37 lines
838 B
Elixir
37 lines
838 B
Elixir
defmodule Day13Part2 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.drop(1)
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|> Enum.map(&parse_schedule/1)
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|> Enum.with_index()
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|> Enum.reject(&match?({:any, _}, &1))
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|> Enum.reduce({0, 1}, &find_sequential_departures/2)
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|> elem(0)
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|> IO.puts()
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end
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def parse_schedule(id) do
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case Integer.parse(id) do
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{id, ""} -> id
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_ -> :any
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end
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end
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# Adapted from voltone's solution ❤️
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# https://elixirforum.com/t/advent-of-code-2020-day-13/36180/6
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defp find_sequential_departures({bus, index}, {t, step}) do
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if rem(t + index, bus) == 0 do
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{t, lcm(step, bus)}
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else
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find_sequential_departures({bus, index}, {t + step, step})
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end
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end
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defp lcm(a, b) do
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div(a * b, Integer.gcd(a, b))
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end
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end
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Day13Part2.run()
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