPosition = [x, y];
  }

  shiftDown() {
    let [x, y] = this.currentPosition;
    if (y === 0) y = this.gridDimension - 1;
    else y = --y;
    this.currentPosition = [x, y];
  }

  shiftLeft() {
    let [x, y] = this.currentPosition;
    if (x === 0) x = this.gridDimension - 1;
    else x = --x;
    this.currentPosition = [x, y];
  }

  shiftRight() {
    let [x, y] = this.currentPosition;
    if (x === this.gridDimension - 1) x = 0;
    else x = ++x;
    this.currentPosition = [x, y];
  }

  getCompassHeading() {
    return this.compassPoints[this.direction % this.compassPoints.length];
  }

  shiftRoverPosition() {
    const moveDirection = this.getCompassHeading();
    if (moveDirection === "N") this.shiftUp();
    else if (moveDirection === "S") this.shiftDown();
    else if (moveDirection === "E") this.shiftRight();
    else if (moveDirection === "W") this.shiftLeft();
  }

  translateInstructions(instructions) {
    instructions.forEach(instruction => {
      if (instruction === "L") this.direction--;
      else if (instruction === "R") this.direction++;
      else if (instruction === "M") this.shiftRoverPosition();
      else throw new Error("Invalid instruction provided");
    });
  }
}

module.exports = {
  Rover
};
```

We interact with the `Rover` instance by calling the `move` method, this method takes 1 parameter, a string of instructions. This string is split into the individual characters and passed as an array into the `translateInstructions` function. Each instruction is checked and if the command is to move left (L), we add 1 from the current `direction`. If the command is to move right (R), we add one to the current `direction`. If the command is to move, we call the `shiftRoverPosition` method and finally, if the instruction is not recognised, we throw and error. The `shiftRoverPosition` method calls the `getCompassHeading` method which is where we try to get our value from the compass headings:

```javascript
getCompassHeading() {
  return this.compassPoints[this.direction % this.compassPoints.length];
}
```

If we get back a `N`, `E`, `S` or `W`, we move up, right, down or left respectively, in practice this merely means altering the `x` and `y` coordinates of the rover.

## Conclusions

I actually did this Kata as part of an interview a while back and this was my solution. I will say though that this isn't the whole Kata, it is a stripped down version that the company I interviewed at used for their tech interview pair-programming session. I recommend trying it out yourself to see what you can come up with or extending the functionality for your rover to make it do even more than just move around a grid, why not give it a try and see what you come up with?