Day 15

Setup

Mix.install([
  {:kino, "~> 0.4.1"},
  {:libgraph, "~> 0.7"}
])

input = Kino.Input.textarea("Please provide your input: ")

input =
  input
  |> Kino.Input.read()
  |> String.split("\n")

defmodule AOC.DayFifteen do
  def solve(grid) do
    graph = Graph.new()

    graph =
      Enum.reduce(grid, graph, fn {{x, y}, _weight}, acc ->
        edge_1 = grid[{x, y + 1}]
        edge_2 = grid[{x + 1, y}]

        cond do
          nil != edge_1 and nil != edge_2 ->
            Graph.add_edges(
              acc,
              [
                Graph.Edge.new({x, y}, {x, y + 1}, weight: edge_1),
                Graph.Edge.new({x, y}, {x + 1, y}, weight: edge_2)
              ]
            )

          nil != edge_1 ->
            Graph.add_edge(
              acc,
              Graph.Edge.new({x, y}, {x, y + 1}, weight: edge_1)
            )

          nil != edge_2 ->
            Graph.add_edge(
              acc,
              Graph.Edge.new({x, y}, {x + 1, y}, weight: edge_2)
            )

          true ->
            acc
        end
      end)

    {{min, _}, {max, _}} = Enum.min_max(grid)

    [_head | tail] =
      graph
      |> Graph.dijkstra(min, max)

    tail
    |> Enum.reduce(0, fn point, sum -> sum + grid[point] end)
  end

  def inc(x, n) do
    cond do
      n + x > 9 -> n + x - 9
      :else -> n + x
    end
  end
end

Part 1

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

AOC.DayFifteen.solve(grid)

{_, {{max, _}, _}} = Enum.min_max(grid)
max = max + 1

grid =
  grid
  |> Enum.reduce(%{}, fn {{x, y}, cost}, acc ->
    Map.merge(acc, %{
      {x, y} => cost,
      {x + max, y} => AOC.DayFifteen.inc(cost, 1),
      {x + max * 2, y} => AOC.DayFifteen.inc(cost, 2),
      {x + max * 3, y} => AOC.DayFifteen.inc(cost, 3),
      {x + max * 4, y} => AOC.DayFifteen.inc(cost, 4)
    })
  end)
  |> Enum.reduce(%{}, fn {{x, y}, cost}, acc ->
    Map.merge(acc, %{
      {x, y} => cost,
      {x, y + max} => AOC.DayFifteen.inc(cost, 1),
      {x, y + max * 2} => AOC.DayFifteen.inc(cost, 2),
      {x, y + max * 3} => AOC.DayFifteen.inc(cost, 3),
      {x, y + max * 4} => AOC.DayFifteen.inc(cost, 4)
    })
  end)

AOC.DayFifteen.solve(grid)