Day 8

Deps & preparation

Mix.install([
  {:kino, "~> 0.4.0"}
])

input = Kino.Input.textarea("Please paste your input file:")

Input

# [ [[first_part], [second]], [[first_part], [second]] ]
lines =
  Kino.Input.read(input)
  |> String.split(["\n", "|"], trim: true)
  |> Enum.map(&String.split/1)
  |> Enum.chunk_every(2)

Part 1

digit_lines = Enum.map(lines, &Enum.at(&1, 1))

digit_lines
|> Enum.map(fn digits ->
  digits
  |> Enum.group_by(&String.length/1)
  |> Map.take([2, 3, 4, 7])
  |> Enum.count()
end)
|> Enum.sum()

Part 2

# 1,4,7,8 -> 9,3 -> 5,0 -> 2,6
create_map = fn rosseta ->
  %{2 => [one], 3 => [seven], 4 => [four], 5 => p_235, 6 => p_069, 7 => [eight]} =
    rosseta
    |> Enum.map(&String.to_charlist/1)
    |> Enum.map(&Enum.sort/1)
    |> Enum.group_by(&length/1)

  nine = Enum.find(p_069, fn d -> length(d -- four) == 2 end)
  three = Enum.find(p_235, fn d -> length(d -- one) == 3 end)
  five = Enum.find(p_235 -- [three], fn d -> length(nine -- d) == 1 end)
  zero = Enum.find(p_069 -- [nine], fn d -> length(d -- one) == 4 end)
  two = (p_235 -- [three, five]) |> List.flatten()
  six = (p_069 -- [nine, zero]) |> List.flatten()

  %{
    zero => 0,
    one => 1,
    two => 2,
    three => 3,
    four => 4,
    five => 5,
    six => 6,
    seven => 7,
    eight => 8,
    nine => 9
  }
end

decode = fn map, digits ->
  digits
  |> Enum.map(&String.to_charlist/1)
  |> Enum.map(&Enum.sort/1)
  |> Enum.map(&Map.get(map, &1))
  |> Integer.undigits()
end

# [ [[first_part], [second]], [[first_part], [second]] ]
lines
|> Enum.map(fn line ->
  create_map.(Enum.at(line, 0)) |> decode.(Enum.at(line, 1))
end)
|> Enum.sum()