Day 5

Solution

{:ok, contents} = File.read("#{__DIR__}/inputs/day05.txt")

parse = fn line, sep ->
  line
  |> String.split(sep)
  |> Enum.map(&String.to_integer/1)
end

[raw_rules, raw_pages] =
  contents
  |> String.split("\n\n")
  |> Enum.map(&String.split(&1, "\n", trim: true))

[rules, pages] = [
  Enum.map(raw_rules, &parse.(&1, "|")) |> MapSet.new(),
  Enum.map(raw_pages, &parse.(&1, ","))
]

ordered_page? = fn page ->
  ipage = Enum.with_index(page)
  pairs = for {x, i} <- ipage, {y, j} <- ipage, i < j, do: [x, y]
  Enum.all?(pairs, &amp;(&amp;1 in rules))
end

pages
|> Enum.filter(ordered_page?)
|> Enum.map(&amp;Enum.at(&amp;1, &amp;1 |> length() |> div(2)))
|> Enum.sum()

reorder = fn page ->
  degree = fn n -> Enum.count(page, &([n, &1] in rules)) end
  Enum.sort_by(page, degree)
end

pages
|> Enum.reject(ordered_page?)
|> Enum.map(reorder)
|> Enum.map(&Enum.at(&1, &1 |> length() |> div(2)))
|> Enum.sum()