verbose = False

def indices(lst, element):
	"""
	See <https://stackoverflow.com/questions/6294179/how-to-find-all-occurrences-of-an-element-in-a-list>.
	"""

	result = []
	offset = -1
	while True:
		try:
			offset = lst.index(element, offset+1)
		except ValueError:
			return result
		result.append(offset)

def load_input(filename="input"):
	with open(filename, "r") as fp:
		lines = fp.read().splitlines()

	# Find the empty line/separator.
	sep = lines.index("")

	# Handle rules
	rule_lines = lines[:sep]
	rules = [tuple(map(int, l.split("|"))) for l in rule_lines]

	# Handle lines
	update_lines = lines[sep + 1:]
	updates = [list(map(int, l.split(","))) for l in update_lines]

	return rules, updates

def is_update_legal(update, rules, print_offence=False) -> bool:
	for rule in rules:
		# Find indices of left hand side and right hand side
		lhs, rhs = rule

		if not lhs in update or not rhs in update:
			continue
		# This reminds me of limes inferior and limes superior.
		# Thanks, Mueger.
		if max(indices(update, lhs)) > min(indices(update, rhs)):
			if print_offence:
				print(f"Rule", rule, "offended by update", update)
			return False
	return True

def middle_page_num(update) -> int:
	return update[len(update) // 2]

if __name__ == "__main__":
	rules, updates = load_input()

	indices_legal = []
	middle_sum = 0
	for idx, update in enumerate(updates):
		if is_update_legal(update, rules):
			if verbose:
				print(update, " is legal")
			indices_legal.append(idx)
			middle_sum += middle_page_num(update)
		else:
			if verbose:		
				print(update, " is illegal")

	if verbose:
		print(indices_legal)
	print(f"The sum of middle page numbers of legal updates is {middle_sum}.")