🎄 Year 2024 🔔 Day 07

Mix.install([{:flow, "~> 1.0"}])

Setup

input =
  File.read!("#{__DIR__}/../../../inputs/2024/day07.txt")
  |> String.split("\n", trim: true)
  |> Enum.map(fn line ->
    [result, numbers] = String.split(line, ":", trim: true)

    result = String.to_integer(result)
    numbers = String.split(numbers, " ", trim: true) |> Enum.map(&String.to_integer/1)

    {result, numbers}
  end)

Part 1

defmodule Helper do
  def check_result({result, numbers}), do: check_result(result, numbers)
  def check_result(result, [head]) when result == head, do: true
  def check_result(_result, [_number]), do: false
  def check_result(result, [head | _tail]) when head > result, do: false

  def check_result(result, [h1, h2 | tail]) do
    check_result(result, [h1 * h2 | tail]) or check_result(result, [h1 + h2 | tail])
  end
end

input
|> Enum.filter(&Helper.check_result/1)
|> Enum.map(&elem(&1, 0))
|> Enum.sum()

Part 2

defmodule Helper2 do
  def check_result({result, numbers}), do: check_result(result, numbers)
  def check_result(result, [head]) when result == head, do: true
  def check_result(_result, [_number]), do: false
  def check_result(result, [head | _tail]) when head > result, do: false

  def check_result(result, [h1, h2 | tail]) do
    check_result(result, [h1 * h2 | tail]) or
      check_result(result, [h1 + h2 | tail]) or
      check_result(result, [concat_numbers(h1, h2) | tail])
  end

  defp concat_numbers(a, b), do: String.to_integer("#{a}#{b}")
end

input
|> Enum.filter(&Helper2.check_result/1)
|> Enum.map(&elem(&1, 0))
|> Enum.sum()