From 03de3587bdac1f610368d5d23cdb4a8350f2472d Mon Sep 17 00:00:00 2001
From: dece <shgck@pistache.land>
Date: Fri, 18 Dec 2020 13:02:09 +0100
Subject: [PATCH] Day 18

---
 2020/day18.py | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 2020/day18.py

diff --git a/2020/day18.py b/2020/day18.py
new file mode 100644
index 0000000..f0834dd
--- /dev/null
+++ b/2020/day18.py
@@ -0,0 +1,54 @@
+import re
+import sys
+
+
+SUBEXPR = re.compile(r"\([^\(\)]+\)")
+
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+
+    # Part 1
+    acc = 0
+    for line in lines:
+        while (subexprs := list(SUBEXPR.finditer(line))):
+            for sub in subexprs:
+                sub_text = sub.group(0)
+                value = spacemath(sub_text.strip("()").split())
+                line = line.replace(sub_text, str(value), 1)
+        acc += spacemath(line.split())
+    print("Sum:", acc)
+
+    # Part 2
+    acc = 0
+    for line in lines:
+        while (subexprs := list(SUBEXPR.finditer(line))):
+            for sub in subexprs:
+                sub_text = sub.group(0)
+                value = advanced(sub_text.strip("()").split())
+                line = line.replace(sub_text, str(value), 1)
+        acc += advanced(line.split())
+    print("Sum with advanced math:", acc)
+
+
+def spacemath(tokens):
+    acc = int(tokens[0])
+    for i in range(1, len(tokens) - 1, 2):
+        op, rhs = tokens[i], tokens[i + 1]
+        acc = (acc + int(rhs)) if op == "+" else (acc * int(rhs))
+    return acc
+
+
+def advanced(tokens):
+    while len(tokens) > 1:  # solve additions first
+        try:
+            i = tokens.index("+")
+        except ValueError:
+            break
+        simplification = int(tokens[i - 1]) + int(tokens[i + 1])
+        tokens[i - 1:i + 2] = [simplification]
+    return spacemath(tokens)  # lazily reuse spacemath for multiplication
+
+
+if __name__ == "__main__":
+    main()