2024 day 18 part 2 successful optimization

Use binary search to narrow down where the first byte that causes a failure is

Way too much time spent debugging off-by-one errors in the search though...

lib/y2024/README.md

@@ -46,6 +46,6 @@ day 15, part 1          5.06      197.80 ms     ±0.60%      197.59 ms      201.
 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
 day 17, part 1       45.64 K       21.91 μs    ±15.81%       21.17 μs       37.50 μs
-day 18, part 1         22.34       0.0448 s     ±5.84%       0.0449 s       0.0516 s
-day 18, part 2        0.0329        30.44 s     ±0.00%        30.44 s        30.44 s
+day 18, part 1         22.53       44.38 ms     ±5.11%       44.48 ms       48.54 ms
+day 18, part 2          4.04      247.58 ms     ±3.18%      247.68 ms      260.90 ms
 ```

lib/y2024/day18.ex

@@ -4,26 +4,42 @@ defmodule Y2024.Day18 do
   alias Advent.PathGrid
 
   def part1(bytes, size \\ 70, byte_num \\ 1024) do
+    empty_grid({0, 0}, {size, size})
+    |> get_path(bytes, byte_num, size)
+    |> length()
+    |> Kernel.-(1)
+  end
+
+  def part2(bytes, size \\ 70) do
+    max_bytes = length(bytes)
+    empty_grid = empty_grid({0, 0}, {size, size})
+
+    # Binary search this thing.
+    {row, col} = run_binary_search(empty_grid, bytes, 0, max_bytes, size)
+    "#{row},#{col}"
+  end
+
+  defp run_binary_search(_grid, bytes, val, val, _), do: Enum.at(bytes, val - 1)
+
+  defp run_binary_search(grid, bytes, min, max, size) do
+    at = min + div(max - min, 2)
+
+    if get_path(grid, bytes, at, size) do
+      run_binary_search(grid, bytes, at + 1, max, size)
+    else
+      run_binary_search(grid, bytes, min, at, size)
+    end
+  end
+
+  defp get_path(grid, bytes, byte_num, size) do
     fallen = Enum.take(bytes, byte_num)
 
     graph =
-      Enum.reduce(fallen, empty_grid({0, 0}, {size, size}), fn coord, graph ->
+      Enum.reduce(fallen, grid, fn coord, graph ->
         PathGrid.add_wall(graph, coord)
       end)
 
-    length(Graph.get_shortest_path(graph, {0, 0}, {size, size})) - 1
-  end
-
-  def part2(bytes, size \\ 70) do
-    Enum.reduce_while(bytes, empty_grid({0, 0}, {size, size}), fn {row, col}, graph ->
-      graph = PathGrid.add_wall(graph, {row, col})
-
-      if Graph.get_shortest_path(graph, {0, 0}, {size, size}) == nil do
-        {:halt, "#{row},#{col}"}
-      else
-        {:cont, graph}
-      end
-    end)
+    Graph.get_shortest_path(graph, {0, 0}, {size, size})
   end
 
   @doc """
@@ -49,9 +65,13 @@ defmodule Y2024.Day18 do
 
         [{row - 1, col}, {row, col - 1}]
         |> Enum.reduce(graph, fn neighbour, graph ->
-          graph
-          |> Graph.add_edge({row, col}, neighbour)
-          |> Graph.add_edge(neighbour, {row, col})
+          if Graph.has_vertex?(graph, neighbour) do
+            graph
+            |> Graph.add_edge({row, col}, neighbour)
+            |> Graph.add_edge(neighbour, {row, col})
+          else
+            graph
+          end
         end)
       end)
     end)

test/y2024/day18_test.exs

@@ -12,8 +12,6 @@ defmodule Y2024.Day18Test do
   end
 
   test "verification, part 1", do: assert(Day18.part1_verify() == 436)
-
-  @tag timeout: :infinity
   test "verification, part 2", do: assert(Day18.part2_verify() == "61,50")
 
   def test_data(), do: File.read!("test/y2024/input/day18/sample.txt")