fix: Resolve Day04 grid processing and X-MAS pattern matching

lib/advent_code2024/solutions/day04/READme.md

@@ -69,3 +69,41 @@ For the given puzzle input, calculate and return the total number of times the w
 - Input will be a rectangular grid of letters.
 
 Good luck helping the little Elf! 🌟
+
+## Part 2 Challenge
+
+The Elf looks quizzically at you. Did you misunderstand the assignment?
+
+Looking for the instructions, you flip over the word search to find that this
+isn't actually an XMAS puzzle; it's an X-MAS puzzle in which you're supposed to
+find two MAS in the shape of an X. One way to achieve that is like this:
+
+```txt
+M.S
+.A.
+M.S
+```
+
+Irrelevant characters have again been replaced with `.` in the above diagram.
+Within the X, each MAS can be written forwards or backwards.
+
+Here's the same example from before, but this time all of the X-MASes have been
+kept instead:
+
+```txt
+.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.
+..........
+```
+
+In this example, an X-MAS appears 9 times.
+
+Flip the word search from the instructions back over to the word search side and
+try again. How many times does an X-MAS appear?

lib/advent_code2024/solutions/day04/day4.ex

@@ -3,10 +3,10 @@ defmodule AdventCode2024.Solutions.Day04 do
 
   @default_input "priv/inputs/day04/input.txt"
 
-  def count_word_occurrences(grid, word) do
+  def count_word_occurrences(grid, word) when is_list(grid) do
     grid
     |> Enum.map(&String.graphemes/1)
-    |> find_word(word)
+    |> find_word(String.graphemes(word))
   end
 
   defp find_word(grid, word) do
@@ -51,9 +51,8 @@ defmodule AdventCode2024.Solutions.Day04 do
     end)
   end
 
-  defp find_word?(grid, word, {row, col}, {d_row, d_col}) do
-    word
-    |> String.codepoints()
+  defp find_word?(grid, word_chars, {row, col}, {d_row, d_col}) do
+    word_chars
     |> Enum.with_index()
     |> Enum.all?(fn {char, i} ->
       new_row = row + i * d_row
@@ -86,7 +85,7 @@ defmodule AdventCode2024.Solutions.Day04 do
   def solve_part2(_input_file) do
   end
 
-  def count_x_mas(grid, word \\ "MAS") do
+  def count_x_mas(grid) do
     directions = [
       # Down
       {1, 0},
@@ -98,18 +97,20 @@ defmodule AdventCode2024.Solutions.Day04 do
       {1, -1}
     ]
 
-    grid
+    grid_chars = Enum.map(grid, &String.graphemes/1)
+    
+    grid_chars
     |> 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_xmas_from(grid, word, {row_idx, col_idx}, directions)
+        acc_inner + count_xmas_from(grid_chars, {row_idx, col_idx}, directions)
       end)
     end)
   end
 
-  defp count_xmas_from(grid, _word, {row, col}, directions) do
+  defp count_xmas_from(grid, {row, col}, directions) do
     directions
     |> Enum.reduce(0, fn direction, acc ->
       if find_xmas?(grid, {row, col}, direction) do
@@ -132,7 +133,13 @@ defmodule AdventCode2024.Solutions.Day04 do
     # Check if all positions contain the correct letters for "MAS"
     Enum.with_index(positions)
     |> Enum.all?(fn {{r, c}, idx} ->
-      within_bounds?(grid, r, c) && Enum.at(Enum.at(grid, r), c) == String.at("MAS", idx)
+      within_bounds?(grid, r, c) && 
+      case idx do
+        0 -> Enum.at(Enum.at(grid, r), c) == "M"
+        1 -> Enum.at(Enum.at(grid, r), c) == "A"
+        2 -> Enum.at(Enum.at(grid, r), c) == "S"
+        3 -> Enum.at(Enum.at(grid, r), c) == "S"
+      end
     end)
   end
 end

test/advent_code2024/solutions/day04/day4_test.exs

@@ -20,4 +20,8 @@ defmodule AdventCode2024.Solutions.Day04Test do
     actual_count = Day04.count_word_occurrences(@example_grid, "XMAS")
     assert actual_count == expected_count
   end
+
+  test "finds all occurrences of X-MAS in the grid" do
+    assert Day04.count_x_mas(@example_grid) == 9
+  end
 end