Hacked with elixir quoted expression, this can be called cheating I guess

Since the input is a valid elixir expression, getting AST is just treating input as elixir code, we dont even have to care about the brackets.

defmodule Advent2020.Day18 do
 def input do
   File.read!(Path.expand("day18.txt", :code.priv_dir(:advent_2020)))
   |> String.split("\n", trim: true)
 end

 defp replace(str, []), do: str

 defp replace(str, [{match, replacement} | rest]),
   do: String.replace(str, match, replacement) |> replace(rest)

 def parse_with_replcement(line, replace) do
   {:ok, quoted} = replace(line, replace) |> Code.string_to_quoted()
   quoted
 end

 defp replace_operation(num, _replacement) when is_integer(num), do: num

 defp replace_operation({operator, metadata, [a, b]}, replace) do
   {replace[operator] || operator, metadata,
    [replace_operation(a, replace), replace_operation(b, replace)]}
 end

 def run(replace) do
   revert = Enum.map(replace, fn {m, r} -> {String.to_atom(r), String.to_atom(m)} end)

   Enum.map(input(), fn line ->
     {result, []} =
       parse_with_replcement(line, replace)
       |> replace_operation(revert)
       |> Code.eval_quoted()

     result
   end)
   |> Enum.sum()
 end

 def part_one, do: run([{"*", "-"}])
 def part_two, do: run([{"*", "-"}, {"+", "/"}])
end

I originally parsed each equation into an expression tree to evaluate them - which worked, but then I saw on reddit someone mention the shunting yard algorithm - did a bunch of reading on operator precedence parsing and stack based calculators and was able to refactor my code in a really satisfying way. The implementation of the algorithm and the evaluator could use some clean up, but it works, and it works about 10 times faster then the expression tree approach

I spent most of the time in part 1 trying to get the recursive descent parser to make the evaluating order left-to-right. When I solved that part 2 was easy.

Here is my solution.

Such a pragmatic solution , I was so tired when tackling this and spent more-time-than-I-want-to-admit solving for merging the numbers back after splitting with String.graphemes() due to those parenthesis without whitespace

Well, if it gets the job done… Nice trick!