39 lines
1.3 KiB
Elixir
39 lines
1.3 KiB
Elixir
defmodule Day10Part2 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.map(&String.to_integer/1)
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|> count_arrangements()
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|> IO.puts()
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end
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# Entry point. Start with one arrangement: the outlet.
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def count_arrangements(list), do: count_arrangements(%{0 => 1}, [0 | Enum.sort(list)])
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# Termination point. We've checked all the adapters. The number of arrangments
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# to the last adapter is the answer.
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def count_arrangements(arrangements, [last]), do: arrangements[last]
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# Check each adapter in turn by comparing it to the next three adapters.
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def count_arrangements(arrangements, [current | rest]) do
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count_for_adapter(current, Enum.take(rest, 3), arrangements)
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|> count_arrangements(rest)
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end
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# Compare the next three adapters to the current adapter.
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# For each checked adapter, if the joltage difference is 3 or less, we can connect them,
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# so add the number of arrangements up to the current adapter to that adapter.
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def count_for_adapter(current, next_three, arrangements) do
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Enum.reduce(next_three, arrangements, fn adapter, arrangements ->
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if adapter - current <= 3 do
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current_count = arrangements[current]
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Map.update(arrangements, adapter, current_count, &(&1 + current_count))
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else
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arrangements
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end
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end)
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end
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end
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Day10Part2.run()
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