Day 04

Mix.install([:kino_aoc])

Kino.configure(inspect: [charlists: :as_lists])

Input

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

parsed = puzzle_input
  |> String.split("\n")
  |> Stream.with_index()
  |> Enum.map(fn {line, iy} -> 
    {
      iy,
      line
      |> String.split("", trim: true)
      |> Stream.with_index()
      |> Enum.map(fn {char, ix} -> {ix, char} end)
      |> Map.new()
    }
  end)
  |> Map.new()

Part 1

defmodule Part1 do
  def count_matches(parsed) do
    Enum.reduce(parsed, 0, fn {iy, line}, acc -> 
      acc + Enum.reduce(line, 0, fn 
        {ix, "X"}, acc2 -> acc2 + matches_xmas?(parsed, iy, ix)
        _, acc2 -> acc2
      end)  
    end)
  end

  def matches_xmas?(parsed, iy, ix) do
    directions = [
      {0, 1}, {1, 0}, {0, -1}, {-1, 0}, {1, 1}, {1, -1}, {-1, 1}, {-1, -1}
    ]

    Enum.reduce(directions, 0, fn {dy, dx}, acc -> 
      acc + (if parsed[iy + dy][ix + dx] == "M" and
        parsed[iy + 2*dy][ix + 2*dx] == "A" and 
        parsed[iy + 3*dy][ix + 3*dx] == "S", do: 1, else: 0)
    end)
  end
end

Part1.count_matches(parsed)

Part 2

defmodule Part2 do
  def count_matches(parsed) do
    Enum.reduce(parsed, 0, fn {iy, line}, acc -> 
      acc + Enum.reduce(line, 0, fn {ix, char}, acc2 -> 
        acc2 + if char == "A" and matches_xmas?(parsed, iy, ix), do: 1, else: 0
      end)  
    end)
  end

  def matches_xmas?(parsed, iy, ix) do
    directions = [
      {1, 1}, {1, -1}, {-1, 1}, {-1, -1}
    ]

    2 <= Enum.reduce(directions, 0, fn {dy, dx}, acc -> 
      acc + if parsed[iy + dy][ix + dx] == "M" and parsed[iy - dy][ix - dx] == "S", do: 1, else: 0
    end)
  end
end

Part2.count_matches(parsed)