AoC - Day 1a

Secret Entrance

Part 1

You arrive at the secret entrance to the North Pole base ready to start decorating. Unfortunately, the password seems to have been changed, so you can't get in. A document taped to the wall helpfully explains:

"Due to new security protocols, the password is locked in the safe below. Please see the attached document for the new combination."

The safe has a dial with only an arrow on it; around the dial are the numbers 0 through 99 in order. As you turn the dial, it makes a small click noise as it reaches each number.

The attached document (your puzzle input) contains a sequence of rotations, one per line, which tell you how to open the safe. A rotation starts with an L or R which indicates whether the rotation should be to the left (toward lower numbers) or to the right (toward higher numbers). Then, the rotation has a distance value which indicates how many clicks the dial should be rotated in that direction.

So, if the dial were pointing at 11, a rotation of R8 would cause the dial to point at 19. After that, a rotation of L19 would cause it to point at 0.

Because the dial is a circle, turning the dial left from 0 one click makes it point at 99. Similarly, turning the dial right from 99 one click makes it point at 0.

So, if the dial were pointing at 5, a rotation of L10 would cause it to point at 95. After that, a rotation of R5 could cause it to point at 0.

The dial starts by pointing at 50.

Solution

const fileContent = require("fs").readFileSync(
  require("path").join(process.cwd(), "day1.txt"),
  "utf8"
);
const lines = fileContent.split("\n");
let count = 0;
const DIAL_LENGTH = 100;

lines.reduce((counter, line) => {
  const direction = line.at(0);
  const steps = parseInt(line.slice(1));
  if (direction === "R") {
    const result = (counter + steps) % DIAL_LENGTH;
    result === 0 ? count++ : null;
    return result;
  } else {
    const result =
      (((counter - steps) % DIAL_LENGTH) + DIAL_LENGTH) % DIAL_LENGTH;
    result === 0 ? count++ : null;
    return result;
  }
}, 50);