Day 20

Mix.install([{:utils, path: "#{__DIR__}/utils"}, {:heap, "~> 3.0"}])

Solution

defmodule Day20 do
  def nbrs({x, y}), do: [{x + 1, y}, {x - 1, y}, {x, y + 1}, {x, y - 1}]

  def path(grid, curr, i, path) do
    path = Map.put(path, curr, i)

    if Map.get(grid, curr) == ?E do
      path
    else
      bad = fn nbr -> Map.has_key?(path, nbr) or Map.get(grid, nbr) == ?# end
      [nbr] = curr |> nbrs() |> Enum.reject(bad)
      path(grid, nbr, i + 1, path)
    end
  end

  def cheats(path, {x, y}, d) do
    curr = Map.get(path, {x, y})

    for i <- -d..d, j <- -d..d, t <- 2..d, abs(i) + abs(j) == t do
      Map.get(path, {x + i, y + j}, 0) - curr - t
    end
    |> Enum.reject(&amp;(&amp;1 <= 0))
  end
end

{grid, oob?} = Utils.read_grid("day20")
start = grid |> Enum.find(fn {_, v} -> v == ?S end) |> elem(0)
path = Day20.path(Map.new(grid), start, 0, %{})

path
|> Map.keys()
|> Enum.flat_map(&amp;Day20.cheats(path, &amp;1, 20))
|> Enum.count(&amp;(&amp;1 >= 100))