From c8b9636957dfe6b4bda712c15fd8b75b539e77ec Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Mon, 21 Dec 2020 12:04:32 +0100
Subject: [PATCH] 2020: d21: ex2: add solution

---
 2020/d21/ex2/ex2.py | 81 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100755 2020/d21/ex2/ex2.py

diff --git a/2020/d21/ex2/ex2.py b/2020/d21/ex2/ex2.py
new file mode 100755
index 0000000..4856636
--- /dev/null
+++ b/2020/d21/ex2/ex2.py
@@ -0,0 +1,81 @@
+#!/usr/bin/env python
+
+import functools
+import re
+import sys
+from collections import defaultdict
+from copy import deepcopy
+from typing import Dict, List, Set, Tuple
+
+
+def parse(raw: List[str]) -> Tuple[List[Set[str]], List[Set[str]]]:
+    def parse_ingredients(line: str) -> Set[str]:
+        pos = line.find(" (contains ")
+        if pos != -1:
+            line = line[:pos]
+        return set(line.split())
+
+    def parse_allergens(line: str) -> Set[str]:
+        pos = line.find("(contains ")
+        if pos == -1:
+            return set()
+        return set(re.findall("([^ ]+)[,\\)]", line))
+
+    ingredients = []
+    allergens = []
+
+    for line in raw:
+        ingredients.append(parse_ingredients(line))
+        allergens.append(parse_allergens(line))
+
+    return ingredients, allergens
+
+
+def find_allergens(
+    ingredients: List[Set[str]], allergens: List[Set[str]]
+) -> Dict[str, Set[str]]:
+    all_ingredients = functools.reduce(lambda lhs, rhs: lhs | rhs, ingredients)
+    possibilities: Dict[str, Set[str]] = defaultdict(lambda: deepcopy(all_ingredients))
+
+    for ing, all in zip(ingredients, allergens):
+        for allergen in all:
+            possibilities[allergen] &= set(ing)
+
+    return dict(possibilities)
+
+
+def cross_eliminate(possibilities: Dict[str, Set[str]]) -> None:
+    while True:
+        eliminated = False
+        for pos in possibilities:
+            if len(possibilities[pos]) != 1:
+                continue
+            for other_pos in possibilities:
+                if other_pos == pos:
+                    continue
+                if len(possibilities[other_pos] & possibilities[pos]) == 0:
+                    continue
+                eliminated = True
+                possibilities[other_pos] -= possibilities[pos]
+        if not eliminated:
+            break
+
+
+def solve(raw: List[str]) -> str:
+    ingredients, allergens = parse(raw)
+    possibilities = find_allergens(ingredients, allergens)
+    cross_eliminate(possibilities)
+    matches = [
+        (ingredient.pop(), allergen) for allergen, ingredient in possibilities.items()
+    ]
+    matches.sort(key=lambda tup: tup[1])
+    return ",".join(ingredient for ingredient, __ in matches)
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()