Day 24 Parts 1 and 2

README.md

@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="978" alt="image" src="https://user-images.githubusercontent.com/498229/102721772-43d60700-4340-11eb-9ccc-baa6479559f8.png">
+<img width="978" alt="image" src="https://user-images.githubusercontent.com/498229/103093696-6321ac80-463e-11eb-933c-8c6b23d15ee0.png">
 
 ## Strategy
 

day24/README

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+Day 24 Notes
+
++--------+
+| Part 1 |
++--------+
+
+$ elixir day24part1.exs
+326
+
+Thoughts:
+
+Interesting. Need to work out how to implement a hexagonal grid.
+
+Use Enum.chunk_while to parse the variable-length `e`, `ne`, `s', `sw` etc. input tokens.
+
+I decided to do it like this: Use x, y coordinates, but horizontally adjacent cells are 2 x units
+apart. Diagonally adjacent cells are 1 x unit and 1 y unit apart.
+
+   / \ (0,0)
+  |   |
+ / \ / \ (1, -1)
+|   |   |
+ \ / \ / 
+  ↑
+(-1, -1)
+
+Define a tranlation vector for each direction, then we can walk the directions, and take the sum
+of all the translations.
+
+Store the black cells in a MapSet, and count them once we're done.
+
++--------+
+| Part 2 |
++--------+
+
+$ elixir day24part2.exs
+3979
+
+Thoughts:
+
+More complicated now, because we actually have to iterate all the black/white cells and their
+neighbours. Calculating the grid extremities looks complicated due to the hexagonal properties.
+However, this is very similar to Day 17 Part 2, so I will just adapt that solution for here too:
+
+1. For all current black squares, check the neighbours to see what its next state should be.
+   - additionally keep track of which white squares we identified.
+2. As white cells must have a black neighbour to be flipped, we know that it's fine to only
+   check white cells that are next to a black cell. So iterate the white cells we encountered
+   in step 1, and check their neighbours too.
+
+
++------------------+
+| Overall Thoughts |
++------------------+
+
+Let another game of life. 3rd time this year? This one was a bit more interesting due to the
+hexagonal nature, but part two was almost the same as Day 17. Maybe that's a good reflection
+on my day 17 implementation, though.

day24/day24part1.exs

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+defmodule Cell do
+  defstruct [:x, :y]
+end
+
+defmodule Day24Part1 do
+  def run do
+    File.read!("input")
+    |> String.split("\n", trim: true)
+    |> Enum.map(&parse_line/1)
+    |> Enum.map(&to_cell/1)
+    |> Enum.reduce(MapSet.new(), fn cell, black ->
+      if MapSet.member?(black, cell) do
+        MapSet.delete(black, cell)
+      else
+        MapSet.put(black, cell)
+      end
+    end)
+    |> Enum.count()
+    |> IO.puts()
+  end
+
+  def parse_line(str) do
+    str
+    |> String.graphemes()
+    |> Enum.chunk_while(
+      nil,
+      fn
+        "n", nil -> {:cont, :north}
+        "s", nil -> {:cont, :south}
+        "e", :north -> {:cont, :northeast, nil}
+        "e", :south -> {:cont, :southeast, nil}
+        "e", nil -> {:cont, :east, nil}
+        "w", :north -> {:cont, :northwest, nil}
+        "w", :south -> {:cont, :southwest, nil}
+        "w", nil -> {:cont, :west, nil}
+      end,
+      fn acc -> {:cont, acc} end
+    )
+  end
+
+  def to_cell(directions) do
+    Enum.reduce(directions, %Cell{x: 0, y: 0}, &move/2)
+  end
+
+  def move(:east, %Cell{x: x, y: y}), do: %Cell{x: x + 2, y: y}
+  def move(:west, %Cell{x: x, y: y}), do: %Cell{x: x - 2, y: y}
+  def move(:northeast, %Cell{x: x, y: y}), do: %Cell{x: x + 1, y: y + 1}
+  def move(:northwest, %Cell{x: x, y: y}), do: %Cell{x: x - 1, y: y + 1}
+  def move(:southeast, %Cell{x: x, y: y}), do: %Cell{x: x + 1, y: y - 1}
+  def move(:southwest, %Cell{x: x, y: y}), do: %Cell{x: x - 1, y: y - 1}
+end
+
+Day24Part1.run()

day24/day24part2.exs

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+defmodule Cell do
+  defstruct [:x, :y]
+end
+
+defmodule Day24Part2 do
+  def run do
+    File.read!("input")
+    |> String.split("\n", trim: true)
+    |> Enum.map(&parse_line/1)
+    |> Enum.reduce(MapSet.new(), fn cell, black ->
+      if MapSet.member?(black, cell) do
+        MapSet.delete(black, cell)
+      else
+        MapSet.put(black, cell)
+      end
+    end)
+    |> flip_times(100)
+    |> Enum.count()
+    |> IO.puts()
+  end
+
+  def parse_line(str) do
+    str
+    |> String.graphemes()
+    |> Enum.chunk_while(
+      nil,
+      fn
+        "n", nil -> {:cont, :north}
+        "s", nil -> {:cont, :south}
+        "e", :north -> {:cont, :northeast, nil}
+        "e", :south -> {:cont, :southeast, nil}
+        "e", nil -> {:cont, :east, nil}
+        "w", :north -> {:cont, :northwest, nil}
+        "w", :south -> {:cont, :southwest, nil}
+        "w", nil -> {:cont, :west, nil}
+      end,
+      fn acc -> {:cont, acc} end
+    )
+    |> Enum.reduce(%Cell{x: 0, y: 0}, &move/2)
+  end
+
+  def move(:east, %Cell{x: x, y: y}), do: %Cell{x: x + 2, y: y}
+  def move(:west, %Cell{x: x, y: y}), do: %Cell{x: x - 2, y: y}
+  def move(:northeast, %Cell{x: x, y: y}), do: %Cell{x: x + 1, y: y + 1}
+  def move(:northwest, %Cell{x: x, y: y}), do: %Cell{x: x - 1, y: y + 1}
+  def move(:southeast, %Cell{x: x, y: y}), do: %Cell{x: x + 1, y: y - 1}
+  def move(:southwest, %Cell{x: x, y: y}), do: %Cell{x: x - 1, y: y - 1}
+
+  # Below code adapted from Day 17 Part 2
+
+  def flip_times(cells, 0), do: cells
+  def flip_times(cells, count), do: flip_times(flip(cells), count - 1)
+
+  def flip(cells) do
+    {flipped, whites} =
+      Enum.reduce(cells, {MapSet.new(), MapSet.new()}, fn cell, {flipped, whites} ->
+        {black_count, whites} =
+          Enum.reduce(neighbours(cell), {0, whites}, fn neighbour, {black_count, whites} ->
+            if MapSet.member?(cells, neighbour) do
+              {black_count + 1, whites}
+            else
+              {black_count, MapSet.put(whites, neighbour)}
+            end
+          end)
+
+        flipped =
+          if black_count == 0 or black_count > 2,
+            do: flipped,
+            else: MapSet.put(flipped, cell)
+
+        {flipped, whites}
+      end)
+
+    Enum.reduce(whites, flipped, fn white, flipped ->
+      black_neighbours =
+        white |> neighbours() |> MapSet.new() |> MapSet.intersection(cells) |> Enum.count()
+
+      if black_neighbours == 2, do: MapSet.put(flipped, white), else: flipped
+    end)
+  end
+
+  def neighbours(cell) do
+    [:east, :west, :northeast, :northwest, :southeast, :southwest]
+    |> Enum.map(&move(&1, cell))
+  end
+end
+
+Day24Part2.run()

day24/input

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