From 874ab7acad1c3f88d3117ae2713ea66d758289a1 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Fri, 18 Dec 2020 10:10:08 +0100
Subject: [PATCH] 2020: d18: ex2: add solution

---
 2020/d18/ex2/ex2.py | 79 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100755 2020/d18/ex2/ex2.py

diff --git a/2020/d18/ex2/ex2.py b/2020/d18/ex2/ex2.py
new file mode 100755
index 0000000..a83161c
--- /dev/null
+++ b/2020/d18/ex2/ex2.py
@@ -0,0 +1,79 @@
+#!/usr/bin/env python
+
+import sys
+from typing import List
+
+"""
+E : T [ * T ]*
+
+T : G [ + G ]*
+
+G : '(' E ')' | CONSTANT
+"""
+
+
+def parse_g(expr: List[str]) -> int:
+    top = expr.pop(0)
+    if top == "(":
+        ans = parse_e(expr)
+        assert expr.pop(0) == ")"
+        return ans
+    return int(top)
+
+
+def parse_t(expr: List[str]) -> int:
+    lhs = parse_g(expr)
+    while len(expr) and expr[0] == "+":
+        expr.pop(0)
+        rhs = parse_g(expr)
+        lhs += rhs
+    return lhs
+
+
+def parse_e(expr: List[str]) -> int:
+    lhs = parse_t(expr)
+    while len(expr) and expr[0] == "*":
+        expr.pop(0)
+        rhs = parse_t(expr)
+        lhs *= rhs
+    return lhs
+
+
+def parse_infix(input: List[str]) -> int:
+    """
+    Parses the given string in infix notation.
+    """
+    ans = parse_e(input)
+    assert len(input) == 0
+    return ans
+
+
+def tokenize(expr: str) -> List[str]:
+    res = []
+
+    def split_tok(tok: str) -> None:
+        if "(" not in tok and ")" not in tok:
+            res.append(tok)
+        if "(" in tok:
+            res.append("(")
+            split_tok(tok[1:])
+        if ")" in tok:
+            split_tok(tok[:-1])
+            res.append(")")
+
+    for tok in expr.split():
+        split_tok(tok)
+    return res
+
+
+def solve(raw: List[str]) -> int:
+    return sum(parse_infix(tokenize(expr)) for expr in raw)
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()