87 lines
2.2 KiB
Elixir
87 lines
2.2 KiB
Elixir
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defmodule Day8 do
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def part1({instructions, network}) do
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instructions
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|> Stream.cycle()
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|> Enum.reduce_while({"AAA", 0}, fn
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_side, {"ZZZ", count} -> {:halt, count}
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side, {node, count} -> {:cont, {elem(network[node], side), count + 1}}
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end)
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end
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# Runs forever.
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# Need a better answer. Start at the end and validate can get there or something?
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def part2({instructions, network}) do
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starting_nodes =
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network
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|> Map.keys()
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|> Enum.filter(&match?(<<_::binary-2, "A">>, &1))
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instructions
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|> Stream.cycle()
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|> Enum.reduce_while({starting_nodes, 0}, fn side, {current_nodes, count} ->
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if Enum.all?(current_nodes, &match?(<<_::binary-2, "Z">>, &1)) do
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{:halt, count}
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else
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next_nodes = Enum.map(current_nodes, fn node -> elem(network[node], side) end)
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{:cont, {next_nodes, count + 1}}
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end
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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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[instructions, nodes] = String.split(input, "\n\n")
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instructions =
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instructions
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|> String.graphemes()
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|> Enum.map(fn
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"L" -> 0
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"R" -> 1
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end)
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network =
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for <<key::binary-3, " = (", left::binary-3, ", ", right::binary-3, ")\n" <- nodes>>,
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into: %{},
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do: {key, {left, right}}
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{instructions, network}
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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("#{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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