2023 day 17, part 2

That took longer than it should have...
sevenseacat · Dec 17, 2023 · d354af0 · d354af0
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diff --git a/README.md b/README.md
@@ -5,7 +5,7 @@
 My Elixir solutions for [Advent of Code](https://adventofcode.com/) (all years).
 
 <!-- stars start -->
-<p><a href="./lib/y2023/"><img src="https://img.shields.io/static/v1?label=2023&message=33%20stars&style=for-the-badge&color=yellow" alt="33 stars" /></a><br />
+<p><a href="./lib/y2023/"><img src="https://img.shields.io/static/v1?label=2023&message=34%20stars&style=for-the-badge&color=yellow" alt="34 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 />
 <a href="./lib/y2020/"><img src="https://img.shields.io/static/v1?label=2020&message=39%20stars&style=for-the-badge&color=yellow" alt="39 stars" /></a><br />

diff --git a/lib/y2023/README.md b/lib/y2023/README.md
@@ -2,7 +2,7 @@
 
 My Elixir solutions for [Advent of Code 2023](https://adventofcode.com/2023).
 
-<!-- stars 2023 start --><img src="https://img.shields.io/static/v1?label=2023&message=33%20stars&style=for-the-badge&color=yellow" alt="33 stars" /><!-- stars 2023 end -->
+<!-- stars 2023 start --><img src="https://img.shields.io/static/v1?label=2023&message=34%20stars&style=for-the-badge&color=yellow" alt="34 stars" /><!-- stars 2023 end -->
 
 ## Benchmarks
 
@@ -47,5 +47,6 @@ day 15, part 1        1.97 K        0.51 ms    ±58.77%        0.47 ms        1.
 day 15, part 2        0.39 K        2.57 ms     ±9.13%        2.56 ms        3.05 ms
 day 16, part 1         97.00       10.31 ms    ±14.20%       10.08 ms       13.65 ms
 day 16, part 2          2.07      483.12 ms     ±2.51%      482.96 ms      503.06 ms
-day 17, part 1          1.42      704.79 ms     ±2.71%      713.38 ms      719.67 ms
+day 17, part 1          1.28      782.35 ms     ±3.06%      789.96 ms      811.18 ms
+day 17, part 2          0.30     3345.17 ms     ±2.14%     3345.17 ms     3395.69 ms
 ```
diff --git a/lib/y2023/day17.ex b/lib/y2023/day17.ex
@@ -4,21 +4,18 @@ defmodule Y2023.Day17 do
   alias Advent.Grid
 
   def part1(grid) do
-    find_best_path(grid, {1, 1})
+    find_best_path(grid, {1, 1}, 0..3)
   end
 
-  # @doc """
-  # iex> Day17.part2("update or delete me")
-  # "update or delete me"
-  # """
-  # def part2(input) do
-  #   input
-  # end
+  def part2(grid) do
+    find_best_path(grid, {1, 1}, 4..10)
+  end
 
-  defp find_best_path(grid, {row, col}) do
+  defp find_best_path(grid, {row, col}, length_range) do
     PriorityQueue.new()
-    |> add_to_queue([[{row, col, nil, 0}]])
-    |> search(grid, Grid.size(grid), MapSet.new())
+    |> add_to_queue([[{row, col, :down, 0}]])
+    |> add_to_queue([[{row, col, :right, 0}]])
+    |> search(grid, Grid.size(grid), MapSet.new(), length_range)
   end
 
   defp add_to_queue(queue, states) do
@@ -27,42 +24,42 @@ defmodule Y2023.Day17 do
     end)
   end
 
-  defp search(queue, grid, destination, cache) do
-    do_search(PriorityQueue.pop(queue), grid, destination, cache)
+  defp search(queue, grid, destination, cache, length_range) do
+    do_search(PriorityQueue.pop(queue), grid, destination, cache, length_range)
   end
 
-  defp do_search({:empty, _queue}, _grid, _destination, _cache) do
+  defp do_search({:empty, _queue}, _grid, _destination, _cache, _straight_line_length) do
     raise("No path found")
   end
 
-  defp do_search(
-         {{:value, [{row, col, _direction, count} | _rest]}, _queue},
-         _grid,
-         {row, col},
-         _cache
-       ) do
-    # Winner winner chicken dinner.
-    count
-  end
+  defp do_search({{:value, path}, queue}, grid, destination, cache, length_range) do
+    {row, col, _, count} = hd(path)
 
-  defp do_search({{:value, path}, queue}, grid, destination, cache) do
-    if MapSet.member?(cache, cache_key(path)) do
-      # Been here before at a lower heat loss value, ignore
-      search(queue, grid, destination, cache)
+    if {row, col} == destination && passes_final_straight_check?(path, length_range) do
+      # Winner winner chicken dinner.
+      # Grid.display(grid, Enum.map(path, fn {row, col, _, _} -> {row, col} end))
+      count
     else
-      cache = MapSet.put(cache, cache_key(path))
-
-      queue
-      |> add_to_queue(valid_moves(path, grid))
-      |> search(grid, destination, cache)
+      cache_key = cache_key(path)
+
+      if MapSet.member?(cache, cache_key) do
+        # Been here before at a lower heat loss value, ignore
+        search(queue, grid, destination, cache, length_range)
+      else
+        cache = MapSet.put(cache, cache_key)
+
+        queue
+        |> add_to_queue(valid_moves(path, grid, length_range))
+        |> search(grid, destination, cache, length_range)
+      end
     end
   end
 
-  defp valid_moves([{row, col, direction, value} | _rest] = path, grid) do
+  defp valid_moves([{row, col, direction, value} | _rest] = path, grid, length_range) do
     [{row - 1, col, :up}, {row + 1, col, :down}, {row, col - 1, :left}, {row, col + 1, :right}]
     |> Enum.filter(&in_grid?(grid, &1))
     |> Enum.reject(&backwards?(direction, &1))
-    |> Enum.reject(&too_far_straight?(path, &1))
+    |> Enum.filter(&passes_straight_check?(path, &1, length_range))
     |> Enum.map(fn {row, col, dir} ->
       [{row, col, dir, value + Map.fetch!(grid, {row, col})} | path]
     end)
@@ -84,12 +81,34 @@ defmodule Y2023.Day17 do
     {from, to} in [{:left, :right}, {:right, :left}, {:up, :down}, {:down, :up}]
   end
 
-  defp too_far_straight?([{_, _, dir, _}, {_, _, dir, _}, {_, _, dir, _} | _rest], {_, _, dir}) do
-    true
+  defp passes_straight_check?(
+         [{_, _, from_dir, _} | _rest] = path,
+         {_, _, to_dir},
+         length_range
+       ) do
+    _min..max = length_range
+
+    straight = straight_line_length(path)
+
+    cond do
+      from_dir == to_dir -> straight < max
+      from_dir != to_dir -> straight in length_range
+    end
+  end
+
+  defp passes_final_straight_check?(path, length_range) do
+    straight_line_length(path) in length_range
   end
 
-  defp too_far_straight?(_, _), do: false
+  defp straight_line_length(path) do
+    {_, _, direction, _} = hd(path)
+
+    path
+    |> Enum.take_while(fn {_, _, dir, _} -> dir == direction end)
+    |> length
+  end
 
+  @spec parse_input(binary()) :: any()
   def parse_input(input) do
     input
     |> Grid.new()
@@ -98,5 +117,5 @@ defmodule Y2023.Day17 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
diff --git a/test/y2023/day17_test.exs b/test/y2023/day17_test.exs
@@ -4,7 +4,7 @@ defmodule Y2023.Day17Test do
   doctest Day17
 
   test "verification, part 1", do: assert(Day17.part1_verify() == 886)
-  # test "verification, part 2", do: assert(Day17.part2_verify() == "update or delete me")
+  test "verification, part 2", do: assert(Day17.part2_verify() == 1055)
 
   @sample_input """
   2413432311323
@@ -26,4 +26,24 @@ defmodule Y2023.Day17Test do
     actual = Day17.parse_input(@sample_input) |> Day17.part1()
     assert 102 == actual
   end
+
+  test "part 2, sample 1" do
+    actual = Day17.parse_input(@sample_input) |> Day17.part2()
+    assert 94 == actual
+  end
+
+  test "part 2, sample 2" do
+    actual =
+      """
+      111111111111
+      999999999991
+      999999999991
+      999999999991
+      999999999991
+      """
+      |> Day17.parse_input()
+      |> Day17.part2()
+
+    assert 71 == actual
+  end
 end