Day 03 Advent of Code implemented in JS and Python with TDD (https://adventofcode.com/2015/day/3)

day03.js

const fs = require("fs");

function main() {
  const path = __dirname + "/input";
  let input = fs.readFileSync(path, { encoding: "utf8" });

  console.log("Day 3, part 1: ", part1(input)); // 2572
  console.log("Day 3, part 2: ", part2(input)); // 2631
}

if (require.main === module) {
  main();
}

function part1(input) {
  const directions = parseInput(input); // an array of characters '^', 'v' '<', '>'
  // Set of String (house coordinates)
  const houses = new Set();
  // Zeroth house gets a present
  let [x, y] = [0, 0];
  let houseCoords = [x, y].join(); // becomes `${x},${y}`
  houses.add(houseCoords);
  for (direction of directions) {
    // Go to next house
    switch (direction) {
      case '^':
        x--;
        break;
      case 'v':
        x++;
        break;
      case '<':
        y--;
        break;
      case '>':
        y++;
        break;
      default:
        throw new Error(`Unrecognized character ${direction}`);
    }
    houseCoords = [x, y].join();
    houses.add(houseCoords);
  }
  return houses.size;
}

function part2(input) {
  const directions = parseInput(input); // an array of characters '^', 'v' '<', '>'
  // Set of String (house coordinates)
  const houses = new Set();
  // Zeroth house gets two presents
  let [xSanta, ySanta, xRobo, yRobo] = [0, 0, 0, 0];
  let houseCoords = [xSanta, ySanta].join(); // becomes `${xSanta},${ySanta}`
  houses.add(houseCoords);
  for (let i = 0; i < directions.length; i++) {
    let [x, y] = (i % 2 === 0 ? [xRobo, yRobo] : [xSanta, ySanta]);
    const direction = directions[i];
    // Go to next house
    switch (direction) {
      case '^':
        x--;
        break;
      case 'v':
        x++;
        break;
      case '<':
        y--;
        break;
      case '>':
        y++;
        break;
      default:
        throw new Error(`Unrecognized character ${direction}`);
    }
    if (i % 2 === 0) {
      [xRobo, yRobo] = [x, y];
    } else {
      [xSanta, ySanta] = [x, y];
    }
    houseCoords = [x, y].join();
    houses.add(houseCoords);
  }
  return houses.size;
}

/**
 * @param {string} input - The puzzle input as a single line string
 * @returns {Array<String>} An array of single character strings, one of '^', 'v'. '<' or '>'
 */
function parseInput(input) {
  const directions = input.trim().split("\n");
  // input file is only one line
  return directions[0].split('');
}

module.exports = {
  parseInput,
  part1,
  part2,
};

day03.test.js

const { expect } = require("chai");
const { parseInput, part1, part2 } = require("./day03.js");

describe("day03", () => {
  describe("parseInput", () => {
    it("should work with example input", () => {
      const input = "v>v<vv\n"; // random; first n chars in input
      const expected = ['v', '>', 'v', '<', 'v', 'v'];
      expect(parseInput(input)).to.deep.equal(expected);
    })
  });

  describe("part1", () => {
    it("should work for example 1", () => {
      const input = ">";
      const expected = 2;
      expect(part1(input)).to.equal(expected);
    });

    it("should work for example 2", () => {
      const input = "^>v<";
      const expected = 4;
      expect(part1(input)).to.equal(expected);
    });

    it("should work for example 3", () => {
      const input = "^v^v^v^v^v";
      const expected = 2;
      expect(part1(input)).to.equal(expected);
    });
  });

  describe("part2", () => {
    it("should work for example 1", () => {
      const input = "^v";
      const expected = 3;
      expect(part2(input)).to.equal(expected);
    });

    it("should work for example 2", () => {
      const input = "^>v<";
      const expected = 3;
      expect(part2(input)).to.equal(expected);
    });

    it("should work for example 3", () => {
      const input = "^v^v^v^v^v";
      const expected = 11;
      expect(part2(input)).to.equal(expected);
    });
  });
});