2024 day 9 part 2 - this needs optimization

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=436%20stars&style=for-the-badge&color=green" alt="436 stars" /></p>
-<p><a href="./lib/y2024/"><img src="https://img.shields.io/static/v1?label=2024&message=17%20stars&style=for-the-badge&color=orange" alt="17 stars" /></a><br />
+<p><img src="https://img.shields.io/static/v1?label=Total&message=437%20stars&style=for-the-badge&color=green" alt="437 stars" /></p>
+<p><a href="./lib/y2024/"><img src="https://img.shields.io/static/v1?label=2024&message=18%20stars&style=for-the-badge&color=orange" alt="18 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=17%20stars&style=for-the-badge&color=orange" alt="17 stars" /><!-- stars 2024 end -->
+<!-- stars 2024 start --><img src="https://img.shields.io/static/v1?label=2024&message=18%20stars&style=for-the-badge&color=orange" alt="18 stars" /><!-- stars 2024 end -->
 
 ## Benchmarks
 
@@ -31,4 +31,5 @@ day 07, part 2         14.76       67.73 ms     ±1.35%       67.51 ms       71.
 day 08, part 1        1.07 K        0.93 ms     ±6.91%        0.90 ms        1.07 ms
 day 08, part 2        0.83 K        1.20 ms     ±7.78%        1.17 ms        1.38 ms
 day 09, part 1          9.27      107.87 ms     ±0.84%      107.61 ms      111.83 ms
+day 09, part 2          0.23         4.43 s     ±0.12%         4.43 s         4.43 s
 ```

lib/y2024/day09.ex

@@ -42,13 +42,64 @@ defmodule Y2024.Day09 do
     |> elem(1)
   end
 
-  # @doc """
-  # iex> Day09.part2("update or delete me")
-  # "update or delete me"
-  # """
-  # def part2(input) do
-  #   input
-  # end
+  @doc """
+  iex> Day09.part2(Integer.digits(2333133121414131402))
+  2858
+  """
+  def part2(input) do
+    input
+    |> Enum.reduce({0, true, []}, fn num, {file_id, fill?, list} ->
+      type = if fill?, do: :fill, else: :gap
+      next_file_id = if fill?, do: file_id + 1, else: file_id
+      {next_file_id, !fill?, [%{type: type, file_id: file_id, num: num} | list]}
+    end)
+    |> elem(2)
+    |> Enum.reverse()
+    |> defrag([])
+    |> Enum.reduce({0, 0}, fn record, {index, acc} ->
+      if record.type == :gap do
+        {index + record.num, acc}
+      else
+        acc =
+          Enum.reduce(0..(record.num - 1), acc, fn sub_index, acc ->
+            acc + (index + sub_index) * record.file_id
+          end)
+
+        {index + record.num, acc}
+      end
+    end)
+    |> elem(1)
+  end
+
+  defp defrag([], list), do: Enum.reverse(list)
+
+  defp defrag([%{type: :fill} = head | tail], list) do
+    defrag(tail, [head | list])
+  end
+
+  defp defrag([%{type: :gap, num: num} = head | tail], list) do
+    filler =
+      Enum.find(Enum.reverse(tail), fn %{num: check_num, type: type} ->
+        type == :fill && check_num <= num
+      end)
+
+    if filler do
+      list = [filler | list]
+
+      tail =
+        Enum.map(tail, fn record ->
+          if record == filler, do: %{type: :gap, num: filler.num}, else: record
+        end)
+
+      if num - filler.num == 0 do
+        defrag(tail, list)
+      else
+        defrag([%{type: :gap, num: num - filler.num} | tail], list)
+      end
+    else
+      defrag(tail, [head | list])
+    end
+  end
 
   def parse_input(input) do
     input
@@ -58,5 +109,5 @@ defmodule Y2024.Day09 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/day09_test.exs

@@ -4,5 +4,5 @@ defmodule Y2024.Day09Test do
   doctest Day09
 
   test "verification, part 1", do: assert(Day09.part1_verify() == 6_241_633_730_082)
-  # test "verification, part 2", do: assert(Day09.part2_verify() == "update or delete me")
+  test "verification, part 2", do: assert(Day09.part2_verify() == 6_265_268_809_555)
 end