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