Day - 09

Setup

Mix.install([
  {:kino, "~> 0.4.1"},
  {:vega_lite, "~> 0.1.2"}
])

alias VegaLite, as: Vl

input = Kino.Input.textarea("Please enter your Input")

lines =
  input
  |> Kino.Input.read()
  |> String.split()

grid =
  for {line, row} <- Enum.with_index(lines),
      {number, col} <- Enum.with_index(String.to_charlist(line)),
      into: %{} do
    {{row, col}, number - ?0}
  end

low_points =
  grid
  |> Enum.filter(fn {{row, col}, value} ->
    up = grid[{row - 1, col}]
    down = grid[{row + 1, col}]
    left = grid[{row, col - 1}]
    right = grid[{row, col + 1}]
    value < up and value < down and value < left and value < right
  end)

Part 1

low_points
|> Enum.map(fn {_, value} -> value + 1 end)
|> Enum.sum()

Part 2

defmodule Recursion do
  def basin(point, grid) do
    basin(MapSet.new(), point, grid)
  end

  defp basin(set, {row, col} = point, grid) do
    if grid[point] in [9, nil] or point in set do
      set
    else
      set
      |> MapSet.put(point)
      |> basin({row - 1, col}, grid)
      |> basin({row + 1, col}, grid)
      |> basin({row, col - 1}, grid)
      |> basin({row, col + 1}, grid)
    end
  end
end

low_points
|> Enum.map(fn {point, _} ->
  point
  |> Recursion.basin(grid)
  |> MapSet.size()
end)
|> Enum.sort(:desc)
|> Enum.take(3)
|> Enum.product()

Visualizations

Vl.new(width: 800, height: 800)
|> Vl.data_from_values(Enum.map(grid, fn {{x, y}, h} -> %{"x" => x, "y" => y, "h" => h} end))
|> Vl.mark(:circle, size: 65, opacity: 0.8)
|> Vl.encode_field(:x, "x", type: :quantitative, axis: false)
|> Vl.encode_field(:y, "y", type: :quantitative, axis: false)
|> Vl.encode_field(:color, "h", type: :quantitative, scale: [range: ["red", "white", "green"]])