🎄 Year 2020 🔔 Day 10

Parse input

filename = "#{__DIR__}/../../../inputs/2020/day10.txt"

parsed_input_stream =
  File.stream!(filename)
  |> Enum.map(&String.trim/1)
  |> Enum.map(&String.to_integer/1)
  |> Enum.sort()
  |> then(&Enum.concat([[0], &1, [List.last(&1) + 3]]))

Part One

parsed_input_stream
|> Enum.chunk_every(2, 1, :discard)
|> Enum.map(fn [a, b] -> b - a end)
|> Enum.frequencies()
|> then(fn %{1 => ones_count, 3 => threes_count} -> ones_count * threes_count end)

Part two

[head | tail] = parsed_input_stream

group_permutation_counts = %{
  1 => 1,
  2 => 1,
  3 => 2,
  4 => 4,
  5 => 7
}

Enum.reduce(tail, {[head], []}, fn e, {acc, result} ->
  if List.first(acc) == e - 3 do
    {[e], [acc | result]}
  else
    {[e | acc], result}
  end
end)
|> then(fn {acc, result} -> [acc | result] end)
|> Enum.map(&Enum.count/1)
|> Enum.map(&Map.fetch!(group_permutation_counts, &1))
|> Enum.product()