Day 10

Mix.install([{:utils, path: "#{__DIR__}/utils"}])

Solution

defmodule Day10 do
  defp nbrs({x, y}), do: [{x + 1, y}, {x - 1, y}, {x, y + 1}, {x, y - 1}]

  def peaks(lookup, curr) do
    val = lookup[curr]

    if val == ?9 do
      MapSet.new([curr])
    else
      curr
      |> nbrs()
      |> Enum.filter(&(Map.get(lookup, &1, -1) == val + 1))
      |> Enum.map(&peaks(lookup, &1))
      |> Enum.reduce(MapSet.new(), &MapSet.union/2)
    end
  end

  def rating(lookup, curr) do
    val = lookup[curr]

    if val == ?9 do
      1
    else
      curr
      |> nbrs()
      |> Enum.filter(&(Map.get(lookup, &1, -1) == val + 1))
      |> Enum.map(&rating(lookup, &1))
      |> Enum.sum()
    end
  end
end

{grid, oob?} = Utils.read_grid("day10")
trailheads = grid |> Enum.filter(fn {_, v} -> v == ?0 end) |> Enum.map(&elem(&1, 0))
lookup = Map.new(grid)

# Part 1
trailheads
|> Enum.map(&Day10.peaks(lookup, &1))
|> Enum.map(&Enum.count/1)
|> Enum.sum()
|> IO.inspect()

# Part 2
trailheads
|> Enum.map(&Day10.rating(lookup, &1))
|> Enum.sum()