advent-of-code/2020/20/solution1.js

const fs = require("fs");

const input = fs.readFileSync("input", "utf-8").trim();

const tiles = Object.fromEntries(
  input
    .split("\n\n")
    .map((tile) => tile.split("\n"))
    .map((line) => [line[0].match("[0-9]+")[0], line.slice(1)])
);

// console.log(tiles);

const tileEdges = {};

for (const [id, tile] of Object.entries(tiles)) {
  const n = tile[0];
  const w = tile.map((line) => line[0]).join("");
  const e = tile.map((line) => line[line.length - 1]).join("");
  const s = tile[tile.length - 1];
  tileEdges[id] = [n, w, e, s];
}

// console.log(tileEdges);

const edgeMap = {};

const reverse = (str) => str.split("").reverse().join("");
const isCanonical = (edge) => edge < reverse(edge);

for (const [id, edges] of Object.entries(tileEdges)) {
  for (const edge of edges) {
    const key = isCanonical(edge) ? edge : reverse(edge);
    if (typeof edgeMap[key] === "undefined") {
      edgeMap[key] = [];
    }
    edgeMap[key].push(id);
    if (edgeMap[key].length > 2) {
      console.warn(
        "Assumption does not hold: One edge is present in more than two tiles"
      );
    }
  }
}

// console.log(edgeMap);

const edgeTiles = new Set();
const cornerTiles = new Set();

for (const ids of Object.values(edgeMap)) {
  if (ids.length === 1) {
    const id = ids[0];
    if (edgeTiles.has(id)) {
      edgeTiles.delete(id);
      cornerTiles.add(id);
    } else {
      edgeTiles.add(id);
    }
  }
}

console.log(Array.from(cornerTiles).reduce((a, b) => a * b, 1));