🎄 Year 2023 🔔 Day 18

Setup

lines =
  File.read!("#{__DIR__}/../../../inputs/2023/day18.txt")
  |> String.split("\n", trim: true)

Part 1

corners =
  lines
  |> Enum.map(fn line ->
    [dir, count, _hex] = String.split(line, " ", trim: true)
    count = String.to_integer(count)

    case dir do
      "U" -> {0, count}
      "D" -> {0, -count}
      "L" -> {-count, 0}
      "R" -> {count, 0}
    end
  end)
  |> then(&[{0, 0} | &1])
  |> Enum.scan({0, 0}, fn {ax, ay}, {bx, by} -> {ax + bx, ay + by} end)

shoelace_result =
  corners
  |> Enum.chunk_every(2, 1, :discard)
  |> Enum.reduce(0, fn [{ax, ay}, {bx, by}], acc ->
    acc + ax * by - ay * bx
  end)
  |> then(&abs(div(&1, 2)))

perimeter =
  corners
  |> Enum.chunk_every(2, 1, :discard)
  |> Enum.reduce(0, fn [{ax, ay}, {bx, by}], acc ->
    acc + abs(ax - bx) + abs(ay - by)
  end)

pick_result = shoelace_result + div(perimeter, 2) + 1

Part 2

corners =
  lines
  |> Enum.map(fn line ->
    [_dir, _count, hex] = String.split(line, " ", trim: true)
    "(#" <> <<count::binary-size(5), dir::binary-size(1)>> <> ")" = hex
    count = String.to_integer(count, 16)

    case dir do
      "0" -> {count, 0}
      "1" -> {0, count}
      "2" -> {-count, 0}
      "3" -> {0, -count}
    end
  end)
  |> then(&[{0, 0} | &1])
  |> Enum.scan({0, 0}, fn {ax, ay}, {bx, by} -> {ax + bx, ay + by} end)

shoelace_result =
  corners
  |> Enum.chunk_every(2, 1, :discard)
  |> Enum.reduce(0, fn [{ax, ay}, {bx, by}], acc ->
    acc + ax * by - ay * bx
  end)
  |> then(&abs(div(&1, 2)))

perimeter =
  corners
  |> Enum.chunk_every(2, 1, :discard)
  |> Enum.reduce(0, fn [{ax, ay}, {bx, by}], acc ->
    acc + abs(ax - bx) + abs(ay - by)
  end)

pick_result = shoelace_result + div(perimeter, 2) + 1