39 lines
1.1 KiB
Elixir
39 lines
1.1 KiB
Elixir
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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Day9Part1.run()
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