AdventOfCode/2021/day16.exs

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Elixir
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defmodule Day16 do
def part1(input) do
{_type, _rest, _count, version_sum} = decode_packet(input, 0, 0)
version_sum
end
@literal_type 4
@literal_part 1
@literal_end 0
@total_length_type 0
@sub_packets_length_type 1
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defp decode_packet(<<version::3, @literal_type::3, rest::bits>>, count, version_sum) do
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decode_literal(rest, <<>>, count + 6, version_sum + version)
end
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defp decode_packet(<<version::3, operator_type::3, rest::bits>>, count, version_sum) do
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{sub_packets, rest, count, version_sum} =
case rest do
<<@total_length_type::1, length::15, rest::bits>> ->
decode_sub_packets_by(
:length,
length,
[],
rest,
count + 3 + 3 + 1 + 15,
version_sum + version
)
<<@sub_packets_length_type::1, num_sub_packets::11, rest::bits>> ->
decode_sub_packets_by(
:quantity,
num_sub_packets,
[],
rest,
count + 3 + 3 + 1 + 11,
version_sum + version
)
end
{{:operator, operator_type, sub_packets}, rest, count, version_sum}
end
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defp decode_literal(<<@literal_part::1, group::bits-4, rest::bits>>, acc, count, version_sum) do
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decode_literal(rest, <<acc::bits, group::bits>>, count + 5, version_sum)
end
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defp decode_literal(<<@literal_end::1, group::bits-4, rest::bits>>, acc, count, version_sum) do
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literal_binary = <<acc::bits, group::bits>>
<<literal::size(bit_size(literal_binary))>> = literal_binary
{{:literal, literal}, rest, count + 5, version_sum}
end
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defp decode_sub_packets_by(_method, 0, decoded_packets, bits, count, version_sum) do
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{Enum.reverse(decoded_packets), bits, count, version_sum}
end
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defp decode_sub_packets_by(method, remaining, decoded_packets, bits, count, version_sum) do
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{decoded_packet, rest, packet_size, version_sum} = decode_packet(bits, 0, version_sum)
remaining =
case method do
:length -> remaining - packet_size
:quantity -> remaining - 1
end
decode_sub_packets_by(
method,
remaining,
[decoded_packet | decoded_packets],
rest,
count + packet_size,
version_sum
)
end
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def part2(input) do
{packet, _rest, _count, _version_sum} = decode_packet(input, 0, 0)
evaluate(packet)
end
defp evaluate({:literal, literal}), do: literal
defp evaluate({:operator, op, args}) do
args
|> Enum.map(&evaluate/1)
|> calculate(op)
end
defp calculate(args, 0), do: Enum.sum(args)
defp calculate(args, 1), do: Enum.reduce(args, &Kernel.*/2)
defp calculate(args, 2), do: Enum.min(args)
defp calculate(args, 3), do: Enum.max(args)
defp calculate([a, b], 5), do: if(a > b, do: 1, else: 0)
defp calculate([a, b], 6), do: if(a < b, do: 1, else: 0)
defp calculate([a, b], 7), do: if(a == b, do: 1, else: 0)
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def input do
with [input_filename] <- System.argv(),
{:ok, input} <- File.read(input_filename) do
input
|> String.trim()
|> Base.decode16!()
else
_ -> :error
end
end
#######################
# HERE BE BOILERPLATE #
#######################
def(run) do
case input() do
:error -> print_usage()
input -> run_parts_with_timer(input)
end
end
defp run_parts_with_timer(input) do
run_with_timer(1, fn -> part1(input) end)
run_with_timer(2, fn -> part2(input) end)
end
defp run_with_timer(part, fun) do
{time, result} = :timer.tc(fun)
IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
IO.puts("#{result}\n")
end
defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
defp format_time(μsec), do: "#{μsec / 1_000_000}s"
defp print_usage do
IO.puts("Usage: elixir day16.exs input_filename")
end
end
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Day16.run()