this post was submitted on 04 Dec 2025
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Advent Of Code

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An unofficial home for the advent of code community on programming.dev! Other challenges are also welcome!

Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

Everybody Codes is another collection of programming puzzles with seasonal events.

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๐ŸŽ„ - 2025 DAY 4 SOLUTIONS - ๐ŸŽ„ (programming.dev)
submitted 2 weeks ago by CameronDev@programming.dev to c/advent_of_code@programming.dev
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Day 4: Printing Department

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[โ€“] Jayjader@jlai.lu 1 points 1 week ago

(Browser-based) Javascript

This was a good opportunity to refresh my grasp on the math involved in losslessly stuffing a tuple into a single number. JS-in-the-browser has Sets and Maps but no Tuples, and Arrays are indexed on their id / memory handle instead of their value contents, so if you want to put coordinates into a set or map and have the collection behave as expected you need to serialize the coordinates into a primitive type. Stuff it into a string if you don't want to think too hard. For this specific problem we don't even need to be able to compute the original coordinates (just count the unique removed points) but implementing that computation was a handy way to verify the "serializer" was working correctly.

Seeing as the record tuple proposal was withdrawn in February of this year this is still a technique worth knowing when working with coords in JS.

Code

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 });
}