add day21a

23/elixir/lib/days/day21.ex

@@ -0,0 +1,67 @@
+defmodule AOC.Day21 do
+  # use AOC.Day, day: 21, input: "example"
+  use AOC.Day, day: 21
+
+  def parse_input(lines) do
+    map =
+      lines
+      |> Enum.with_index()
+      |> Enum.flat_map(fn {line, y} ->
+        line
+        |> String.to_charlist()
+        |> Enum.with_index()
+        |> Enum.map(fn {tile, x} -> {{y, x}, tile} end)
+      end)
+      |> Enum.into(%{})
+
+    start_position =
+      map
+      |> Enum.find(fn {_, tile} -> tile == ?S end)
+      |> elem(0)
+
+    {Map.put(map, start_position, ?.), start_position}
+  end
+
+  def walk(map, start_position, steps) do
+    map
+    |> do_walk(start_position, steps)
+    |> Enum.count()
+  end
+
+  def do_walk(map, position, _) when not is_map_key(map, position), do: []
+  def do_walk(map, position, _) when :erlang.map_get(position, map) == ?#, do: []
+  def do_walk(_map, position, 0), do: [position]
+
+  def do_walk(map, {y, x} = position, steps_left) do
+    case :ets.lookup(:steps_memo, {position, steps_left}) do
+      [] ->
+        reachable =
+          [{y - 1, x}, {y, x + 1}, {y + 1, x}, {y, x - 1}]
+          |> Enum.flat_map(fn neighbor_position ->
+            do_walk(map, neighbor_position, steps_left - 1)
+          end)
+          |> Enum.uniq()
+
+        :ets.insert(:steps_memo, {{position, steps_left}, reachable})
+
+        reachable
+
+      [{_, reachable}] ->
+        reachable
+    end
+  end
+
+  def part1({map, start_position}) do
+    :ets.new(:steps_memo, [:named_table])
+
+    map
+    |> walk(start_position, 64)
+    |> IO.inspect()
+
+    "TODO"
+  end
+
+  def part2(_input) do
+    "TODO"
+  end
+end