Advent of Code 2021 - Day 5Hydrothermal Venture

| Problem statement | Source code | Tags: Grid walking

Part 1

It happens that the coordinates are less than 1000, so this is doable with a fixed-size 2D array of size 1000x1000 without any kind of coordinate compression (coordinate compression won't work for part 2 with diagonal lines anyway). Each cell in the grid represents how many lines pass through that point.

if (x1 === x2) {
  for (let y = Math.min(y1, y2); y <= Math.max(y1, y2); y++) {
    grid[y][x1]++;
  }
} else if (y1 === y2) {
  for (let x = Math.min(x1, x2); x <= Math.max(x1, x2); x++) {
    grid[y1][x]++;
  }
}

Part 2

We just need to add another case for diagonal lines. This can be done by calculating the step for x and y separately, then iterating from the start point to the end point using those steps.

const dirX = x2 > x1 ? 1 : -1;
const dirY = y2 > y1 ? 1 : -1;
for (let x = x1, y = y1; x !== x2 + dirX; x += dirX, y += dirY) {
  grid[y][x]++;
}