Day 5

Very helpful array transposition from this SO answer.

Part 1

The crates and the moves are pretty different, so we'll start by breaking them up.

I want to squash the crates down to a transposed 2D array with no empty spaces, so we can just move the first index around n times. Our move method does just this - shifts from n times and pushes the shifted value to to.

I want the moves format to just be n moves with from and to as array indexes, so we match out those numbers, parse them, then decrement the from and to values by one, since they're 1-indexed in the puzzle.

Then, for each of our moves, we just call move. Grab the first element, join them together, and we have our message.

const [c,m] = document.querySelector('pre').innerText.split('\n\n');
const crates = c.split('\n')
  .map(l => l
    .split(/(.{4})/g)
    .filter(c => c !== '')
    .map(c => c.trim() === '' ? '' : c.trim().replace(/[\[\]]/g, '')))
  .reduce((prev, next) => next
    .map((item, i) => (prev[i] || []).concat(next[i])), [])
    .map(s => s.slice(0,8)
    .filter(s => s !== ''));
crates.move = (n, from, to) => { for (let i = 0; i < n; i++) { to = [from.shift(), ...to]} };
const moves =  m.trim()
  .split('\n')
  .map(l => l.match(/move (\d+) from (\d+) to (\d+)/))
  .map(l => [parseInt(l[1]),parseInt(l[2])-1,parseInt(l[3])-1]);
moves.forEach(m => crates.move(...m));
crates.map(l => l[0]).join('');

Reload the inputs page.

Part 2

It's funny, this is the original solution I had for 1 that I was stumped on for a whille - I forgot that I'd need to use .reverse() to work with splices in part 1.

This process is basically the same, but we use splice to grab multiple values and prepend to in order, instead of individually shifting values.

const [c,m] = document.querySelector('pre').innerText.split('\n\n');
const crates = c.split('\n')
  .map(l => l
    .split(/(.{4})/g)
    .filter(c => c !== '')
    .map(c => c.trim() === '' ? '' : c.trim().replace(/[\[\]]/g, '')))
  .reduce((prev, next) => next
    .map((item, i) => (prev[i] || []).concat(next[i])), [])
    .map(s => s.slice(0,8)
        .filter(s => s !== ''));
crates.multiMove = (n, from, to) => crates[to].splice(0, 0, ...crates[from].splice(0, n > crates[from].length ? crates[from].length : n))
const moves =  m.trim()
  .split('\n')
  .map(l => l.match(/move (\d+) from (\d+) to (\d+)/))
  .map(l => [parseInt(l[1]),parseInt(l[2])-1,parseInt(l[3])-1]);
moves.forEach(m => crates.multiMove(...m));
crates.map(l => l[0]).join('');