My beginner’s solution, Day12 part 1. Hot Springs
It took a minute but it returned the correct answer 
defmodule Day12 do
def part1(input) do
parse(input)
|> Enum.map(&possible_arrangements/1)
|> Enum.sum
end
def possible_arrangements({springs, numbers}) do
possibilities(springs)
|> Enum.filter(&(valid?(&1, numbers)))
|> Enum.count |> tap(&IO.puts(&1))
end
def possibilities(springs, possible \\ [[]])
def possibilities([], possible), do: possible
def possibilities([:unknown | springs], possible) do
variant1 = Enum.map(possible, fn x -> x ++ [:damaged] end)
variant2 = Enum.map(possible, fn x -> x ++ [:operational] end)
possibilities(springs, variant1 ++ variant2)
end
def possibilities([:damaged | springs], possible) do
variant1 = Enum.map(possible, fn x -> x ++ [:damaged] end)
possibilities(springs, variant1)
end
def possibilities([:operational | springs], possible) do
variant2 = Enum.map(possible, fn x -> x ++ [:operational] end)
possibilities(springs, variant2)
end
def valid?(springs, numbers) do
springs
|> Enum.chunk_by(&(&1))
|> Enum.reject(fn s -> Enum.all?(s, &(&1 == :operational)) end)
|> Enum.map(&Enum.count/1)
|> Kernel.==(numbers)
end
def parse(raw_records) do
raw_records
|> String.split("\n")
|> Enum.map(&parse_line/1)
end
def parse_line(raw_line) do
[raw_chars, raw_numbers] = String.split(raw_line)
numbers = raw_numbers
|> String.split(",")
|> Enum.map(&String.to_integer/1)
springs = raw_chars
|> String.graphemes
|> Enum.map(&(case &1 do
"." -> :operational
"#" -> :damaged
"?" -> :unknown
end))
{springs, numbers}
end
end
