Advent-of-Code-2022/12/solution.mjs

import { readFileSync } from 'node:fs';

const input = readFileSync('input', 'utf-8');

function getMap() {
  return input
    .split('\n')
    .filter(line => line)
    .map((line, y) => line
      .split('')
      .map((elevation, x) => {
        const isStart = elevation === 'S';
        const isEnd = elevation === 'E';
        elevation = elevation.replaceAll('S', 'a');
        elevation = elevation.replaceAll('E', 'z');
        const height = elevation.charCodeAt(0) - 97;
        return {
          x,
          y,
          height,
          isStart,
          isEnd,
          path: null
        };
      })
    );
}

const cardinals = [
  [0, -1], // North
  [1, 0],  // East
  [0, 1],  // South
  [-1, 0]  // West
];

// Recursively walk through map stopping whenever we encounter a square with a
// shorter path than ours
function solve(map, x, y, canMoveToSquare, path = []) {
  const square = map[y][x];
  path = [...path, [x, y]];

  if (square.path && square.path.length <= path.length) {
    return;
  }

  square.path = path;

  cardinals
    .map(c => map[y + c[1]]?.[x + c[0]])
    // Filter undefined squares
    .filter(s => s)
    // Filter squares we can't reach
    .filter(s => canMoveToSquare(square, s))
    .forEach(s => solve(map, s.x, s.y, canMoveToSquare, [...path]));
}

function shortestPathStartToEnd() {
  const map = getMap();
  const start = map.flat().find(square => square.isStart);
  solve(
    map,
    start.x,
    start.y,
    (from, to) => to.height <= from.height + 1
  );
  const end = map.flat().find(square => square.isEnd);
  return end.path;
}

function shortestPathEndToA() {
  const map = getMap();
  const end = map.flat().find(square => square.isEnd);
  solve(
    map,
    end.x,
    end.y,
    (from, to) => to.height >= from.height - 1
  );

  const squareWithShortestDistance = map
    .flat()
    .filter(square => square.path)
    .filter(square => square.height === 0)
    .sort((s1, s2) => s1.path.length - s2.path.length)[0];

  return squareWithShortestDistance.path;
}


console.log('This will take a minute...');

let path;

path = shortestPathStartToEnd();
console.log(`The least steps from start to end is: ${path.length - 1}`);

path = shortestPathEndToA();
console.log(`The least steps from 'a' to end is: ${path.length - 1}`);