Day 6. Part 2 took 15 seconds to run... hm. But it works! Elixir is comfortable, but maybe not very exciting

day6/data.txt

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day6/main.exs

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+defmodule Day6 do
+  def run do
+    map =
+      File.read!("data.txt")
+      |> String.split("\n")
+      |> Enum.drop(-1)
+      |> Enum.map(&String.graphemes(&1))
+
+    height = length(map)
+    width = length(hd(map))
+
+    # find initial location
+    idx = Enum.concat(map) |> Enum.find_index(&(&1 != "#" and &1 != "."))
+    location = {div(idx, height), rem(idx, width)}
+
+    initialState = %{
+      map: map,
+      visited: MapSet.new(),
+      location: location,
+      bearing: 0,
+      height: height,
+      width: width
+    }
+
+    visited = walk(initialState, false)
+    IO.puts("Part 1: #{MapSet.size(visited)}")
+
+    # for each visited location, try putting an obstacle there
+    num_positions =
+      Enum.reduce(visited, 0, fn {row, col}, acc ->
+        # generate new map with obstacle in the path
+        new_row = List.replace_at(Enum.at(map, row), col, "#")
+        new_map = List.replace_at(map, row, new_row)
+        acc + walk(%{initialState | map: new_map}, true)
+      end)
+
+    IO.puts("Part 2: #{num_positions}")
+  end
+
+  defp walk(state, detect_loops?) do
+    # if we're detecting loops, and this location & bearing is already in our map, get out
+    if detect_loops? and MapSet.member?(state.visited, {state.location, state.bearing}) do
+      1
+    else
+      # add current location to set
+      visited =
+        if detect_loops? do
+          # if we're detecting loops, also record the bearing
+          MapSet.put(state.visited, {state.location, state.bearing})
+        else
+          MapSet.put(state.visited, state.location)
+        end
+
+      {row, col} = state.location
+
+      # grab next location
+      {next_row, next_col} =
+        case state.bearing do
+          0 -> {row - 1, col}
+          1 -> {row, col + 1}
+          2 -> {row + 1, col}
+          3 -> {row, col - 1}
+        end
+
+      in_bounds =
+        next_row >= 0 and next_row < state.height and
+          next_col >= 0 and next_col < state.width
+
+      if !in_bounds do
+        # if not in bounds, end recursion
+        if detect_loops? do
+          # if we're detecting loops, we didn't find one. return 0
+          0
+        else
+          # otherwise return the list of visited locations
+          visited
+        end
+      else
+        cell = Enum.at(state.map, next_row) |> Enum.at(next_col)
+
+        new_state =
+          if cell == "#" do
+            # keep location the same, turn 90° right
+            new_bearing = Integer.mod(state.bearing + 1, 4)
+            %{state | visited: visited, bearing: new_bearing}
+          else
+            %{state | visited: visited, location: {next_row, next_col}}
+          end
+
+        walk(new_state, detect_loops?)
+      end
+    end
+  end
+end
+
+Day6.run()

day6/test.txt

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+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...