2024 day 16, part 2

Feels like I'm doing some unnecessary stuff to figure out all of the paths
non-exponentially but hey it's quick

.gitattributes

@@ -1 +1,2 @@
 lib/y*/input/day*.txt filter=git-crypt diff=git-crypt
+test/y*/input/day*.txt filter=git-crypt diff=git-crypt

README.md

@@ -5,8 +5,8 @@
 My Elixir solutions for [Advent of Code](https://adventofcode.com/) (all years).
 
 <!-- stars start -->
-<p><img src="https://img.shields.io/static/v1?label=Total&message=449%20stars&style=for-the-badge&color=green" alt="449 stars" /></p>
-<p><a href="./lib/y2024/"><img src="https://img.shields.io/static/v1?label=2024&message=30%20stars&style=for-the-badge&color=yellow" alt="30 stars" /></a><br />
+<p><img src="https://img.shields.io/static/v1?label=Total&message=450%20stars&style=for-the-badge&color=green" alt="450 stars" /></p>
+<p><a href="./lib/y2024/"><img src="https://img.shields.io/static/v1?label=2024&message=31%20stars&style=for-the-badge&color=yellow" alt="31 stars" /></a><br />
 <a href="./lib/y2023/"><img src="https://img.shields.io/static/v1?label=2023&message=44%20stars&style=for-the-badge&color=green" alt="44 stars" /></a><br />
 <a href="./lib/y2022/"><img src="https://img.shields.io/static/v1?label=2022&message=%E2%AD%90%EF%B8%8F%2050%20stars%20%E2%AD%90%EF%B8%8F&style=for-the-badge&color=brightgreen" alt="50 stars" /></a><br />
 <a href="./lib/y2021/"><img src="https://img.shields.io/static/v1?label=2021&message=46%20stars&style=for-the-badge&color=green" alt="46 stars" /></a><br />

lib/y2024/README.md

@@ -2,7 +2,7 @@
 
 My Elixir solutions for [Advent of Code 2024](https://adventofcode.com/2024).
 
-<!-- stars 2024 start --><img src="https://img.shields.io/static/v1?label=2024&message=30%20stars&style=for-the-badge&color=yellow" alt="30 stars" /><!-- stars 2024 end -->
+<!-- stars 2024 start --><img src="https://img.shields.io/static/v1?label=2024&message=31%20stars&style=for-the-badge&color=yellow" alt="31 stars" /><!-- stars 2024 end -->
 
 ## Benchmarks
 
@@ -43,5 +43,6 @@ day 13, part 1       1294.57        0.77 ms     ±4.01%        0.76 ms        0.
 day 14, part 1        516.12        1.94 ms     ±8.66%        1.85 ms        2.30 ms
 day 14, part 2          1.41      707.27 ms     ±0.37%      707.76 ms      710.36 ms
 day 15, part 1          5.06      197.80 ms     ±0.60%      197.59 ms      201.50 ms
-day 16, part 1         13.98       71.55 ms     ±6.08%       73.25 ms       80.54 ms
+day 16, part 1         12.91       77.43 ms     ±5.49%       77.14 ms       86.71 ms
+day 16, part 2         13.07       76.51 ms     ±5.00%       76.09 ms       85.69 ms
 ```

lib/y2024/day16.ex

@@ -4,24 +4,24 @@ defmodule Y2024.Day16 do
   alias Advent.PathGrid
 
   def part1(path_grid) do
-    from = Enum.find(path_grid.units, &(&1.identifier == "S")).position
-    to = Enum.find(path_grid.units, &(&1.identifier == "E")).position
+    do_parts(path_grid) |> elem(0)
+  end
 
-    {_path, score} = find_lowest_score(path_grid.graph, from, to)
-    score
+  def part2(path_grid) do
+    do_parts(path_grid)
+    |> elem(1)
+    |> MapSet.size()
   end
 
-  # @doc """
-  # iex> Day16.part2("update or delete me")
-  # "update or delete me"
-  # """
-  # def part2(input) do
-  #   input
-  # end
+  defp do_parts(path_grid) do
+    from = Enum.find(path_grid.units, &(&1.identifier == "S")).position
+    to = Enum.find(path_grid.units, &(&1.identifier == "E")).position
+    find_lowest_scores(path_grid.graph, from, to)
+  end
 
-  defp find_lowest_score(graph, from, to) do
+  defp find_lowest_scores(graph, from, to) do
     queue = queue_next_states(PriorityQueue.new(), {from, :east, [{from, :east}], 0}, graph)
-    do_search(PriorityQueue.pop(queue), graph, to, MapSet.new([{from, :east}]))
+    do_search(PriorityQueue.pop(queue), graph, to, %{{from, :east} => {0, []}})
   end
 
   defp queue_next_states(queue, {coord, facing, path, score}, graph) do
@@ -48,13 +48,53 @@ defmodule Y2024.Day16 do
 
   defp do_search({{:value, {coord, facing, path, score}}, queue}, graph, to, seen) do
     if coord == to do
-      {path, score}
+      # We've found a best path! We might have had multiple branches that combined
+      # together to get to this point though
+      {score, get_all_paths(path, seen)}
     else
-      if MapSet.member?(seen, {coord, facing}) do
-        do_search(PriorityQueue.pop(queue), graph, to, seen)
-      else
-        queue = queue_next_states(queue, {coord, facing, path, score}, graph)
-        do_search(PriorityQueue.pop(queue), graph, to, MapSet.put(seen, {coord, facing}))
+      case Map.get(seen, {coord, facing}) do
+        {^score, old_path} ->
+          # We've found an alternate way of getting to this point
+          # We can cut this path (but record that it existed)
+          seen = Map.put(seen, {coord, facing}, {score, [path | old_path]})
+          do_search(PriorityQueue.pop(queue), graph, to, seen)
+
+        # We've seen a better way of getting to this point (skip)
+        {old_score, _old_path} when old_score < score ->
+          do_search(PriorityQueue.pop(queue), graph, to, seen)
+
+        _ ->
+          # Either the first time we've seen this coordinate, or this is a better way
+          # of getting to it (actions are the same)
+          seen = Map.put(seen, {coord, facing}, {score, path})
+          queue = queue_next_states(queue, {coord, facing, path, score}, graph)
+          do_search(PriorityQueue.pop(queue), graph, to, seen)
+      end
+    end
+  end
+
+  # This feels rather unnecessary but I can't think of a better way to do it
+  # A BFS over all of the points in the path to build all of the ways we got there
+  defp get_all_paths(path, seen) do
+    do_get_all_paths(path, [], MapSet.new(), seen)
+  end
+
+  defp do_get_all_paths([], [], path, _seen), do: path
+
+  defp do_get_all_paths([], next, path, seen) do
+    do_get_all_paths(List.flatten(next), [], path, seen)
+  end
+
+  defp do_get_all_paths([{coord, _} = one | rest], next, path, seen) do
+    if MapSet.member?(path, coord) do
+      do_get_all_paths(rest, next, path, seen)
+    else
+      case Map.fetch(seen, one) do
+        :error ->
+          do_get_all_paths(rest, next, MapSet.put(path, coord), seen)
+
+        {:ok, {_score, points}} ->
+          do_get_all_paths(rest, [points | next], MapSet.put(path, coord), seen)
       end
     end
   end
@@ -64,5 +104,5 @@ defmodule Y2024.Day16 do
   end
 
   def part1_verify, do: input() |> parse_input() |> part1()
-  # def part2_verify, do: input() |> parse_input() |> part2()
+  def part2_verify, do: input() |> parse_input() |> part2()
 end

test/y2024/day16_test.exs

@@ -8,8 +8,13 @@ defmodule Y2024.Day16Test do
     assert test_data("sample2") |> Day16.parse_input() |> Day16.part1() == 11048
   end
 
+  test "part2" do
+    assert test_data("sample1") |> Day16.parse_input() |> Day16.part2() == 45
+    assert test_data("sample2") |> Day16.parse_input() |> Day16.part2() == 64
+  end
+
   test "verification, part 1", do: assert(Day16.part1_verify() == 91464)
-  # test "verification, part 2", do: assert(Day16.part2_verify() == "update or delete me")
+  test "verification, part 2", do: assert(Day16.part2_verify() == 494)
 
   def test_data(name), do: File.read!("test/y2024/input/#{name}.txt")
 end