aoc/2020/21/21.py

def main():
	with open("input.txt") as f:
		lines = f.read().split('\n')
	foods = [s[:-1].split(' (contains ') for s in lines]
	# map allergens to possible ingredients
	possible = [
		(allergen, set(food[0].split(' ')))
		for food in foods
		for allergen in food[1].split(', ')
		]
	# get sets of all allergens and all ingredients
	allergens = {p[0] for p in possible}
	ingredients = set.union(*[p[1] for p in possible])
	# find which ingredients are always listed for a given allergen
	possible = {
		a:set.intersection(*[
			s[1]
			for s in possible
			if s[0] == a]
		) for a in allergens
		}
	# reduce
	while any({i for i in possible if len(possible[i]) > 1}):
		for b in {j for j in possible if len(possible[j]) > 1}:
			for a in {i for i in possible if len(possible[i]) == 1}:
				possible[b] -= possible[a]
	# find sets of allergenic and non-allergenic
	unsafe = set.union(*list(possible.values()))
	safe = ingredients - unsafe
	#
	print('|non-allergen occurrences| =', sum(
		food[0].split().count(s)
		for food in foods
		for s in safe)
		)
	print('allergens:', ','.join(
		[
			list(possible[a])[0]
			for a in sorted(list(possible))
		])
		)

	"""
	all_ingredients = set()
	all_allergens = set()
	may_contain = {}
	unsafe_ingredients = {}
	foods = []
	for line in lines:
		ingredients, contains = line.split(' (contains ')
		ingredients = ingredients.split(' ')
		contains = contains[:-1].split(', ') # remove trailing ")" then split
		all_ingredients = all_ingredients | set(ingredients)
		all_allergens = all_allergens | set(contains)
	"""

if __name__ == "__main__":
	main()