53 lines
1.7 KiB
Python
53 lines
1.7 KiB
Python
#!/usr/bin/env python3
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from random import randint
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from math import log2
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class lfsr(object):
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def __init__(self, state, taps):
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self.state = state
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self.taps = taps
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# getters and setters (private attributes)
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def get_state(self):
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return self.__state
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def set_state(self, value):
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self.__state = value
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state = property(fget=get_state, fset=set_state, doc="lfsr state (current bits value in lfsr)")
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def get_taps(self):
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return self.__taps
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def set_taps(self, valeur):
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self.__taps = valeur
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taps = property(fget=get_taps, fset=set_taps, doc="lfsr taps (bit positions affecting next state)")
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def shift(self):
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'''
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calculate next state and return the output bit
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'''
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feedback = sum(self.state[tap] for tap in self.taps) % 2
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output = self.state[-1]
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self.state = [feedback] + self.state[:-1]
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return output
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def test_lfsr17():
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key = [randint(0, 1) for _ in range(16)] # first 16 bits
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key.append(1) # prevent initial state from being {0}^17
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taps = [16-0, 16-14]
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lfsr17 = lfsr(key, taps)
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states = [lfsr17.state]
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for _ in range(2**17-2):
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lfsr17.shift()
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states.append(lfsr17.state)
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sorted_states = sorted(states, key=lambda x: tuple(x))
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for i in range(2**17-2):
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if sorted_states[i] == sorted_states[i+1]: # compare each state with the next state in the sorted list, if 2 states are identical they should be next to each other
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print(f'state {sorted_states[i]} appears at least 2 times')
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return False
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n_state = len(sorted_states)
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n_state_log = log2(n_state+1)
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print(f'all {n_state} = 2^({n_state_log})-1 states are different')
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return True
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test_lfsr17()
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