Advent of code 2024 - Day 7

Mix.install([
  {:kino, "~> 0.14.2"}
])

Part 1

https://adventofcode.com/2024/day/7

textarea = Kino.Input.textarea("Give input:")

equations =
  Kino.Input.read(textarea)
  |> String.split("\n", trim: true)
  |> Enum.map(fn line -> [total, values_line] = String.split(line, ": ") 
        { String.to_integer(total), Enum.map(String.split(values_line), &String.to_integer/1) }
     end)

:ok

defmodule Solve do
  defp equation(_total, [], _oper), do: []
  defp equation(total, [total], oper), do: Enum.reverse(oper)
  defp equation(_total, [_val], _oper), do: []

  defp equation(total, [val1, val2 | other], oper) do
    plus = equation(total, [val1 + val2 | other], ["+" | oper])
    multi = equation(total, [val1 * val2 | other], ["*" | oper])

    if plus != [] and multi != [] do
      # two solutions
      [plus, multi]
    else
      plus ++ multi
    end
  end

  def equation({total, values}), do: equation(total, values, [])
end

IO.inspect Solve.equation({45, [3, 5, 3]})
IO.inspect Solve.equation({24, [3, 5, 3]})
IO.inspect Solve.equation({18, [3, 5, 3]})
IO.inspect Solve.equation({3267, [81, 40, 27]})
IO.inspect Solve.equation({6537, [1, 81, 40, 27, 2, 3]})

Enum.filter(equations, fn equation -> Solve.equation(equation) != [] end) 
|> Enum.map(fn {total, _values} -> total end)
|> Enum.sum

Part 2

defmodule SolvePart2 do
  defp equation(_total, [], _oper), do: []
  defp equation(total, [total], oper), do: Enum.reverse(oper)
  defp equation(_total, [_val], _oper), do: []

  # when negative numbers and 0 are not in the input, apply this optimalization:
  # defp equation(total, [val | _other], _oper) when val > total, do: []

  defp equation(total, [val1, val2 | other], oper) do
    plus = equation(total, [val1 + val2 | other], ["+" | oper])
    multi = equation(total, [val1 * val2 | other], ["*" | oper])

    concat =
      equation(total, [String.to_integer(to_string(val1) <> to_string(val2)) | other], [
        "|" | oper
      ])

    cond do
      plus != [] and multi != [] and concat != [] ->
        [plus, multi, concat]

      concat == [] and plus != [] and multi != [] ->
        [plus, multi]

      concat == [] ->
        plus ++ multi

      plus != [] ->
        [plus, concat]

      multi != [] ->
        [multi, concat]

      true ->
        concat
    end
  end

  def equation({total, values}), do: equation(total, values, [])
end

Enum.filter(equations, fn equation -> SolvePart2.equation(equation) != [] end)
|> Enum.map(fn {total, _values} -> total end)
|> Enum.sum()