109 lines
3 KiB
Elixir
Executable file
109 lines
3 KiB
Elixir
Executable file
#!/usr/bin/env elixir
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defmodule Day8 do
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def part1({x, y, frequencies}) do
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Enum.reduce(frequencies, MapSet.new(), fn antennas, antinodes ->
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antennas
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|> find_pairs()
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|> Enum.flat_map(fn pair -> find_antinodes(pair, x, y) end)
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|> MapSet.new()
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|> MapSet.union(antinodes)
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end)
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|> MapSet.size()
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end
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def find_pairs([_]), do: []
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def find_pairs([a | rest]), do: for(b <- rest, do: {a, b}) ++ find_pairs(rest)
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def find_antinodes({{x_a, y_a}, {x_b, y_b}}, max_x, max_y) do
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dx = x_a - x_b
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dy = y_a - y_b
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antinode_1 = {x_a + dx, y_a + dy}
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antinode_2 = {x_b - dx, y_b - dy}
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Enum.filter([antinode_1, antinode_2], fn
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{x, y} -> 0 <= x and x < max_x and 0 <= y and y < max_y
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end)
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end
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def part2({x, y, frequencies}) do
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Enum.reduce(frequencies, MapSet.new(), fn antennas, antinodes ->
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antennas
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|> find_pairs()
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|> Enum.flat_map(fn pair -> find_resonant_harmonics(pair, x, y) end)
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|> MapSet.new()
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|> MapSet.union(antinodes)
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end)
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|> MapSet.size()
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end
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def find_resonant_harmonics({{x_a, y_a}, {x_b, y_b}}, max_x, max_y) do
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dx = x_a - x_b
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dy = y_a - y_b
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[{x_a, y_a}, {x_b, y_b}] ++
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resonate(x_a, y_a, dx, dy, max_x, max_y) ++
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resonate(x_b, y_b, dx * -1, dy * -1, max_x, max_y)
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end
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def resonate(x, y, dx, dy, max_x, max_y, multiplier \\ 1) do
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next_x = x + dx * multiplier
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next_y = y + dy * multiplier
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if next_x < 0 or next_x >= max_x or next_y < 0 or next_y >= max_y do
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[]
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else
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[{next_x, next_y} | resonate(x, y, dx, dy, max_x, max_y, multiplier + 1)]
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end
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end
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def input do
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with [input_filename] <- System.argv(),
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{:ok, input} <- File.read(input_filename) do
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{_, x, y, antennas} =
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for <<char::binary-1 <- input>>, reduce: {0, 0, 0, []} do
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{x, max_x, y, antennas} ->
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case char do
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"." -> {x + 1, max_x, y, antennas}
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"\n" -> {0, x, y + 1, antennas}
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freq -> {x + 1, max_x, y, [{freq, {x, y}} | antennas]}
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end
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end
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{x, y, antennas |> Enum.group_by(&elem(&1, 0), &elem(&1, 1)) |> Map.values()}
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else
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_ -> :error
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end
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end
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#######################
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# HERE BE BOILERPLATE #
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#######################
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def run do
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case input() do
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:error -> print_usage()
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input -> run_parts_with_timer(input)
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end
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end
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defp run_parts_with_timer(input) do
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run_with_timer(1, fn -> part1(input) end)
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run_with_timer(2, fn -> part2(input) end)
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end
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defp run_with_timer(part, fun) do
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{time, result} = :timer.tc(fun)
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IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
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IO.puts("#{inspect(result)}\n")
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end
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defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
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defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
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defp format_time(μsec), do: "#{μsec / 1_000_000}s"
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defp print_usage do
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IO.puts("Usage: elixir day8.exs input_filename")
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end
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end
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Day8.run()
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