Day 9

Setup

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

input = Kino.Input.textarea("Problem input")

input_map =
  input
  |> Kino.Input.read()
  |> String.split("\n")
  |> Enum.map(fn x ->
    String.split(x, "", trim: true)
    |> Enum.map(&String.to_integer/1)
  end)
  |> Enum.reduce({%{}, {0, 0}}, fn row, acc ->
    row
    |> Enum.reduce(acc, fn height, {map, {x, y}} ->
      {Map.put(map, {x, y}, height), {x, y + 1}}
    end)
    |> then(fn {map, {x, _y}} -> {map, {x + 1, 0}} end)
  end)
  |> then(fn {map, _} -> map end)

Part 1

defmodule Solver do
  def low_point?({x, y}, map) do
    top = map[{x - 1, y}] || 10
    bottom = map[{x + 1, y}] || 10
    left = map[{x, y - 1}] || 10
    right = map[{x, y + 1}] || 10

    Enum.all?([top, bottom, left, right], fn adj -> adj > map[{x, y}] end)
  end

  def score({_, height}) do
    height + 1
  end
end

input_map
|> Enum.filter(fn {{x, y}, _height} -> Solver.low_point?({x, y}, input_map) end)
|> Enum.map(&Solver.score/1)
|> Enum.sum()