Day 10

# Note: when making the next template, something like this works well:
#   `cat day04.livemd | sed 's/10/04/' > day04.livemd`
# When inspecting lists of numbers, use "charlists: :as_lists"
#
Mix.install([
  # Join the string so a copy of dayN to dayM doesn't destroy it.
  {:kino, "~> 0.1" <> "4.2"}
])

# Join the string so a copy of dayN to dayM doesn't destroy it.
IEx.Helpers.c("/Users/johnb/dev/2" <> "0" <> "2" <> "4adventOfCode/advent_of_code.ex")
alias AdventOfCode, as: AOC
alias Kino.Input

Installation and Data

input_example = Kino.Input.textarea("Example Data", monospace: true)

input_puzzleInput = Kino.Input.textarea("Puzzle Input", monospace: true)

input_source_select =
  Kino.Input.select("Source", [{:example, "example"}, {:puzzle_input, "puzzle input"}])

data = fn ->
  (Kino.Input.read(input_source_select) == :example &&
     Kino.Input.read(input_example)) ||
    Kino.Input.read(input_puzzleInput)
end

Solution

defmodule Day10 do
  @start_of_trail 0
  
  def parse(text) do
    text
    |> AOC.as_grid_of_digits()
  end

  def original_trailheads(grid, next_value \\ @start_of_trail) do
    AOC.grid_cells(grid)
      |> Enum.filter(fn cell -> grid[cell] == next_value end)
  end

  def solve1(text) do
    grid = parse(text)

    starter_lists = original_trailheads(grid)
      |> Enum.map(fn zero -> [zero] end)
    
    part2 = 1..9
      |> Enum.reduce(starter_lists, fn next_val, paths ->
        paths
        |> Enum.reduce([], fn [cell | _rest] = path, new_paths ->
          longer_paths = AOC.neighbors4(grid, cell)
            |> Enum.filter(fn neighbor -> grid[neighbor] == next_val end)
            |> Enum.map(fn neighbor -> [neighbor | path] end)
  
          new_paths ++ longer_paths
        end)
      end)

    IO.puts("Part 2 answer: #{Enum.count(part2)}")

    part2
    |> Enum.map(fn list -> [List.last(list), List.first(list)] end)
    |> Enum.uniq()
    |> Enum.count()
  end

  def solve2(text) do
    parse(text)
  end
end

# Example:

data.()
|> Day10.solve1()
|> IO.inspect(label: "\n*** Part 1 solution (example: 36)")
# 629

# data.()
# |> Day10.solve2()
# |> IO.inspect(label: "\n*** Part 2 solution (example: 81)")
# 1242