2024 Day 4

Day 4: Ceres Search

Day 4: Ceres Search

Part 1

defmodule AdventOfCode2024Day4Part1 do
  def count_xmas(input) do
    grid = parse_input(input)
    directions = [
      {1, 0},  # 水平
      {0, 1},  # 垂直
      {1, 1},  # 斜め（右下）
      {1, -1}, # 斜め（右上）
      {-1, 0}, # 水平（逆）
      {0, -1}, # 垂直（逆）
      {-1, -1},# 斜め（左上）
      {-1, 1}  # 斜め（左下）
    ]

    Enum.reduce(0..(length(grid) - 1), 0, fn row, acc ->
      Enum.reduce(0..(length(grid) - 1), acc, fn col, acc2 ->
        acc2 + Enum.reduce(directions, 0, fn {dx, dy}, acc3 ->
          acc3 + if match_xmas?(grid, row, col, dx, dy), do: 1, else: 0
        end)
      end)
    end)
  end

  defp parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&String.graphemes/1)
  end

  defp match_xmas?(grid, row, col, dx, dy) do
    word = "XMAS"
    Enum.all?(0..3, fn i ->
      r = row + i * dx
      c = col + i * dy
      r >= 0 and r < length(grid) and c >= 0 and c < length(grid) and Enum.at(grid, r) |> Enum.at(c) == String.at(word, i)
    end)
  end
end

# テストデータ
input = """
MMMSXXMASM
MSAMXMSMSA
AMXSXMAAMM
MSAMASMSMX
XMASAMXAMM
XXAMMXXAMA
SMSMSASXSS
SAXAMASAAA
MAMMMXMMMM
MXMXAXMASX
"""

input
|> AdventOfCode2024Day4Part1.count_xmas()
|> IO.puts()

Part 2

defmodule AdventOfCode2024Day4Part2 do
  def count_xmas(input) do
    grid = parse_input(input)
    size = length(grid)

    Enum.reduce(1..(size - 2), 0, fn row, acc ->
      Enum.reduce(1..(size - 2), acc, fn col, acc2 ->
        acc2 + if match_x_mas?(grid, row, col), do: 1, else: 0
      end)
    end)
  end

  defp parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&String.graphemes/1)
  end

  defp match_x_mas?(grid, row, col) do
    mas_variants = [["M", "A", "S"], ["S", "A", "M"]]

    Enum.any?(mas_variants, fn mas1 ->
      Enum.any?(mas_variants, fn mas2 ->
        match_arms?(grid, row, col, mas1, mas2)
      end)
    end)
  end

  defp match_arms?(grid, row, col, mas1, mas2) do
    [mas1_positions, mas2_positions] = [
      [{-1, -1}, {0, 0}, {1, 1}],
      [{-1, 1}, {0, 0}, {1, -1}]
    ]

    match_arm?(grid, row, col, mas1, mas1_positions) and
      match_arm?(grid, row, col, mas2, mas2_positions)
  end

  defp match_arm?(grid, row, col, [m, a, s], deltas) do
    Enum.zip([m, a, s], deltas)
    |> Enum.all?(fn {char, {dx, dy}} ->
      x = row + dx
      y = col + dy
      in_bounds?(grid, x, y) and Enum.at(grid, x) |> Enum.at(y) == char
    end)
  end

  defp in_bounds?(grid, x, y) do
    x >= 0 and y >= 0 and x < length(grid) and y < length(Enum.at(grid, 0))
  end
end

# 使用例
input = """
MMMSXXMASM
MSAMXMSMSA
AMXSXMAAMM
MSAMASMSMX
XMASAMXAMM
XXAMMXXAMA
SMSMSASXSS
SAXAMASAAA
MAMMMXMMMM
MXMXAXMASX
"""

input
|> AdventOfCode2024Day4Part2.count_xmas()
|> IO.puts()