70 lines
2.0 KiB
Elixir
70 lines
2.0 KiB
Elixir
defmodule Day9Part2 do
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def run do
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input =
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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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invalid_number = Day9Part1.find_invalid(input)
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input
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|> find_weakness(invalid_number)
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|> IO.puts()
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end
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def find_weakness([current | rest], invalid_number) do
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case find_group(rest, [current], current, invalid_number) do
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{:ok, group} -> Enum.min(group) + Enum.max(group)
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_ -> find_weakness(rest, invalid_number)
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end
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end
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def find_group(_list, group, sum, target) when sum == target, do: {:ok, group}
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def find_group(_list, _group, sum, target) when sum > target, do: :too_big
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def find_group(_list, [], _sum, _target), do: :eol
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def find_group([next | rest], group, sum, target) do
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find_group(rest, [next | group], sum + next, target)
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end
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end
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defmodule Day9Part1 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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|> find_invalid()
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|> IO.puts()
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end
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def find_invalid([_ | next] = numbers) do
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{preamble, [number | _]} = Enum.split(numbers, 25)
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sorted_preamble = Enum.sort(preamble)
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max = number - hd(sorted_preamble)
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if sum_in_preamble?(sorted_preamble, max, number) do
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find_invalid(next)
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else
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number
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end
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end
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def sum_in_preamble?([], _max, _number), do: false
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def sum_in_preamble?([preamble_head | _], max, _number) when preamble_head >= max, do: false
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def sum_in_preamble?([preamble_head | preamble_rest], max, number) do
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if forms_pair?(preamble_head, preamble_rest, max, number) do
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true
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else
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sum_in_preamble?(preamble_rest, max, number)
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end
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end
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def forms_pair?(_x, [y | _], max, _number) when y > max, do: false
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def forms_pair?(_x, [], _max, _number), do: false
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def forms_pair?(x, [y | _], _max, number) when x + y == number, do: true
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def forms_pair?(x, [_y | rest], max, number), do: forms_pair?(x, rest, max, number)
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end
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Day9Part2.run()
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