2024 day 20, part 2

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=456%20stars&style=for-the-badge&color=green" alt="456 stars" /></p>
-<p><a href="./lib/y2024/"><img src="https://img.shields.io/static/v1?label=2024&message=37%20stars&style=for-the-badge&color=yellow" alt="37 stars" /></a><br />
+<p><img src="https://img.shields.io/static/v1?label=Total&message=457%20stars&style=for-the-badge&color=green" alt="457 stars" /></p>
+<p><a href="./lib/y2024/"><img src="https://img.shields.io/static/v1?label=2024&message=38%20stars&style=for-the-badge&color=yellow" alt="38 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=37%20stars&style=for-the-badge&color=yellow" alt="37 stars" /><!-- stars 2024 end -->
+<!-- stars 2024 start --><img src="https://img.shields.io/static/v1?label=2024&message=38%20stars&style=for-the-badge&color=yellow" alt="38 stars" /><!-- stars 2024 end -->
 
 ## Benchmarks
 
@@ -50,5 +50,6 @@ day 18, part 1         22.53       44.38 ms     ±5.11%       44.48 ms       48.
 day 18, part 2          4.04      247.58 ms     ±3.18%      247.68 ms      260.90 ms
 day 19, part 1         29.38       34.03 ms     ±0.86%       34.01 ms       35.47 ms
 day 19, part 2         29.36       34.06 ms     ±1.18%       34.03 ms       36.73 ms
-day 20, part 1         13.16       76.00 ms     ±8.60%       75.01 ms       89.74 ms
+day 20, part 1         13.45       74.33 ms     ±6.84%       74.14 ms       83.83 ms
+day 20, part 2          1.30      771.43 ms     ±1.61%      767.13 ms      796.90 ms
 ```

lib/y2024/day20.ex

@@ -3,23 +3,15 @@ defmodule Y2024.Day20 do
 
   alias Advent.PathGrid
 
-  def part1(input) do
+  def parts(input, cheat_size) do
     input
-    |> cheats
+    |> cheats(cheat_size)
     |> Enum.reduce(0, fn {saving, count}, acc ->
       if saving >= 100, do: acc + count, else: acc
     end)
   end
 
-  # @doc """
-  # iex> Day20.part2("update or delete me")
-  # "update or delete me"
-  # """
-  # def part2(input) do
-  #   input
-  # end
-
-  def cheats({graph, from, to}) do
+  def cheats({graph, from, to}, max_cheat_size) do
     # The default path through the grid takes *every floor space*. Use this
     # to our advantage.
     baseline = Graph.get_shortest_path(graph, from, to)
@@ -29,22 +21,21 @@ defmodule Y2024.Day20 do
       |> Enum.with_index()
       |> Map.new()
 
+    cheat_options =
+      for row <- -max_cheat_size..max_cheat_size,
+          col <- -max_cheat_size..max_cheat_size,
+          abs(row) + abs(col) <= max_cheat_size,
+          do: {row, col}
+
     # For each step in the path, see what happens if we knock out walls around it
     # and skip ahead in the path
     Enum.flat_map(baseline, fn {row, col} ->
       from_index = Map.get(map, {row, col})
 
-      Enum.map([{0, -1}, {0, 1}, {-1, 0}, {1, 0}], fn {o_row, o_col} ->
-        to_index = Map.get(map, {row + o_row * 2, col + o_col * 2})
-        maybe_wall = {row + o_row, col + o_col}
+      Enum.map(cheat_options, fn {o_row, o_col} ->
+        to_index = Map.get(map, {row + o_row, col + o_col})
 
         cond do
-          !PathGrid.in_graph?(graph, maybe_wall) ->
-            nil
-
-          PathGrid.floor?(graph, maybe_wall) ->
-            nil
-
           to_index == nil ->
             nil
 
@@ -53,10 +44,10 @@ defmodule Y2024.Day20 do
 
           true ->
             # This will be the saving of time - 2 is the length of the shortcut
-            to_index - from_index - 2
+            to_index - from_index - abs(o_row) - abs(o_col)
         end
       end)
-      |> Enum.reject(&(&1 == nil))
+      |> Enum.reject(&(&1 == nil || &1 == 0))
     end)
     |> Enum.frequencies()
   end
@@ -69,6 +60,6 @@ defmodule Y2024.Day20 do
     {grid.graph, from, to}
   end
 
-  def part1_verify, do: input() |> parse_input() |> part1()
-  # def part2_verify, do: input() |> parse_input() |> part2()
+  def part1_verify, do: input() |> parse_input() |> parts(2)
+  def part2_verify, do: input() |> parse_input() |> parts(20)
 end

test/y2024/day20_test.exs

@@ -36,9 +36,9 @@ defmodule Y2024.Day20Test do
       64 => 1
     }
 
-    assert Day20.parse_input(@sample) |> Day20.cheats() == expected
+    assert Day20.parse_input(@sample) |> Day20.cheats(2) == expected
   end
 
   test "verification, part 1", do: assert(Day20.part1_verify() == 1289)
-  # test "verification, part 2", do: assert(Day20.part2_verify() == "update or delete me")
+  test "verification, part 2", do: assert(Day20.part2_verify() == 982_425)
 end