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Day 2: Dive!

https://adventofcode.com/2021/day/2
Challenge
Now, you need to figure out how to pilot this thing.
It seems like the submarine can take a series of commands like forward 1, down 2, or up 3:
Note that since you're on a submarine, down and up affect your depth, and so they have the opposite result of what you might expect.
The submarine seems to already have a planned course (your puzzle input). You should probably figure out where it's going. For example:
forward 5
down 5
forward 8
up 3
down 8
forward 2
Your horizontal position and depth both start at 0. The steps above would then modify them as follows:
After following these instructions, you would have a horizontal position of 15 and a depth of 10. (Multiplying these together produces 150.)
Calculate the horizontal position and depth you would have after following the planned course. What do you get if you multiply your final horizontal position by your final depth?

Part Two

Based on your calculations, the planned course doesn't seem to make any sense. You find the submarine manual and discover that the process is actually slightly more complicated.
In addition to horizontal position and depth, you'll also need to track a third value, aim, which also starts at 0. The commands also mean something entirely different than you first thought:
Again note that since you're on a submarine, down and up do the opposite of what you might expect: "down" means aiming in the positive direction.
Now, the above example does something different:
After following these new instructions, you would have a horizontal position of 15 and a depth of 60. (Multiplying these produces 900.)
Using this new interpretation of the commands, calculate the horizontal position and depth you would have after following the planned course. What do you get if you multiply your final horizontal position by your final depth?

Part A

We'll track and x and y and update accordingly (with an if statement)
import { strings } from "./util";

let x = 0;
let y = 0;

strings().forEach((line) => {
  const [dir, _dist] = line.split(" ");
  const dist = parseInt(_dist);
  if (dir === "forward") {
    x += dist;
  } else if (dir === "up") {
    y -= dist;
  } else if (dir === "down") {
    y += dist;
  } else {
    throw "Unrecognized dir -- " + dir;
  }
});

console.log(x * y);
I really like ES2015's pattern matching assignments, but it gets annoying when I want to run a function on one of the fields:
const [dir, _dist] = line.split(" ");
const dist = parseInt(_dist);  // bleh

Part B

In addition to horizontal position and depth, you'll also need to track a third value, aim, which also starts at 0. The commands also mean something entirely different than you first thought:

Not too different here. We'll still track an x and y, but introduce an aim. "up" and "down" now update aim instead of y, and forward still updates x, but now also updates y using aim.
import { strings } from "./util";

let x = 0;
let y = 0;
let aim = 0;

strings().forEach((line) => {
  const [dir, _dist] = line.split(" ");
  const dist = parseInt(_dist);
  if (dir === "forward") {
    x += dist;
    y += aim * dist;
  } else if (dir === "up") {
    aim -= dist;
  } else if (dir === "down") {
    aim += dist;
  } else {
    throw "Unrecognized dir -- " + dir;
  }
});

console.log(x * y);