const fs = require("fs");

function setCharAt(str, index, chr) {
  if (index > str.length - 1) return str;
  return str.substring(0, index) + chr + str.substring(index + 1);
}

const input = fs.readFileSync("input", "utf-8");
const rows = input.split("\n");

console.log(rows.slice(0, 10));

let countTrees = 0;

let curX = 0;
for (let curY = 1; curY < rows.length; curY++) {
  curX = (curX + 3) % rows[curY].length;
  const row = rows[curY];
  const field = rows[curY][curX];
  rows[curY] = setCharAt(row, curX, field === "#" ? "X" : "O");
  if (field === "#") countTrees++;
}

console.log(rows.slice(0, 10));
console.log(countTrees);