SMB3 LFSR

smb3_lfsr.py

from time import time  # not 100% necessary, just used to seed the RNG


def nes_lfsr(seed: int) -> int:
    """
    Implementation of the Linear-Feedback Shift Register RNG used by SMB3

    RNG will repeat after 32,767 (0x7FFF) iterations (the maximum 15-bit
    integer value) regardless of the seed 
    """
    if seed <= 0:
        raise ValueError('seed value must be >= 1')
    MASK = 0x7FFF
    rand = seed & MASK # start with the seed value, masked to 15 bits
    while True:
        # XOR the 0th and 8th bits
        xor = ((rand >> 8) & 1) ^ (rand & 1)
        # shift the value over 1 bit, insert XOR value at MSB
        rand = (rand >> 1 | xor << 14) & MASK
        yield rand


seed = int(time())
rng = nes_lfsr(seed)

for _ in range(0x7FFF):
    print(next(rng))