AoC 2024 Day 15

aoc-2024-day-15.js

import { pipe } from "@/lib/utils.js";

// prettier-ignore
const CMD = { "^": [-1, 0], ">": [0, 1], "v": [1, 0], "<": [0, -1], };

const parse = (input, [g, cmds] = input.split("\n\n")) => [
  g.split("\n").map(row => [...row]),
  [...cmds.split("\n").join("")].map(c => CMD[c]),
];

const run = ([grid, commands]) => {
  let py = grid.findIndex(row => row.includes("@"));
  let px = grid[py].indexOf("@");

  commands.forEach(([dy, dx]) => {
    const move = (y, x) => {
      const c = grid[y][x];
      if (".#".includes(c)) {
        return c === ".";
      }
      if (move(y + dy, x + dx)) {
        grid[y + dy][x + dx] = c;
        return true;
      }
      return false;
    };

    if (move(py, px)) {
      grid[py][px] = ".";
      py = py + dy;
      px = px + dx;
    }
  });

  return grid;
};

const score = grid =>
  grid
    .flatMap((row, y) =>
      row.map((c, x) =>
        "[O".includes(c) ? 100 * y + x : 0
      )
    )
    .reduce((sum, v) => sum + v);

export const part1 = pipe(parse, run, score);

const scaleX = ([grid, commands]) => [
  grid.map(row =>
    row.flatMap(
      c => ({ O: ["[", "]"], "@": ["@", "."] }[c] ?? [c, c])
    )
  ),
  commands,
];

const run2 = ([grid, commands]) => {
  let py = grid.findIndex(row => row.includes("@"));
  let px = grid[py].indexOf("@");

  commands.forEach(([dy, dx]) => {
    const testMove = (y, x) => {
      const c = grid[y][x];
      if (".#".includes(c)) return c === ".";
      if (dx || c === "@") return testMove(y + dy, x + dx);
      return (
        testMove(y + dy, x + (c === "[" ? 1 : -1)) &&
        testMove(y + dy, x)
      );
    };

    const move = (y, x) => {
      const c = grid[y][x];
      if ("." === c) return;
      if (dx || c === "@") {
        move(y + dy, x + dx);
        grid[y + dy][x + dx] = c;
      } else {
        const x2 = x + (c === "[" ? 1 : -1);
        move(y + dy, x2);
        move(y + dy, x);
        grid[y + dy][x] = c;
        grid[y + dy][x2] = c === "[" ? "]" : "[";
        grid[y][x2] = ".";
      }
    };

    if (testMove(py, px)) {
      move(py, px);
      grid[py][px] = ".";
      py = py + dy;
      px = px + dx;
    }
  });

  return grid;
};

export const part2 = pipe(parse, scaleX, run2, score);