2023 day 3, part 2

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=5%20stars&style=for-the-badge&color=red" alt="5 stars" /></a><br />
+<p><a href="./lib/y2023/"><img src="https://img.shields.io/static/v1?label=2023&message=6%20stars&style=for-the-badge&color=red" alt="6 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=38%20stars&style=for-the-badge&color=yellow" alt="38 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=5%20stars&style=for-the-badge&color=red" alt="5 stars" /><!-- stars 2023 end -->
+<!-- stars 2023 start --><img src="https://img.shields.io/static/v1?label=2023&message=6%20stars&style=for-the-badge&color=red" alt="6 stars" /><!-- stars 2023 end -->
 
 ## Benchmarks
 
@@ -18,4 +18,6 @@ day 01, part 1        110.07        9.09 ms     ±1.38%        9.07 ms        9.
 day 01, part 2         85.23       11.73 ms     ±0.77%       11.71 ms       11.96 ms
 day 02, part 1        1.17 K        0.86 ms    ±75.22%        0.77 ms        4.03 ms
 day 02, part 2        1.17 K        0.85 ms    ±67.69%        0.79 ms        2.49 ms
+day 03, part 1         30.82       32.45 ms     ±2.29%       32.36 ms       36.61 ms
+day 03, part 2         13.71       72.96 ms     ±0.50%       72.89 ms       73.86 ms
 ```

lib/y2023/day03.ex

@@ -16,7 +16,19 @@ defmodule Y2023.Day03 do
     |> Enum.any?(&Enum.member?(symbol_positions, &1))
   end
 
-  defp calculate_surrounding_positions({row, col}, length) do
+  # 27998156 - too low
+  def part2(%{numbers: numbers, symbols: symbols}) do
+    number_positions = invert_numbers(numbers)
+
+    symbols
+    |> Enum.filter(fn {_position, symbol} -> symbol == "*" end)
+    |> Enum.map(&elem(&1, 0))
+    |> Enum.map(&adjacent_numbers(&1, number_positions))
+    |> Enum.filter(fn list -> length(list) == 2 end)
+    |> Enum.reduce(0, fn [a, b], acc -> acc + a * b end)
+  end
+
+  defp calculate_surrounding_positions({row, col}, length \\ 1) do
     [{row, col - 1}, {row, col + length}] ++
       row_positions(row + 1, col, length) ++ row_positions(row - 1, col, length)
   end
@@ -25,13 +37,25 @@ defmodule Y2023.Day03 do
     Enum.map((col - 1)..(col + length), &{row, &1})
   end
 
-  # @doc """
-  # iex> Day03.part2("update or delete me")
-  # "update or delete me"
-  # """
-  # def part2(input) do
-  #   input
-  # end
+  # For part 1, numbers were useful to store as `%{position -> %{number, length}}`
+  # But for part 2, we want the set of positions each number covers
+  defp invert_numbers(numbers) do
+    numbers
+    |> Enum.reduce([], fn {{row, col}, %{number: number, length: length}}, acc ->
+      positions = Enum.map(col..(col + length - 1), fn col -> {row, col} end) |> MapSet.new()
+      [{number, positions} | acc]
+    end)
+  end
+
+  defp adjacent_numbers(position, numbers) do
+    surrounding_positions = MapSet.new(calculate_surrounding_positions(position))
+
+    numbers
+    |> Enum.reject(fn {_number, positions} ->
+      MapSet.disjoint?(positions, surrounding_positions)
+    end)
+    |> Enum.map(&elem(&1, 0))
+  end
 
   def parse_input(input) do
     input
@@ -74,5 +98,5 @@ defmodule Y2023.Day03 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/y2023/day03_test.exs

@@ -4,7 +4,7 @@ defmodule Y2023.Day03Test do
   doctest Day03
 
   test "verification, part 1", do: assert(Day03.part1_verify() == 536_202)
-  # test "verification, part 2", do: assert(Day03.part2_verify() == "update or delete me")
+  test "verification, part 2", do: assert(Day03.part2_verify() == 78_272_573)
 
   @sample_input """
   467..114..
@@ -25,4 +25,11 @@ defmodule Y2023.Day03Test do
 
     assert actual == expected
   end
+
+  test "part 2" do
+    actual = @sample_input |> Day03.parse_input() |> Day03.part2()
+    expected = 467_835
+
+    assert actual == expected
+  end
 end