ai_advent_code_2024/lib/advent_code2024/solutions/day04/day4.ex

defmodule AdventCode2024.Solutions.Day04 do
  def count_word_occurrences(grid, word) do
    grid
    |> Enum.map(&String.graphemes/1)
    |> find_word(word)
  end

  defp find_word(grid, word) do
    directions = [
      # Right
      {0, 1},
      # Down
      {1, 0},
      # Left
      {0, -1},
      # Up
      {-1, 0},
      # Down-right diagonal
      {1, 1},
      # Up-left diagonal
      {-1, -1},
      # Down-left diagonal
      {1, -1},
      # Up-right diagonal
      {-1, 1}
    ]

    grid
    |> Enum.with_index()
    |> Enum.reduce(0, fn {row, row_idx}, acc ->
      row
      |> Enum.with_index()
      |> Enum.reduce(acc, fn {_, col_idx}, acc_inner ->
        acc_inner + count_word_from(grid, word, {row_idx, col_idx}, directions)
      end)
    end)
  end

  @doc """
  Counts the occurrences of a word in all possible directions from a given starting position in the grid.

  ## Parameters
  - grid: A 2D list representing the word search grid.
  - word: The word to be searched for.
  - row: The starting row index.
  - col: The starting column index.
  - directions: A list of tuples representing the directions to search in.

  ## Returns
  - The number of times the word is found from the given starting position in the grid.
  """
  defp count_word_from(grid, word, {row, col}, directions) do
    directions
    |> Enum.reduce(0, fn direction, acc ->
      if find_word?(grid, word, {row, col}, direction) do
        acc + 1
      else
        acc
      end
    end)
  end

  defp find_word?(grid, word, {row, col}, {d_row, d_col}) do
    word
    |> String.codepoints()
    |> Enum.with_index()
    |> Enum.all?(fn {char, i} ->
      new_row = row + i * d_row
      new_col = col + i * d_col
      within_bounds?(grid, new_row, new_col) && Enum.at(Enum.at(grid, new_row), new_col) == char
    end)
  end

  defp within_bounds?(grid, row, col) do
    row >= 0 && row < length(grid) && col >= 0 && col < length(Enum.at(grid, 0))
  end
end