🎄 Year 2022 🔔 Day 12

Parse input

filename = "#{__DIR__}/../../../inputs/2022/day12.txt"

{coords, {start_coord, end_coord}} =
  File.stream!(filename)
  |> Enum.map(&String.trim/1)
  |> Enum.map(&String.to_charlist/1)
  |> Enum.with_index(fn list, y ->
    Enum.with_index(list, fn e, x -> {{x, y}, e} end)
  end)
  |> Enum.concat()
  |> Enum.map_reduce({nil, nil}, fn e, {start_coord, end_coord} ->
    case e do
      {coord, 83} -> {{coord, 97}, {coord, end_coord}}
      {coord, 69} -> {{coord, 122}, {start_coord, coord}}
      _ -> {e, {start_coord, end_coord}}
    end
  end)

coords = Map.new(coords)

Part One

defmodule Helper do
  def get_adjacent({{x, y}, e}, coords) do
    [{x, y + 1}, {x + 1, y}, {x, y - 1}, {x - 1, y}]
    |> Enum.map(fn coord -> {coord, Map.get(coords, coord, :error)} end)
    |> Enum.filter(fn {_, v} -> v != :error end)
    |> Enum.filter(fn {_, v} -> v <= e + 1 end)
  end
end

starting_state = %{
  steps: 0,
  current_coords: [{start_coord, Map.fetch!(coords, start_coord)}],
  coords: coords
}

Stream.iterate(starting_state, fn acc ->
  %{steps: steps, current_coords: current_coords, coords: coords} = acc
  steps = steps + 1
  coords = coords |> Enum.reject(fn e -> Enum.any?(current_coords, &(&1 == e)) end) |> Map.new()

  current_coords =
    current_coords |> Enum.flat_map(&Helper.get_adjacent(&1, coords)) |> Enum.uniq()

  %{steps: steps, current_coords: current_coords, coords: coords}
end)
|> Enum.find_value(fn %{steps: steps, current_coords: current_coords} ->
  if Enum.any?(current_coords, fn {coord, _} -> coord == end_coord end) || current_coords == [],
    do: steps,
    else: false
end)

Part two

defmodule Helper do
  def get_adjacent({{x, y}, e}, coords) do
    [{x, y + 1}, {x + 1, y}, {x, y - 1}, {x - 1, y}]
    |> Enum.map(fn coord -> {coord, Map.get(coords, coord, :error)} end)
    |> Enum.filter(fn {_, v} -> v != :error end)
    |> Enum.filter(fn {_, v} -> e <= v + 1 end)
  end
end

starting_state = %{
  steps: 0,
  current_coords: [{end_coord, Map.fetch!(coords, end_coord)}],
  coords: coords
}

Stream.iterate(starting_state, fn acc ->
  %{steps: steps, current_coords: current_coords, coords: coords} = acc
  steps = steps + 1
  coords = coords |> Enum.reject(fn e -> Enum.any?(current_coords, &(&1 == e)) end) |> Map.new()

  current_coords =
    current_coords |> Enum.flat_map(&Helper.get_adjacent(&1, coords)) |> Enum.uniq()

  %{steps: steps, current_coords: current_coords, coords: coords}
end)
|> Enum.find_value(fn %{steps: steps, current_coords: current_coords} ->
  if Enum.any?(current_coords, fn {_, v} -> v == 97 end) || current_coords == [],
    do: steps,
    else: false
end)