#!/usr/bin/env elixir
defmodule Day10 do
  def part1({grid, zeros}) do
    zeros
    |> Enum.map(fn point -> point |> find_trails(grid, 0) |> Enum.uniq() |> Enum.count() end)
    |> Enum.sum()
  end

  def find_trails(point, _grid, 9), do: [point]

  def find_trails({x, y}, grid, height) do
    find_neighbours(x, y, height + 1, grid)
    |> Enum.flat_map(fn {point, _} -> find_trails(point, grid, height + 1) end)
  end

  def find_neighbours(x, y, height, grid) do
    grid
    |> Map.take([{x - 1, y}, {x + 1, y}, {x, y - 1}, {x, y + 1}])
    |> Enum.filter(&match?({_, ^height}, &1))
  end

  def part2({grid, zeros}) do
    zeros
    |> Enum.map(fn point -> point |> find_trails(grid, 0) |> Enum.count() end)
    |> Enum.sum()
  end

  def input do
    with [input_filename] <- System.argv(),
         {:ok, input} <- File.read(input_filename) do
      {grid, zeros, _, _} =
        for <<char::binary-1 <- input>>, reduce: {%{}, [], 0, 0} do
          {grid, zeros, x, y} ->
            case char do
              "\n" -> {grid, zeros, 0, y + 1}
              "0" -> {Map.put(grid, {x, y}, 0), [{x, y} | zeros], x + 1, y}
              char -> {Map.put(grid, {x, y}, String.to_integer(char)), zeros, x + 1, y}
            end
        end

      {grid, zeros}
    else
      _ -> :error
    end
  end

  #######################
  # HERE BE BOILERPLATE #
  #######################

  def run do
    case input() do
      :error -> print_usage()
      input -> run_parts_with_timer(input)
    end
  end

  defp run_parts_with_timer(input) do
    run_with_timer(1, fn -> part1(input) end)
    run_with_timer(2, fn -> part2(input) end)
  end

  defp run_with_timer(part, fun) do
    {time, result} = :timer.tc(fun)
    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
    IO.puts("#{inspect(result)}\n")
  end

  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
  defp format_time(μsec), do: "#{μsec / 1_000_000}s"

  defp print_usage do
    IO.puts("Usage: elixir day10.exs input_filename")
  end
end

Day10.run()