From 499a6f145d428a16ad1710b5ecc3349771bf09c8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Aur=C3=A9lien=20Geron?= <ageron@users.noreply.github.com>
Date: Sat, 7 Dec 2024 19:34:39 +1300
Subject: [PATCH] Solve day 4

---
 lib/aoc2024/day_04.ex | 99 ++++++++++++++++++++++++++++++++++++++++---
 test/aoc2024_test.exs | 19 +++++++++
 2 files changed, 113 insertions(+), 5 deletions(-)

diff --git a/lib/aoc2024/day_04.ex b/lib/aoc2024/day_04.ex
index d88421d..8936220 100644
--- a/lib/aoc2024/day_04.ex
+++ b/lib/aoc2024/day_04.ex
@@ -1,13 +1,102 @@
 defmodule Aoc2024.Day04 do
   def parse(input) do
-    input |> String.split("\n", trim: true)
+    input |> String.split("\n", trim: true) |> Enum.map(&String.graphemes/1)
   end
 
-  def part1(_lines) do
-    :todo1
+  defp size(grid) do
+    height = length(grid)
+    width = grid |> Enum.map(&length/1) |> Enum.max()
+    {height, width}
   end
 
-  def part2(_lines) do
-    :todo2
+  def part1(grid) do
+    word_chars = "XMAS" |> String.graphemes()
+    {height, width} = grid |> size()
+
+    -1..1
+    |> Enum.map(fn dy ->
+      -1..1
+      |> Enum.map(fn dx ->
+        if dx == 0 && dy == 0 do
+          0
+        else
+          0..(height - 1)
+          |> Enum.map(fn y ->
+            0..(width - 1)
+            |> Enum.map(fn x ->
+              all_chars_ok =
+                word_chars
+                |> Enum.with_index()
+                |> Enum.all?(fn {char, index} ->
+                  cy = y + index * dy
+                  cx = x + index * dx
+
+                  cond do
+                    cy < 0 ->
+                      false
+
+                    cx < 0 ->
+                      false
+
+                    cy >= height ->
+                      false
+
+                    true ->
+                      row = grid |> Enum.at(cy)
+
+                      if cx >= length(row) do
+                        false
+                      else
+                        row |> Enum.at(cx) == char
+                      end
+                  end
+                end)
+
+              if all_chars_ok do
+                1
+              else
+                0
+              end
+            end)
+            |> Enum.sum()
+          end)
+          |> Enum.sum()
+        end
+      end)
+      |> Enum.sum()
+    end)
+    |> Enum.sum()
+  end
+
+  def part2(grid) do
+    {height, width} = grid |> size()
+
+    0..(height - 3)
+    |> Enum.map(fn y ->
+      0..(width - 3)
+      |> Enum.map(fn x ->
+        center = grid |> Enum.at(y + 1) |> Enum.at(x + 1)
+        upper_left = grid |> Enum.at(y) |> Enum.at(x)
+        lower_right = grid |> Enum.at(y + 2) |> Enum.at(x + 2)
+        upper_right = grid |> Enum.at(y) |> Enum.at(x + 2)
+        lower_left = grid |> Enum.at(y + 2) |> Enum.at(x)
+
+        diag1? =
+          (upper_left == "M" && lower_right == "S") ||
+            (upper_left == "S" && lower_right == "M")
+
+        diag2? =
+          (upper_right == "M" && lower_left == "S") ||
+            (upper_right == "S" && lower_left == "M")
+
+        if center == "A" && diag1? && diag2? do
+          1
+        else
+          0
+        end
+      end)
+      |> Enum.sum()
+    end)
+    |> Enum.sum()
   end
 end
diff --git a/test/aoc2024_test.exs b/test/aoc2024_test.exs
index 5a8eaad..3d3976b 100644
--- a/test/aoc2024_test.exs
+++ b/test/aoc2024_test.exs
@@ -51,4 +51,23 @@ defmodule Aoc2024Test do
     input = "xmul(2,4)&mul[3,7]!^don't()_mul(5,5)\n+mul(32,64](mul(11,8)undo()?mul(8,5))"
     assert Aoc2024.Day03.part2(input) == 48
   end
+
+  test "Day 4" do
+    input = """
+    MMMSXXMASM
+    MSAMXMSMSA
+    AMXSXMAAMM
+    MSAMASMSMX
+    XMASAMXAMM
+    XXAMMXXAMA
+    SMSMSASXSS
+    SAXAMASAAA
+    MAMMMXMMMM
+    MXMXAXMASX
+    """
+
+    parsed_input = Aoc2024.Day04.parse(input)
+    assert Aoc2024.Day04.part1(parsed_input) == 18
+    assert Aoc2024.Day04.part2(parsed_input) == 9
+  end
 end