day16.py

tickets = []
rules = {}
valid_range = set()
total = 0

for l in LINES:
    if l == "your ticket:" or l == "nearby tickets:":
        continue
    if ":" in l:
        n, rule = l.split(": ")
        a, b = rule.split(" or ")
        x1, y1 = [int(z) for z in a.split("-")]
        x2, y2 = [int(z) for z in b.split("-")]
        rules[n] = set(range(x1, y1 + 1)) | set(range(x2, y2 + 1))
        valid_range |= rules[n]
    else:
        ticket = ints(l)
        valid = True
        for field in ticket:
            if field not in valid_range:
                total += field
                valid = False
        if valid:
            tickets.append(ticket)

print(total)

mapping = collections.defaultdict(list)
for field in rules:
    for i in range(len(tickets[0])):
        if all(ticket[i] in rules[field] for ticket in tickets):
            mapping[field].append(i)

fixed = {}
while any(mapping.values()):
    for field, remaining in mapping.items():
        if len(remaining) == 1:
            fixed[field] = remaining[0]
            for other in mapping.values():
                if fixed[field] in other:
                    other.remove(fixed[field])
            break

result = 1
for field in fixed:
    if field.startswith("departure"):
        result *= tickets[0][fixed[field]]

print(result)