function part1(inputText) {
  const gridWidth = inputText.indexOf('\n');
  const lines = inputText.trim().split('\n');
  const gridHeight = lines.length;
  let accessibleRolls = 0;
  for (let y = 0; y < gridHeight; y++) {
    for (let x = 0; x < gridWidth; x++) {
      if (lines[y][x] === '@') {
        let occupiedNeighbors = 0;
        for (const [neighborX, neighborY] of [
            [x - 1, y],
            [x + 1, y],
            [x, y - 1],
            [x, y + 1],
            [x - 1, y - 1],
            [x - 1, y + 1],
            [x + 1, y - 1],
            [x + 1, y + 1],
          ]) {
          if (neighborX < 0 || neighborX >= gridWidth || neighborY < 0 || neighborY >= gridHeight) {
            continue;
          }
          if (lines[neighborY][neighborX] === '@') {
            occupiedNeighbors++;
          }
        }
        if (occupiedNeighbors < 4) {
          accessibleRolls++;
        }
      }
    }
  }
  return accessibleRolls;
}
{
  const start = performance.now()
  const result = part1(document.body.textContent)
  const end = performance.now()
  console.info({day: 4, part: 1, time: end - start, result})
}

function serializeCoords(x, y, gridWidth) {
  const leftShiftAmount = Math.ceil(Math.log10(gridWidth));
  return x * (10 ** leftShiftAmount) + y;
}
/*
{
  const x = 3;
  const y = 4;
  const gridWidth = 13;
  const serialized = serializeCoords(x, y, gridWidth);
  console.debug({ x, y, gridWidth, serialized });
}
*/

function deserializeCoords(serialized, gridWidth) {
  const leftShiftAmount = Math.ceil(Math.log10(gridWidth));
  const x = Math.floor(serialized / (10 ** leftShiftAmount));
  const y = serialized - x * 10 ** leftShiftAmount;
  return [x, y];
}
/*
{
  const serialized = 304;
  const gridWidth = 13;
  const [x, y] = deserializeCoords(serialized, gridWidth);
  console.debug({ serialized, gridWidth, x, y });
}
*/
function part2(inputText) {
  const gridWidth = inputText.indexOf('\n');
  const lines = inputText.trim().split('\n');
  const gridHeight = lines.length;
  let removed = new Set();
  while (true) {
    const toRemove = new Set();
    for (let y = 0; y < gridHeight; y++) {
      for (let x = 0; x < gridWidth; x++) {
        const serialized = serializeCoords(x, y, gridWidth);
        if (lines[y][x] === '@' && !removed.has(serialized)) {
          let occupiedNeighbors = 0;
          for (const [neighborX, neighborY] of [
              [x - 1, y],
              [x + 1, y],
              [x, y - 1],
              [x, y + 1],
              [x - 1, y - 1],
              [x - 1, y + 1],
              [x + 1, y - 1],
              [x + 1, y + 1],
            ]) {
            if (neighborX < 0 || neighborX >= gridWidth || neighborY < 0 || neighborY >= gridHeight) {
              continue;
            }
            const serializedNeighbor = serializeCoords(neighborX, neighborY, gridWidth);
            if (lines[neighborY][neighborX] === '@' && !removed.has(serializedNeighbor)) {
              occupiedNeighbors++;
            }
          }
          if (occupiedNeighbors < 4) {
            toRemove.add(serialized);
          }
        }
      }
    }
    if (toRemove.size === 0) {
      break;
    }
    removed = removed.union(toRemove);
  }
  return removed.size;
}
/*
{
  const exampleText = `..@@.@@@@.
@@@.@.@.@@
@@@@@.@.@@
@.@@@@..@.
@@.@@@@.@@
.@@@@@@@.@
.@.@.@.@@@
@.@@@.@@@@
.@@@@@@@@.
@.@.@@@.@.
`;
  const start = performance.now();
  const result = part2(exampleText);
  const end = performance.now();
  console.info({ day: 4, part: 2, time: end - start, result });
}
*/

{
  const start = performance.now()
  const result = part2(document.body.textContent)
  const end = performance.now()
  console.info({ day: 4, part: 2, time: end - start, result });
}