As you look out the window and notice a heavily-forested continent slowly appear over the horizon, you are interrupted by the child sitting next to you. They're curious if you could help them with their math homework.

Unfortunately, it seems like this "math" follows different rules than you remember.

Read the full puzzle.

using System;
using System.Linq;
using System.Collections.Generic;

namespace AdventOfCode.Y2020.Day18;

[ProblemName("Operation Order")]
class Solution : Solver {

    public object PartOne(string input) => Solve(input, true);
    public object PartTwo(string input) => Solve(input, false);

    long Solve(string input, bool part1) {
        var sum = 0L;
        foreach (var line in input.Split("\n")) {
            // https://en.wikipedia.org/wiki/Shunting-yard_algorithm

            var opStack = new Stack<char>();
            var valStack = new Stack<long>();

            void evalUntil(string ops) {
                while (!ops.Contains(opStack.Peek())) {
                    if (opStack.Pop() == '+') {
                        valStack.Push(valStack.Pop() + valStack.Pop());
                    } else {
                        valStack.Push(valStack.Pop() * valStack.Pop());
                    }
                }
            }

            opStack.Push('(');

            foreach (var ch in line) {
                switch (ch) {
                    case ' ':
                        break;
                    case '*':
                        evalUntil("(");
                        opStack.Push('*');
                        break;
                    case '+':
                        evalUntil(part1 ? "(" : "(*");
                        opStack.Push('+');
                        break;
                    case '(':
                        opStack.Push('(');
                        break;
                    case ')':
                        evalUntil("(");
                        opStack.Pop();
                        break;
                    default:
                        valStack.Push(long.Parse(ch.ToString()));
                        break;
                }
            }

            evalUntil("(");

            sum += valStack.Single();
        }

        return sum;
    }
}

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