#17-bit lfsr---This is one of the two LFSR's used in
#the (broken) Content Scrambling System to copy-protect DVDs,
#and is used in one of the problems in Assignment 2.

#State is implemented as an integer; this will be overridden
#by initstate.

#I used a lot of bitwise operations--shifts and masks and the like--
#rather than the more expensive operations of division and remaindering.
#Probably unnecessary in small examples of this scale.

import bytestuff

state = 1

#The key is a string which we will convert to
#a long integer and then grab the low-order 17 bits
#as a regular int to initialize the register.

def init_state(key):
    global state
    bytelist=bytestuff.ascii_to_bytes(key)
    state=int(bytestuff.bytes_to_long(bytelist)&(0x1ffff))



#update the register and return the new high-order bit
def update_state():
    global state
    lowbit=state&1
    tapbit=((1<<14)&state)>>14
    state=(state>>1)
    highbit=(lowbit^tapbit)<<16
    state = state|highbit
    return lowbit^tapbit

#Get the next byte from the register
def next_byte():
    val=0
    for j in range(8):
        val=2*val+update_state()
    return val

#Return a list of the next k bytes from the register
def get_bytes(k):
    blist=[]
    for j in range(k):
        blist.append(next_byte())
    return blist