part2.js

const { readFileSync } = require("fs");
// Parse Input
const inputFile = "input.txt";
const input = readFileSync(inputFile).toString();
const grid = [];
input
  .trim()
  .split("\n")
  .forEach((line) => grid.push(line.split("")));

let antenaPositions = [];

for (let y = 0; y < grid.length; y++) {
  const line = grid[y];
  for (let x = 0; x < line.length; x++) {
    const element = line[x];
    if (element != ".") {
      antenaPositions.push({
        id: antenaPositions.length,
        type: element,
        x,
        y,
      });
    }

    grid[y][x] = element;
  }
}

function isInGrid(x, y) {
  return x >= 0 && y >= 0 && x < grid[0].length && y < grid.length;
}

const antinodes = [...antenaPositions];
antenaPositions.forEach((a) => {
  antenaPositions
    .filter((ca) => a.type == ca.type && ca.id != a.id)
    .forEach((o) => {
      let xDiff = o.x - a.x;
      let yDiff = o.y - a.y;
      let antinode = { type: a.type, x: a.x - xDiff, y: a.y - yDiff };

      if (xDiff == 0 && yDiff == 0) {
        antinodes.push(antinode);
      } else {
        while (true) {
          if (isInGrid(antinode.x, antinode.y)) {
            if (
              !antinodes.find((n) => n.x == antinode.x && n.y == antinode.y)
            ) {
              antinodes.push(antinode);
            }

            antinode = {
              type: a.type,
              x: antinode.x - xDiff,
              y: antinode.y - yDiff,
            };
          } else {
            break;
          }
        }
      }
    });
});

console.log(antinodes.length);