Edd Mann Developer

Advent of Code 2015 - Day 18 - Like a GIF For Your Yard

On the eighteenth day of Advent of Code 2015 we are asked to re-arrange the light show we created on a previous day.

Part 1

We are supplied with an initial 100x100 grid of light states (on or off) and told that Santa has provided us with rules to follow to produce this new light show. These rules follow the Conway’s Game of Life, like so:

  • A light which is on stays on when 2 or 3 neighbors are on, and turns off otherwise.
  • A light which is off turns on if exactly 3 neighbors are on, and stays off otherwise.

We begin by parsing the provided initial light state grid into a form we can process.

type Light = [x: number, y: number];
type OnLights = CompoundSet<Light>;

const parseGrid = (
  input: string
): [upperBounds: number, lights: OnLights] => {
  const rows = input.split('\n');
  return [
    rows.reduce((on, line, x) => {
      return [...line].reduce(
        (on, light, y) => (light === '#' ? on.add([x, y]) : on),
    }, new CompoundSet<Light>()),

We iterate through the initial grid, building up a CompoundSet of all the lights that are currently on (identified by their x and y coordinates). So as to manage the bounds within future calculations we return the upper-bounds value of the grid (in this case 100) to the callee as well.

From here, we then create a function which supplied with the upper-bounds and the current light coordinates returns all the possible neighbouring light positions.

type Neighbours = (l: Light) => Light[];

const neighboursFor = (upperBounds: number): Neighbours => ([
]: Light): Light[] => {
  const neighbours: Light[] = [];

  for (let dx = -1; dx <= 1; dx++) {
    for (let dy = -1; dy <= 1; dy++) {
      const [nx, ny] = [x + dx, y + dy];
      if (dx === 0 && dy === 0) continue;
      if (nx < 0 || ny < 0) continue;
      if (nx >= upperBounds || ny >= upperBounds) continue;
      neighbours.push([nx, ny]);

  return neighbours;

Their is a little guard logic above to ensure that any adjacent subject light is within the bounds of the grid. Supplying this function partial applied with the desired upper-bounds we are able to begin codifying the animation state transitions.

const animate = (neighboursFor: Neighbours) => (
  lights: OnLights
): OnLights => {
  const onWithNeighours = new CompoundSet<Light>(
      (lights, light) => [...lights, light, ...neighboursFor(light)],

  return new CompoundSet<Light>(
    [...onWithNeighours].filter(light => {
      const active = neighboursFor(light).filter(l => lights.has(l))
      return active === 3 || (active === 2 && lights.has(light));

Based on the provided set of lights that are currenty on, we determine the listing of all these lights including any valid neighbouring light positions. Combined these are all the possible lights that could be returned from this state transition as on. From here, we filter done to only include lights that meet the rules explained above.

Combining all this functionality together (and with the help of the repeat function we created on a previous day), we can iterate over this animation for 100 transitions, and return the desired count of the lights that are on 🌟.

const part1 = (input: string): number => {
  const [upperBounds, lights] = parseGrid(input);
  return repeat(100, animate(neighboursFor(upperBounds)), lights)

Part 2

For part two, we are told that it appears that the lights we bought have stuck corners where-by these lights are always left on. We are told to apply this new characteristic and repeat the 100 iterations from the initial grid state again.

const cornersStuckOn = (upperBounds: number) => (
  lights: OnLights
): OnLights =>
    .add([0, 0])
    .add([0, upperBounds - 1])
    .add([upperBounds - 1, 0])
    .add([upperBounds - 1, upperBounds - 1]);

In a similiar fashion to how we partially applied the neighboursFor function above, we do the same for ensuring that the supplied lights that are on maintain the stuck corners. With this new functionality we can compose all the building blocks together, returning the desired new count of lights that are on 🌟.

const part2 = (input: string): number => {
  const [upperBounds, lights] = parseGrid(input);
  const stuck = cornersStuckOn(upperBounds);
  const action = animate(neighboursFor(upperBounds));
  return repeat(100, (l: OnLights) => stuck(action(l)), stuck(lights))

Upon reflection I did find this solution to be rather slow. I opted for readability and clear problem solving over an optimised answer response. In the future I hope to explore this problem further and improve on its performance characteristics.