Advent of Code - Day 4

Mix.install([
  {:kino_aoc, "~> 0.1"}
])

Problem

https://adventofcode.com/2024/day/4

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2024", "4", System.fetch_env!("LB_AOC_SESSION"))

example_input2 = "....XXMAS.
.SAMXMS...
...S..A...
..A.A.MS.X
XMASAMX.MM
X.....XA.A
S.S.S.S.SS
.A.A.A.A.A
..M.M.M.MM
.X.X.XMASX"

example_input = "MMMSXXMASM
MSAMXMSMSA
AMXSXMAAMM
MSAMASMSMX
XMASAMXAMM
XXAMMXXAMA
SMSMSASXSS
SAXAMASAAA
MAMMMXMMMM
MXMXAXMASX"

defmodule Day4 do
  @all_directions [
    {-1, -1}, {-1, 0}, {-1, 1}, 
    {0, -1},           {0, 1}, 
    {1, -1}, {1, 0}, {1, 1}
  ]
  @diag_directions [
    {-1, -1}, {-1, 1}, 
    {1, -1}, {1, 1}
  ]
  
def to_grid(str) do
    str
    |> String.split("\n", trim: true)
    |> Enum.map(&String.graphemes/1)
  end

  def elem_at(grid, {x, y}) do
    if x > -1 && y > -1 do
      row = grid |> Enum.at(y)
      if row do
        row |> Enum.at(x)
      end
    end
  end

  def check_position(grid, {x, y}, directions, search_chars) do
    goal_length = length(search_chars)

    Enum.filter(directions, fn {dx, dy} ->
      Enum.all?(1..goal_length - 1, fn i ->
        elem_at(grid, {x + dx * i, y + dy * i}) == Enum.at(search_chars, i)
      end)
    end)
  end

  def grid_positions(grid) do
    for y <- 0..(length(grid) - 1),
        x <- 0..(length(Enum.at(grid, 0)) - 1),
        do: {x, y}
  end

  def gen_solve(grid, directions, search_word) do
    search_list = String.graphemes(search_word)
    start_char = Enum.at(search_list, 0)

    grid_positions(grid)
    |> Enum.filter(fn {x, y} -> elem_at(grid, {x, y}) == start_char end)
    |> Enum.reduce([], fn {x, y}, acc ->

      dirs = check_position(grid, {x, y}, directions, search_list) |> Enum.map(fn dir -> {{x,y},dir} end)
      
      acc ++ dirs
    end)
  end

  def solve_part1(str) do
    str |> to_grid() |> gen_solve(@all_directions, "XMAS")  |> length
  end

  def solve_part2(str) do
    str |> to_grid() |> gen_solve(@diag_directions, "MAS") |> Enum.map(fn {{x, y}, {dx, dy}} ->
        {x + dx, y + dy}
      end) |> Enum.frequencies() |> Enum.filter(fn {_k,v} ->
      v >= 2
      end) |> length
  end
end

grid = Day4.to_grid(example_input2)

Day4.solve_part1(puzzle_input)

Part 2

example_input3 = ".M.S......
..A..MSMS.
.M.S.MAA..
..A.ASMSM.
.M.S.M....
..........
S.S.S.S.S.
.A.A.A.A..
M.M.M.M.M.
.........."

Day4.solve_part2(example_input3)

Day4.solve_part2(puzzle_input)