Advent of Code 2024 - Day 4

Learning Questions & Discussions Coding Challenges
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4

I feel part 2 is actually easier.

defmodule AoC2024.Day04 do
  def part_1(grid) do
    directions =
      for di <- -1..1,
          dj <- -1..1,
          di != 0 or dj != 0,
          do: {di, dj}

    directions
    |> Enum.map(fn {di, dj} ->
      Enum.count(Map.keys(grid), fn coord ->
        coord
        |> Stream.iterate(fn {i, j} -> {i + di, j + dj} end)
        |> Stream.take(4)
        |> Enum.map(&grid[&1])
        |> Kernel.==(~c"XMAS")
      end)
    end)
    |> Enum.sum()
  end

  def part_2(grid) do
    Enum.count(grid, fn
      {{i, j}, ?A} ->
        [grid[{i - 1, j - 1}], grid[{i + 1, j + 1}]] in [~c"MS", ~c"SM"] and
        [grid[{i - 1, j + 1}], grid[{i + 1, j - 1}]] in [~c"MS", ~c"SM"]
        
      _ ->
        false
    end)
  end
end

where grid is built like this:

charlists = puzzle_input |> String.split() |> Enum.map(&String.to_charlist/1)

grid =
  for {row, i} <- Enum.with_index(charlists),
      {char, j} <- Enum.with_index(row),
      into: %{},
      do: {{i, j}, char}
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