Day 18

day18/part2.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+import re
+
+lines = (x.strip().replace(" ", "") for x in open("input.txt"))
+
+
+def calc(a, b, pop):
+    return a+b if pop == "+" else a*b
+
+
+def pres(pop):
+    if pop == "+":
+        return 2
+    if pop == "*":
+        return 1
+    return 0
+
+
+def evaluate(line):
+    stack = []
+    postfix = [0]
+    pos = -1
+    for char in "("+line+")":
+        pos += 1
+        if char == '(':
+            stack.append("(")
+        elif char == ')':
+            while True:
+                pop = stack.pop()
+                if pop == '(':
+                    break
+                a, b = postfix.pop(), postfix.pop()
+                postfix.append(calc(a, b, pop))
+        elif char in "0123456789":
+            postfix[-1] = postfix[-1] * 10 + int(char)
+        elif char in "*+":
+            while True:
+                pop = stack.pop()
+                if pres(pop) <= pres(char):
+                    stack.append(pop)
+                    stack.append(char)
+                    postfix.append(0)
+                    break
+                a, b = postfix.pop(), postfix.pop()
+                postfix.append(calc(a, b, pop))
+        else:
+            raise SystemError("Invalid char: "+char)
+    return postfix[0]
+
+
+print(sum(evaluate(line) for line in lines))