aoc2020_18.py

class Evaluator:
    def __init__(self, string):
        self.tokens = re.findall(r"\d+|\*|\+|\(|\)", string)
        self.pos = 0

    def next(self):
        self.pos += 1
        return self.tokens[self.pos - 1]
        
    def consume(self, token):
        if self.pos < len(self.tokens) and  self.tokens[self.pos] == token:
            self.next()
            return True
        else:
            return False
    
    def evaluate_term(self):
        if self.consume("("):
            subexpr = self.evaluate()
            assert self.consume(")")
            return subexpr
        else:
            return int(self.next())
        
    def evaluate(self):
        result = self.evaluate_term()
        while True:
            if self.consume("+"):
                result += self.evaluate_term()
            elif self.consume("*"):
                result *= self.evaluate_term()
            else:
                break
        return result
    
assert Evaluator("2 * 3 + (4 * 5)").evaluate() == 26
assert Evaluator("5 + (8 * 3 + 9 + 3 * 4 * 3)").evaluate() == 437
assert Evaluator("5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))").evaluate() == 12240
assert Evaluator("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2").evaluate() == 13632

class Evaluator2(Evaluator):
    def parse_sum(self):
        result = self.evaluate_term()
        while self.consume("+"):
            result += self.evaluate_term()
        return result
        
    def evaluate(self):
        result = self.parse_sum()
        while self.consume("*"):
            result *= self.parse_sum()
        return result
    
assert Evaluator2("1 + (2 * 3) + (4 * (5 + 6))").evaluate() ==  51
assert Evaluator2("2 * 3 + (4 * 5)").evaluate() == 46
assert Evaluator2("5 + (8 * 3 + 9 + 3 * 4 * 3)").evaluate() == 1445
assert Evaluator2("5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))").evaluate() == 669060
assert Evaluator2("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2 ").evaluate() == 23340