Day 7

Solution

{:ok, contents} = File.read("#{__DIR__}/inputs/day07.txt")

parse = fn line ->
  [raw_target | raw_numbers] = String.split(line)
  target = raw_target |> String.trim(":") |> String.to_integer()
  numbers = raw_numbers |> Enum.map(&String.to_integer/1)
  {target, numbers}
end

equations = contents
|> String.split("\n", trim: true)
|> Enum.map(parse)

defmodule Day07 do
  defp concat(a, b) do
    [a, b] |> Enum.flat_map(&Integer.digits/1) |> Integer.undigits()
  end

  defp all_results_reversed([x]), do: [x]

  defp all_results_reversed([x | xs]) do
    prev = all_results_reversed(xs)
    Enum.map(prev, &(x + &1)) ++ Enum.map(prev, &(x * &1)) ++ Enum.map(prev, &concat(&1, x))
  end

  def all_results(line) do
    line |> Enum.reverse() |> all_results_reversed()
  end
end

equations
|> Enum.filter(fn {target, numbers} -> target in Day07.all_results(numbers) end)
|> Enum.map(fn {target, _} -> target end)
|> Enum.sum()