Day 9

Mix.install([
  {:kino, "~> 0.6.2"}
])

Input

input = Kino.Input.textarea("In put:")

map =
  input
  |> Kino.Input.read()
  |> String.split("\n", trim: true)
  |> Enum.map(fn x -> String.to_charlist(x) |> Enum.map(&(&1 - ?0)) end)
  |> IO.inspect()

m =
  for {line, l} <- Enum.with_index(map),
      {val, c} <- Enum.with_index(line) do
    {{l, c}, val}
  end
  |> Enum.into(%{})

low_points =
  Enum.filter(m, fn {{x, y}, h} ->
    [{x - 1, y}, {x, y - 1}, {x + 1, y}, {x, y + 1}]
    |> Enum.all?(fn c ->
      is_nil(m[c]) or m[c] > h
    end)
  end)

Part 1

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

Part 2

neighbors = fn {x, y} ->
  [{x - 1, y}, {x, y - 1}, {x + 1, y}, {x, y + 1}]
end

grow_basin_rec = fn
  d, c, rec ->
    if c in d or m[c] in [9, nil] do
      d
    else
      Enum.reduce(
        neighbors.(c),
        [c | d],
        &rec.(&2, &1, rec)
      )
    end
end

recurse_basin = fn {c, _} -> grow_basin_rec.([], c, grow_basin_rec) end

Enum.map(
  low_points,
  recurse_basin
)
|> Enum.map(&length/1)
|> Enum.sort()
|> Enum.reverse()
|> Enum.take(3)
|> Enum.reduce(&(&1 * &2))