Day 10

Helpers

defmodule Helpers do
  def data() do
    "data/day10.txt"
    |> File.stream!()
    |> Stream.map(&String.trim/1)
  end

  @opens ~w{ ( [ < { }

  def check(str, stack \\ [])
  def check(<<>>, []), do: :ok
  def check(<<>>, stack) when stack != [], do: {:error, :eos, stack}
  def check(<>, stack) when c in @opens, do: check(rest, [c | stack])
  def check(<<")", rest::binary>>, ["(" | stack]), do: check(rest, stack)
  def check(<<"]", rest::binary>>, ["[" | stack]), do: check(rest, stack)
  def check(<<">", rest::binary>>, ["<" | stack]), do: check(rest, stack)
  def check(<<"}", rest::binary>>, ["{" | stack]), do: check(rest, stack)
  def check(<>, _stack), do: {:error, :unexpected, c}

  def error_score(")"), do: 3
  def error_score("]"), do: 57
  def error_score("}"), do: 1197
  def error_score(">"), do: 25137

  def complete_score("("), do: 1
  def complete_score("["), do: 2
  def complete_score("{"), do: 3
  def complete_score("<"), do: 4
  def complete_scores(stack), do: stack |> Enum.reduce(0, &amp;(&amp;2 * 5 + complete_score(&amp;1)))

  def mid(list), do: list |> Enum.at(list |> length() |> div(2))
end

Part 1

import Helpers

data()
|> Stream.map(&check/1)
|> Stream.filter(&match?({:error, :unexpected, _actual}, &1))
|> Stream.map(fn {:error, :unexpected, actual} -> actual end)
|> Stream.map(&error_score/1)
|> Enum.sum()

Part 2

import Helpers

data()
|> Stream.map(&check/1)
|> Stream.filter(&match?({:error, :eos, _stack}, &1))
|> Stream.map(fn {:error, :eos, stack} -> stack end)
|> Stream.map(&complete_scores/1)
|> Enum.to_list()
|> Enum.sort()
|> mid()