AdventOfCode/2020/elixir/day9/day9part2.exs

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Elixir
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2020-12-09 11:03:34 +00:00
defmodule Day9Part2 do
def run do
input =
File.read!("input")
|> String.split("\n", trim: true)
|> Enum.map(&String.to_integer/1)
invalid_number = Day9Part1.find_invalid(input)
input
|> find_weakness(invalid_number)
|> IO.puts()
end
def find_weakness([current | rest], invalid_number) do
case find_group(rest, [current], current, invalid_number) do
{:ok, group} -> Enum.min(group) + Enum.max(group)
_ -> find_weakness(rest, invalid_number)
end
end
def find_group(_list, group, sum, target) when sum == target, do: {:ok, group}
def find_group(_list, _group, sum, target) when sum > target, do: :too_big
def find_group(_list, [], _sum, _target), do: :eol
def find_group([next | rest], group, sum, target) do
find_group(rest, [next | group], sum + next, target)
end
end
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)
2020-12-09 11:03:34 +00:00
end
Day9Part2.run()