Solve 2016/13

Author: code-shoily

lib/2016/day_13.ex

@@ -0,0 +1,86 @@
+defmodule AdventOfCode.Y2016.Day13 do
+  @moduledoc """
+  --- Day 13: A Maze of Twisty Little Cubicles ---
+  Problem Link: https://adventofcode.com/2016/day/13
+  Difficulty: m
+  Tags: graph implicit-graph shortest-path a-star bfs
+  """
+  import Bitwise
+  alias AdventOfCode.Helpers.InputReader
+  alias Yog.Pathfinding.AStar
+  alias Yog.Traversal
+
+  def input, do: InputReader.read_from_file(2016, 13)
+
+  def run(input \\ input()) do
+    fav = parse(input)
+
+    {run_1(fav), run_2(fav)}
+  end
+
+  defp run_1(fav) do
+    # Target coordinate: (31, 39)
+    # Finding shortest path using A* search from (1, 1).
+    target = {31, 39}
+
+    successors = fn pos ->
+      open_neighbors(pos, fav) |> Enum.map(fn n -> {n, 1} end)
+    end
+
+    heuristic = fn {x, y} ->
+      abs(x - target_x(target)) + abs(y - target_y(target))
+    end
+
+    {:ok, dist} =
+      AStar.implicit_a_star(
+        from: {1, 1},
+        is_goal: fn pos -> pos == target end,
+        successors_with_cost: successors,
+        heuristic: heuristic
+      )
+
+    dist
+  end
+
+  defp run_2(fav) do
+    # BFS to find all unique nodes reachable within 50 steps.
+    Traversal.implicit_fold(
+      from: {1, 1},
+      using: :breadth_first,
+      initial: MapSet.new(),
+      successors_of: fn pos -> open_neighbors(pos, fav) end,
+      with: fn acc, pos, meta ->
+        new_acc = MapSet.put(acc, pos)
+
+        # Stop exploring depth > 50.
+        action = if meta.depth >= 50, do: :stop, else: :continue
+        {action, new_acc}
+      end
+    )
+    |> MapSet.size()
+  end
+
+  defp is_wall?(x, y, fav) do
+    if x < 0 or y < 0 do
+      true
+    else
+      val = x * x + 3 * x + 2 * x * y + y + y * y + fav
+      rem(count_ones(val), 2) != 0
+    end
+  end
+
+  defp open_neighbors({x, y}, fav) do
+    [{x + 1, y}, {x - 1, y}, {x, y + 1}, {x, y - 1}]
+    |> Enum.filter(fn {nx, ny} -> not is_wall?(nx, ny, fav) end)
+  end
+
+  defp count_ones(0), do: 0
+  defp count_ones(n), do: (n &&& 1) + count_ones(n >>> 1)
+
+  defp target_x({x, _}), do: x
+  defp target_y({_, y}), do: y
+
+  def parse(data \\ input()) do
+    data |> String.trim() |> String.trim_trailing(".") |> String.to_integer()
+  end
+end

priv/input_files/2016_13.txt

@@ -0,0 +1 @@
+1350

test/2016/day_13_test.exs

@@ -0,0 +1,12 @@
+defmodule AdventOfCode.Y2016.Day13Test do
+  @moduledoc false
+
+  use ExUnit.Case, async: true
+  @moduletag :y1613
+
+  alias AdventOfCode.Y2016.Day13, as: Solution
+
+  test "Year 2016, Day 13 run/1" do
+    assert Solution.run() == {92, 124}
+  end
+end