Advent of Code 2021 day 16 part 1

AoC_2021_16_1.py

def version_sum(packet):
    if int(packet, 2) == 0:  # padding with zeroes
        return 0, None

    version = int(packet[:3], 2)
    ptype = int(packet[3:6], 2)

    if ptype == 4:  # literal packet
        rest = packet[6:]
        while rest[0] == "1":
            rest = rest[5:]
        return version, rest[5:]

    else:  # operator packet
        ltype = packet[6]
        rest = packet[22:] if ltype == "0" else packet[18:]
        subtotals = 0
        while rest:
            subt, rest = version_sum(rest)
            subtotals += subt
        return version + subtotals, None


hexstr = open("input_16.txt").read().strip()
packet = "".join(f"{int(ch, 16):04b}" for ch in hexstr)
result, _ = version_sum(packet)
print(result)

I'd prefer group[-4:] instead of group[1:5].

I agree. Edited the source accordingly (revision 5).

Revision 6 is a major update that:

• has no need to calculate literal values (just consumes the literal packet)
• recurses the parsing without passing along the subtotal of versions