aoc 9 lame rope

const fs = require('fs')

// const input = fs.readFileSync('test.txt').toString().trim()
const input = fs.readFileSync('input.txt').toString().trim()

const range = (a) => [...Array(a).keys()]
const abs = Math.abs
const follow = (head, tail) => abs(head[0] - tail[0]) >= 2 || abs(head[1] - tail[1]) >= 2 ? head : null
const move = (rope, [hx, hy]) => {
  const head = rope.head.at(-1)
  const tail = rope.tail.at(-1)
  const newHead = [head[0] + hx, head[1] + hy]

  return {
    head: [...rope.head, newHead],
    tail: follow(newHead, tail) ? [...rope.tail, [...head]] : rope.tail,
  }
}

const moves = {
  U: (current, count) => range(count).reduce((acc) => move(acc, [1, 0]), current),
  D: (current, count) => range(count).reduce((acc) => move(acc, [-1, 0]), current),
  L: (current, count) => range(count).reduce((acc) => move(acc, [0, -1]), current),
  R: (current, count) => range(count).reduce((acc) => move(acc, [0, 1]), current),
}

const pairs = input.split('\n').map((pair) => pair.split(' '))
  .reduce((acc, [direction, count]) => moves[direction](acc, +count), {
    head: [[0, 0]],
    tail: [[0, 0]],
  })

console.log(new Set(pairs.tail.map((pairs) => pairs.join('_'))).size)

const getModifiers = ([hx, hy], [tx, ty]) => {
  const x = abs(hx - tx) === 2 ? (hx - tx) / 2 :
    abs(hy - ty) === 2 ?
      (abs(hx - tx) === 1 ? hx - tx : 0)
      : 0
  const y = abs(hy - ty) === 2 ? (hy - ty) / 2 :
    abs(hx - tx) === 2 ?
      (abs(hy - ty) === 1 ? hy - ty : 0)
      : 0

  return [x, y]
}
const move2 = (rope, [hx, hy]) => {
  const head = [rope.knots[0][0] + hx, rope.knots[0][1] + hy]
  const knots = rope.knots.reduce((acc, knot, i) => {
    if (i === 0) {
      acc[i] = head
      return acc
    }

    const prev = acc[i - 1]
    const [dx, dy] = getModifiers(prev, acc[i])
    acc[i] = follow(prev, acc[i]) ? [acc[i][0] + dx, acc[i][1] + dy] : acc[i]
    return acc
  }, rope.knots.map(a => a.slice()))
  return {
    knots: knots,
    tail: [...rope.tail, knots.at(-1)],
  }
}

const moves2 = {
  U: (current, count) => range(count).reduce((acc) => move2(acc, [1, 0]), current),
  D: (current, count) => range(count).reduce((acc) => move2(acc, [-1, 0]), current),
  L: (current, count) => range(count).reduce((acc) => move2(acc, [0, -1]), current),
  R: (current, count) => range(count).reduce((acc) => move2(acc, [0, 1]), current),
}

const pairs2 = input.split('\n').map((pair) => pair.split(' '))
  .reduce((acc, [direction, count]) => moves2[direction](acc, +count), {
    knots: range(10).map(() => [0, 0]),
    tail: [],
  })

console.log(new Set(pairs2.tail.map((s) => s.join('_'))).size)