aoc2020day18.ex

defmodule Day18 do
  def read_input do
    File.read!("input/day18.txt")
    |> String.split("\n", trim: true)
  end

  def solve_part2(expr) do
    cond do
      String.contains?(expr, "(") ->
        Regex.replace(~r/\(([^()]+)\)/, expr, fn _, expr -> solve_part2(expr) end)
        |> solve_part2

      String.contains?(expr, "+") ->
        Regex.replace(~r/(\d+) \+ (\d+)/, expr, fn
          _, op1, op2 -> Integer.to_string(String.to_integer(op1) + String.to_integer(op2))
        end)
        |> solve_part2

      String.contains?(expr, "*") ->
        Regex.replace(~r/(\d+) \* (\d+)/, expr, fn
          _, op1, op2 -> Integer.to_string(String.to_integer(op1) * String.to_integer(op2))
        end)
        |> solve_part2
        
        true -> expr
    end
  end

  def solve_part1(expr) do
    cond do
      String.contains?(expr, "(") ->
        Regex.replace(~r/\(([^()]+)\)/, expr, fn _, expr -> solve_part1(expr) end)
        |> solve_part1

      String.match?(expr, ~r/^(\d+) \+ (\d+)/) ->
        Regex.replace(~r/^(\d+) \+ (\d+)/, expr, fn
          _, op1, op2 -> Integer.to_string(String.to_integer(op1) + String.to_integer(op2))
        end)
        |> solve_part1

      String.match?(expr, ~r/^(\d+) \* (\d+)/) ->
        Regex.replace(~r/^(\d+) \* (\d+)/, expr, fn
          _, op1, op2 -> Integer.to_string(String.to_integer(op1) * String.to_integer(op2))
        end)
        |> solve_part1

      true -> expr
    end
  end

  def part1 do
    read_input()
    |> Enum.map(&solve_part1/1)
    |> Enum.map(&String.to_integer/1)
    |> Enum.sum
    |> IO.inspect
  end

  def part2 do
    read_input()
    |> Enum.map(&solve_part2/1)
    |> Enum.map(&String.to_integer/1)
    |> Enum.sum
    |> IO.inspect
  end
end