40 lines
1.1 KiB
Elixir
40 lines
1.1 KiB
Elixir
defmodule Day9Part1 do
|
|
def run do
|
|
File.read!("input")
|
|
|> String.split("\n", trim: true)
|
|
|> Enum.map(&String.to_integer/1)
|
|
|> find_invalid()
|
|
|> IO.puts()
|
|
end
|
|
|
|
def find_invalid([_ | next] = numbers) do
|
|
{preamble, [number | _]} = Enum.split(numbers, 25)
|
|
sorted_preamble = Enum.sort(preamble)
|
|
max = number - hd(sorted_preamble)
|
|
|
|
if sum_in_preamble?(sorted_preamble, max, number) do
|
|
find_invalid(next)
|
|
else
|
|
number
|
|
end
|
|
end
|
|
|
|
def sum_in_preamble?([], _max, _number), do: false
|
|
def sum_in_preamble?([preamble_head | _], max, _number) when preamble_head >= max, do: false
|
|
|
|
def sum_in_preamble?([preamble_head | preamble_rest], max, number) do
|
|
if forms_pair?(preamble_head, preamble_rest, max, number) do
|
|
true
|
|
else
|
|
sum_in_preamble?(preamble_rest, max, number)
|
|
end
|
|
end
|
|
|
|
def forms_pair?(_x, [y | _], max, _number) when y > max, do: false
|
|
def forms_pair?(_x, [], _max, _number), do: false
|
|
def forms_pair?(x, [y | _], _max, number) when x + y == number, do: true
|
|
def forms_pair?(x, [_y | rest], max, number), do: forms_pair?(x, rest, max, number)
|
|
end
|
|
|
|
Day9Part1.run()
|