aoc/2020/02/02.exs

regex = ~r{\A(\d+)-(\d+) (\w): (\w+)\z}

list = File.read!("input.txt")
|> String.split("\n")
|> Enum.map(&Regex.run(regex, &1, capture: :all_but_first)) # "all but first" removes matching substring, leaving only capture groups
|> Enum.map(
	fn [n1, n2, character, password] ->
		{
			String.to_integer(n1),
			String.to_integer(n2),
			character,
			String.graphemes(password) # into array of letters
		}
	end
)

IO.puts "Part 1: "
list
|> Enum.filter(
	fn {minimum, maximum, character, password} ->
		count = Enum.count(password, &(&1 == character))
		minimum <= count and count <= maximum
	end
)
|> Enum.count()
|> IO.puts()

IO.puts "Part 2: "
list
|> Enum.filter(
	fn {p1, p2, character, password} ->
		first = Enum.at(password, p1-1) == character
		second = Enum.at(password, p2-1) == character
		first != second # exclusive or is equivalent to two things not matching (not both true or both false)
	end
)
|> Enum.count()
|> IO.puts()