105 lines
2.1 KiB
Elixir
105 lines
2.1 KiB
Elixir
import AOC
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aoc 2024, 4 do
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@directions ~w[n ne e se s sw w nw]a
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def p1(input) do
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grid = to_grid(input)
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{xs, ys} = bounds(grid)
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pos_and_dirs = for x <- xs, y <- ys, dir <- @directions, do: {{x, y}, dir}
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Enum.count(pos_and_dirs, fn {pos, dir} ->
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spells_xmas?(grid, pos, dir)
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end)
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end
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def p2(input) do
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grid = to_grid(input)
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{xl..xh, yl..yh} = bounds(grid)
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xs = (xl + 1)..(xh - 1)
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ys = (yl + 1)..(yh - 1)
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positions = for x <- xs, y <- ys, do: {x, y}
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Enum.count(positions, &x_mas?(grid, &1))
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end
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def spells_xmas?(grid, pos, dir) do
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spells?(grid, pos, dir, "XMAS")
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end
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def x_mas?(grid, pos) do
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~w[ne se sw nw]a
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|> Enum.map(&{&1, move(pos, &1)})
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|> Enum.count(fn {op_dir, pos} ->
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dir =
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case op_dir do
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:ne -> :sw
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:nw -> :se
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:se -> :nw
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:sw -> :ne
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end
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spells?(grid, pos, dir, "MAS")
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end) == 2
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end
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def spells?(grid, pos, dir, word) do
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letters = String.split(word, "", trim: true)
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grid
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|> walk(pos, dir)
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|> Stream.zip_with(letters, &(&1 == &2))
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|> Enum.all?()
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end
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defp to_grid(input) do
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grid =
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input
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|> String.split("\n")
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|> Enum.map(fn line ->
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line
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|> String.split("", trim: true)
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|> List.to_tuple()
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end)
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|> List.to_tuple()
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grid
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end
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def bounds(grid) do
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ys = 0..(tuple_size(grid) - 1)
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xs = 0..(tuple_size(elem(grid, 0)) - 1)
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{xs, ys}
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end
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def walk(grid, pos, dir) do
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pos
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|> indexes(dir)
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|> Stream.map(&get(grid, &1))
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end
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def indexes(pos, dir) do
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Stream.iterate(pos, &move(&1, dir))
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end
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def move({x, y}, :n), do: {x, y - 1}
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def move({x, y}, :e), do: {x + 1, y}
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def move({x, y}, :s), do: {x, y + 1}
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def move({x, y}, :w), do: {x - 1, y}
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def move(pos, :ne), do: pos |> move(:n) |> move(:e)
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def move(pos, :se), do: pos |> move(:s) |> move(:e)
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def move(pos, :sw), do: pos |> move(:s) |> move(:w)
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def move(pos, :nw), do: pos |> move(:n) |> move(:w)
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def get(grid, {x, y}) do
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{xs, ys} = bounds(grid)
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if x in xs and y in ys do
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elem(elem(grid, y), x)
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end
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end
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end
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