solve 2024 day 4

2024/README.md

@@ -2,7 +2,7 @@
 
 |  S  |  M  |  T  |  W  |  T  |  F  |  S  |
 | :-: | :-: | :-: | :-: | :-: | :-: | :-: |
-| [1] | [2] | [3] |   4 |   5 |   6 |   7 |
+| [1] | [2] | [3] | [4] |   5 |   6 |   7 |
 |   8 |   9 |  10 |  11 |  12 |  13 |  14 |
 |  15 |  16 |  17 |  18 |  19 |  20 |  21 |
 |  22 |  23 |  24 |  25 |     |     |     |
@@ -10,3 +10,4 @@
 [1]: ./lib/2024/1.ex
 [2]: ./lib/2024/2.ex
 [3]: ./lib/2024/3.ex
+[4]: ./lib/2024/4.ex

2024/lib/2024/4.ex

@@ -0,0 +1,105 @@
+import AOC
+
+aoc 2024, 4 do
+  @directions ~w[n ne e se s sw w nw]a
+
+  def p1(input) do
+    grid = to_grid(input)
+    {xs, ys} = bounds(grid)
+
+    pos_and_dirs = for x <- xs, y <- ys, dir <- @directions, do: {{x, y}, dir}
+
+    Enum.count(pos_and_dirs, fn {pos, dir} ->
+      spells_xmas?(grid, pos, dir)
+    end)
+  end
+
+  def p2(input) do
+    grid = to_grid(input)
+    {xl..xh, yl..yh} = bounds(grid)
+
+    xs = (xl + 1)..(xh - 1)
+    ys = (yl + 1)..(yh - 1)
+
+    positions = for x <- xs, y <- ys, do: {x, y}
+
+    Enum.count(positions, &x_mas?(grid, &1))
+  end
+
+  def spells_xmas?(grid, pos, dir) do
+    spells?(grid, pos, dir, "XMAS")
+  end
+
+  def x_mas?(grid, pos) do
+    ~w[ne se sw nw]a
+    |> Enum.map(&{&1, move(pos, &1)})
+    |> Enum.count(fn {op_dir, pos} ->
+      dir =
+        case op_dir do
+          :ne -> :sw
+          :nw -> :se
+          :se -> :nw
+          :sw -> :ne
+        end
+
+      spells?(grid, pos, dir, "MAS")
+    end) == 2
+  end
+
+  def spells?(grid, pos, dir, word) do
+    letters = String.split(word, "", trim: true)
+
+    grid
+    |> walk(pos, dir)
+    |> Stream.zip_with(letters, &(&1 == &2))
+    |> Enum.all?()
+  end
+
+  defp to_grid(input) do
+    grid =
+      input
+      |> String.split("\n")
+      |> Enum.map(fn line ->
+        line
+        |> String.split("", trim: true)
+        |> List.to_tuple()
+      end)
+      |> List.to_tuple()
+
+    grid
+  end
+
+  def bounds(grid) do
+    ys = 0..(tuple_size(grid) - 1)
+    xs = 0..(tuple_size(elem(grid, 0)) - 1)
+
+    {xs, ys}
+  end
+
+  def walk(grid, pos, dir) do
+    pos
+    |> indexes(dir)
+    |> Stream.map(&get(grid, &1))
+  end
+
+  def indexes(pos, dir) do
+    Stream.iterate(pos, &move(&1, dir))
+  end
+
+  def move({x, y}, :n), do: {x, y - 1}
+  def move({x, y}, :e), do: {x + 1, y}
+  def move({x, y}, :s), do: {x, y + 1}
+  def move({x, y}, :w), do: {x - 1, y}
+  def move(pos, :ne), do: pos |> move(:n) |> move(:e)
+  def move(pos, :se), do: pos |> move(:s) |> move(:e)
+  def move(pos, :sw), do: pos |> move(:s) |> move(:w)
+  def move(pos, :nw), do: pos |> move(:n) |> move(:w)
+
+  def get(grid, {x, y}) do
+    {xs, ys} = bounds(grid)
+
+    if x in xs and y in ys do
+      elem(elem(grid, y), x)
+    end
+  end
+end

README.md

@@ -15,7 +15,7 @@
 | [2021] | **43/50** 🌟 | Elixir |
 | [2022] | **14/50** 🌟 | Elixir, Haskell |
 | [2023] | **19/50** 🌟 | Elixir, Haskell |
-| [2024] | **6/50** 🌟  | Elixir |
+| [2024] | **8/50** 🌟  | Elixir |
 | **Total** | **162** 🌟 | |
 
 [2015]: ./2015