2023 day 15, part 2

That was suspiciously easy...

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=29%20stars&style=for-the-badge&color=yellow" alt="29 stars" /></a><br />
+<p><a href="./lib/y2023/"><img src="https://img.shields.io/static/v1?label=2023&message=30%20stars&style=for-the-badge&color=yellow" alt="30 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 />

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=29%20stars&style=for-the-badge&color=yellow" alt="29 stars" /><!-- stars 2023 end -->
+<!-- stars 2023 start --><img src="https://img.shields.io/static/v1?label=2023&message=30%20stars&style=for-the-badge&color=yellow" alt="30 stars" /><!-- stars 2023 end -->
 
 ## Benchmarks
 
@@ -43,4 +43,6 @@ day 13, part 1        150.89        6.63 ms     ±5.72%        6.62 ms        7.
 day 13, part 2          3.68      271.37 ms     ±1.28%      270.72 ms      283.72 ms
 day 14, part 1         18.61       53.73 ms     ±9.41%       53.71 ms       67.81 ms
 day 14, part 2          0.63     1575.73 ms     ±3.34%     1569.17 ms     1644.07 ms
+day 15, part 1        1.97 K        0.51 ms    ±58.77%        0.47 ms        1.38 ms
+day 15, part 2        0.39 K        2.57 ms     ±9.13%        2.56 ms        3.05 ms
 ```

lib/y2023/day15.ex

@@ -2,8 +2,7 @@ defmodule Y2023.Day15 do
   use Advent.Day, no: 15
 
   @doc """
-  iex> Day15.part1(['rn=1', 'cm-', 'qp=3', 'cm=2', 'qp-', 'pc=4', 'ot=9',
-  ...>  'ab=5', 'pc-', 'pc=6', 'ot=7'])
+  iex> Day15.part1(~W(rn=1 cm- qp=3 cm=2 qp- pc=4 ot=9 ab=5 pc- pc=6 ot=7))
   1320
   """
   def part1(input) do
@@ -12,41 +11,87 @@ defmodule Y2023.Day15 do
     |> Enum.sum()
   end
 
-  # @doc """
-  # iex> Day15.part2("update or delete me")
-  # "update or delete me"
-  # """
-  # def part2(input) do
-  #   input
-  # end
+  @doc """
+  iex> Day15.part2(~W(rn=1 cm- qp=3 cm=2 qp- pc=4 ot=9 ab=5 pc- pc=6 ot=7))
+  145
+  """
+  def part2(input) do
+    input
+    |> Enum.reduce(%{}, &place_lens/2)
+    |> to_focussing_power()
+  end
+
+  defp place_lens(lens, map) do
+    {label, op} = parse_lens(lens)
+    box_no = hash(label)
+
+    case op do
+      "-" ->
+        Map.update(map, box_no, [], fn box ->
+          Enum.reject(box, fn {x, _} -> x == label end)
+        end)
+
+      {"=", focal_length} ->
+        entry = {label, focal_length}
+
+        Map.update(map, box_no, [entry], fn list ->
+          index = Enum.find_index(list, fn {x, _} -> x == label end)
+
+          if index do
+            List.replace_at(list, index, entry)
+          else
+            list ++ [entry]
+          end
+        end)
+    end
+  end
+
+  defp parse_lens(string) do
+    if String.contains?(string, "-") do
+      label = String.split(string, "-") |> hd()
+      {label, "-"}
+    else
+      [label, num] = String.split(string, "=", parts: 2)
+      {label, {"=", String.to_integer(num)}}
+    end
+  end
+
+  defp to_focussing_power(map) do
+    Enum.reduce(map, 0, fn {box_no, lenses}, acc ->
+      Enum.reduce(lenses, {1, acc}, fn {_label, length}, {index, acc2} ->
+        {index + 1, acc2 + (box_no + 1) * index * length}
+      end)
+      |> elem(1)
+    end)
+  end
 
   @doc """
-  iex> Day15.hash('rn=1')
+  iex> Day15.hash("rn=1")
   30
 
-  iex> Day15.hash('cm-')
+  iex> Day15.hash("cm-")
   253
 
-  iex> Day15.hash('qp=3')
+  iex> Day15.hash("qp=3")
   97
   """
   def hash(string) do
     string
+    |> String.to_charlist()
     |> Enum.reduce(0, fn char, acc ->
       rem((acc + char) * 17, 256)
     end)
   end
 
   @doc """
   iex> Day15.parse_input("rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7")
-  ['rn=1', 'cm-', 'qp=3', 'cm=2', 'qp-', 'pc=4', 'ot=9', 'ab=5', 'pc-', 'pc=6', 'ot=7']
+  ~W(rn=1 cm- qp=3 cm=2 qp- pc=4 ot=9 ab=5 pc- pc=6 ot=7)
   """
   def parse_input(input) do
     input
     |> String.split(",", trim: true)
-    |> Enum.map(&String.to_charlist/1)
   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/y2023/day15_test.exs

@@ -4,5 +4,5 @@ defmodule Y2023.Day15Test do
   doctest Day15
 
   test "verification, part 1", do: assert(Day15.part1_verify() == 513_172)
-  # test "verification, part 2", do: assert(Day15.part2_verify() == "update or delete me")
+  test "verification, part 2", do: assert(Day15.part2_verify() == 237_806)
 end