Solution to problem 18 in Python

src/2020/P18.py

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+# --- Day 18: Operation Order ---
+
+# 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.
+
+# The homework (your puzzle input) consists of a series of expressions that
+# consist of addition (+), multiplication (*), and parentheses ((...)). Just
+# like normal math, parentheses indicate that the expression inside must be
+# evaluated before it can be used by the surrounding expression. Addition still
+# finds the sum of the numbers on both sides of the operator, and
+# multiplication still finds the product.
+
+# However, the rules of operator precedence have changed. Rather than
+# evaluating multiplication before addition, the operators have the same
+# precedence, and are evaluated left-to-right regardless of the order in which
+# they appear.
+
+# For example, the steps to evaluate the expression 1 + 2 * 3 + 4 * 5 + 6 are
+# as follows:
+
+# 1 + 2 * 3 + 4 * 5 + 6
+#   3   * 3 + 4 * 5 + 6
+#       9   + 4 * 5 + 6
+#          13   * 5 + 6
+#              65   + 6
+#                  71
+
+# Parentheses can override this order; for example, here is what happens if
+# parentheses are added to form 1 + (2 * 3) + (4 * (5 + 6)):
+
+# 1 + (2 * 3) + (4 * (5 + 6))
+# 1 +    6    + (4 * (5 + 6))
+#      7      + (4 * (5 + 6))
+#      7      + (4 *   11   )
+#      7      +     44
+#             51
+
+# Here are a few more examples:
+
+#     2 * 3 + (4 * 5) becomes 26.
+#     5 + (8 * 3 + 9 + 3 * 4 * 3) becomes 437.
+#     5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4)) becomes 12240.
+#     ((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2 becomes 13632.
+
+# Before you can help with the homework, you need to understand it yourself.
+# Evaluate the expression on each line of the homework; what is the sum of the
+# resulting values?
+
+with open("files/P18.txt", "r") as f:
+    homework = [line for line in f.read().strip().split("\n")]
+
+
+def eval(n1, n2, op) -> int:
+    if op == "+":
+        return n1 + n2
+    elif op == "*":
+        return n1 * n2
+
+
+def simplify(expr) -> str:
+    while len(expr) > 1:
+        n1 = int(expr.pop())
+        op = expr.pop()
+        n2 = int(expr.pop())
+        result = eval(n1, n2, op)
+        expr.append(str(result))
+
+    return expr[0]
+
+
+def part_1() -> None:
+    total = 0
+    for line in homework:
+        line = line.replace("(", "( ")
+        line = line.replace(")", " )")
+        expr = line.split()
+
+        stack = []
+        for char in expr:
+            if char.isdigit() or char in ["+", "*", "("]:
+                stack.append(char)
+            elif char == ")":
+                ordered_stack = []
+                while stack[len(stack) - 1] != "(":
+                    ordered_stack.append(stack.pop())
+                stack.pop()
+
+                result = simplify(ordered_stack)
+                stack.append(result)
+        stack.reverse()
+        result = simplify(stack)
+        total += int(result)
+
+    print(f"The result of evaluating my homework is {total}")
+
+
+if __name__ == "__main__":
+    part_1()