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diff --git a/2020/aoc2020-d21.py b/2020/aoc2020-d21.py
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+#advent of code 2020
+#day 21
+#part 2 very similar to ticket problem from the same year
+
+ingredients = [];
+AllergyDict = dict();
+PossibleAllergens = set();
+AssignedAllergens = dict();
+
+PuzzleInput = open("21.in","r");
+for line in PuzzleInput:
+	line = line.replace("\n","");
+	FoodIngr, FoodAllr = line[:-1].split(" (contains ");
+	FoodIngr = FoodIngr.split(" ");
+	FoodAllr = FoodAllr.split(", ");
+	ingredients.extend(FoodIngr);	
+	for a in FoodAllr:
+		if a in AllergyDict:
+			AllergyDict[a].intersection_update(set(FoodIngr));
+		else:
+			AllergyDict[a] = set(FoodIngr);
+PuzzleInput.close();
+
+for al in AllergyDict:
+	print(al, AllergyDict[al]);
+	PossibleAllergens.update(AllergyDict[al]);
+
+p1 = len([i for i in ingredients if i not in PossibleAllergens]);
+print("part 1 =",p1);
+
+while len(AssignedAllergens) != len(AllergyDict):
+	#print(len(AssignedAllergens),"/", len(AllergyDict)); #in case it takes long
+	for al in AllergyDict:
+		if len(AllergyDict[al]) == 1:
+			found = AllergyDict[al].pop();
+			AssignedAllergens[al] = found;
+		else:
+			AllergyDict[al].difference_update( AssignedAllergens.values() );
+
+p2 = ",".join([AssignedAllergens[allergen] for allergen in sorted(AssignedAllergens.keys())]);
+print("part 2 =",p2);